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Meteor Blaze Components

Blaze Components for Meteor are a system for easily developing complex UI elements that need to be reused around your Meteor app. You can use them in ES2015, vanilla JavaScript, and CoffeeScript.

See live tutorial for an introduction.

Adding this package to your Meteor application adds BlazeComponent and BlazeComponentDebug classes into the global scope.

Both client and server side.

Table of contents

Installation

meteor remove blaze-html-templates
meteor remove templating
meteor add peerlibrary:blaze-components

If you get an error that two packages are trying to handle *.html files, you have to make sure that your app or a package does not depend on the templating packages (often through the blaze-html-templates package). Blaze Components supersedes the templating package and provides its functionality as well.

Blaze Components compile HTML template files both on the client and server side. If you have previously been adding HTML files on the server side as well, you might want to limit those to the client side only.

Additional packages

Components

While Blaze Components are built on top of Blaze, Meteor's a powerful library for creating live-updating user interfaces, its public API and semantics are different with the goal of providing extensible and composable components through unified and consistent interface.

This documentation assumes familiarity with Blaze and its concepts of templates, template helpers, data contexts, and reactivity, but we will also turn some of those concepts around. For a gentle introduction to Blaze Components see the tutorial.

A Blaze Component is defined as a class providing few methods Blaze Components system will call to render a component and few methods which will be called through a lifetime of a component. See the reference for the list of all methods used and/or provided by Blaze Components.

A basic component might look like the following (using the reactive-field) package as well).

class ExampleComponent extends BlazeComponent {
  // Life-cycle hook to initialize component's state.
  onCreated() {
    // It is a good practice to always call super.
    super.onCreated();
    this.counter = new ReactiveField(0);
  }

  // Mapping between events and their handlers.
  events() {
    // It is a good practice to always call super.
    return super.events().concat({
      // You could inline the handler, but the best is to make
      // it a method so that it can be extended later on.
      'click .increment': this.onClick
    });
  }

  onClick(event) {
    this.counter(this.counter() + 1);
  }

  // Any component's method is available as a template helper in the template.
  customHelper() {
    if (this.counter() > 10) {
      return "Too many times";
    }
    else if (this.counter() === 10) {
      return "Just enough";
    }
    else {
      return "Click more";
    }
  }
}

// Register a component so that it can be included in templates. It also
// gives the component the name. The convention is to use the class name.
ExampleComponent.register('ExampleComponent');
<!-- By default a template with the component's name will be used for the content. -->
<template name="ExampleComponent">
  <button class="increment">Click me</button>
  {{! You can include subtemplates to structure your templates. }}
  {{> subTemplate}}
</template>

<!-- We use camelCase to distinguish it from the component's template. -->
<template name="subTemplate">
  {{! You can access component's properties. }}
  <p>Counter: {{counter}}</p>
  {{! And component's methods. }}
  <p>Message: {{customHelper}}</p>  
</template>

You can see how to register a component, define a template, define a life-cycle hook, event handlers, and a custom helper as a component method.

All template helpers, methods, event handlers, life-cycle hooks have this bound to the component.

JavaScript and CoffeeScript support

While documentation is in ES2015, Blaze Components are designed to be equally easy to use with vanilla JavaScript and CoffeeScript classes as well.

Example above in vanilla JavaScript:

var ExampleComponent = BlazeComponent.extendComponent({
  onCreated: function () {
    Object.getPrototypeOf(ExampleComponent).prototype.onCreated.call(this);
    this.counter = new ReactiveField(0);
  },

  events: function () {
    return Object.getPrototypeOf(ExampleComponent).prototype.events.call(this).concat({
      'click .increment': this.onClick
    });
  },

  onClick: function (event) {
    this.counter(this.counter() + 1);
  },

  customHelper: function () {
    if (this.counter() > 10) {
      return "Too many times";
    }
    else if (this.counter() === 10) {
      return "Just enough";
    }
    else {
      return "Click more";
    }
  }
}).register('ExampleComponent');

Example in CoffeeScript:

class ExampleComponent extends BlazeComponent
  @register 'ExampleComponent'

  onCreated: ->
    super
    @counter = new ReactiveField 0

  events: ->
    super.concat
      'click .increment': @onClick

  onClick: (event) ->
    @counter @counter() + 1

  customHelper: ->
    if @counter() > 10
      "Too many times"
    else if @counter() is 10
      "Just enough"
    else
      "Click more"

Accessing data context

Blaze Components are designed around the separation of concerns known as model–view–controller (MVC). Controller and its logic is implemented through a component. View is described through a template. And model is provided as a data context to a controller, a component.

Data context is often reactive. It often comes from a database using Meteor's reactive stack. Often as data context changes, components stays rendered and just how it is rendered changes.

When accessing values in a template, first component methods are searched for a property (with possible mixins), then global template helpers, and lastly the data context.

You can provide a data context to a component when you are including it in the template.

Examples:

{{> MyComponent}}

{{#with dataContext}}
  {{> MyComponent}}
{{/with}}

{{#each documents}}
  {{> MyComponent}}
{{/each}}

{{> MyComponent dataContext}}

{{> MyComponent a='foo' b=helper}}

dataContext, documents and helper are template helpers, component's methods. If they are reactive, the data context is reactive.

You can access provided data context in your component's code through reactive data and currentData methods. There is slight difference between those two. The former always returns component's data context, while the latter returns the data context from where it was called. It can be different in template helpers and event handlers.

Example:

<template name="Buttons">
  <button>Red</button>
  {{color1}}
  {{#with color='blue'}}
    <button>Blue</button>
    {{color2}}
  {{/with}}
</template>

If top-level data context is {color: "red"}, then currentData inside a color1 component method (template helper) will return {color: "red"}, but inside a color2 it will return {color: "blue"}. Similarly, click event handler on buttons will by calling currentData get {color: "red"} as the data context for red button, and {color: "blue"} for blue button. In all cases data will return {color: "red"}.

Because data and currentData are both component's methods you can access them in a template as well. This is useful when you want to access a data context property which is shadowed by a component's method.

Example:

<template name="Colors">
  {{color}}
  {{#with color='blue'}}
    {{! If "color" is the component's method, this would resolve to the method, and not to the data context. }}
    {{color}}
    {{! If you want to make sure you are calling the component's method. }}
    {{component.color}}
    {{! To access the component's data context from an inner data context, use "data". }}
    {{data.color}}
    {{! Or first access the component. }}
    {{component.data.color}}
    {{! To access the current data context through the component method. }}
    {{currentData.color}}
    {{! Alternatively, you can also use keyword "this". }}
    {{this.color}}
  {{/with}}
</template>

See Spacebars documentation for more information how to specify and work with the data context in templates.

Specifying a data context to a component in the code will be provided through the renderComponent method which is not yet public.

Passing arguments

Blaze Components automatically instantiate an instance of a component when needed. In most cases you pass data to a component as its data context, but sometimes you want to pass arguments to component's constructor. You can do that as well with the special args syntax:

{{> MyComponent args 'foo' key='bar'}}

Blaze Components will call MyComponent's constructor with arguments foo and Spacebars.kw({key: 'bar'}) when instantiating the component's class. Keyword arguments are wrapped into Spacebars.kw.

Compare:

{{> MyComponent key='bar'}}

MyComponent's constructor is called without any arguments, but the data context of a component is set to {key: 'bar'}.

{{> MyComponent}}

MyComponent's constructor is called without any arguments and the data context is kept as it is.

When you want to use a data context and when arguments depends on your use case and code structure. Sometimes your class is not used only as a component and requires some arguments to the constructor.

A general rule of thumb is that if you want the component to persist while data used to render the component is changing, use a data context. But if you want to reinitialize the component itself if your data changes, then pass that data through arguments. Component is always recreated when any argument changes. In some way arguments configure the life-long properties of a component, which then uses data context reactively when rendering.

Another look at it is from the MVC perspective. Arguments configure the controller (component), while data context is the model. If data is coming from the database, it should probably be a data context.

Passing arguments to a component method which returns a component to be included, something like {{> getComponent args 'foo' key='bar'}} is not yet possible.

Life-cycle hooks

There are multiple stages in the life of a component. In the common case it starts with a class which is instantiated, rendered, and destroyed.

Life-cycle hooks are called in order:

  1. Class constructor
  2. mixinParent (mixins only) – called on a mixin after it has been created to associate it with its component
  3. onCreated – called once a component is being created before being inserted into DOM
  4. onRendered – called once a rendered component is inserted into DOM
  5. onDestroyed – called once a component was removed from DOM and is being destroyed

The suggested use is that most of the component related initialization should be in onCreated and constructor should be used for possible other uses of the same class. constructor does receive optional arguments though.

Mixins share life-cycle with the component and their life-cycle hooks are called automatically by Blaze Components.

Life-cycle of a component is is the common case linked with its life in the DOM. But you can create an instance of a component which you can keep a reference to and reuse it multiple times, thus keeping its state between multiple renderings. You can do this using the renderComponent method which is not yet public.

Handling events

There are two ways to handle DOM events in Blaze Components. The first is by using event maps in a similar way how you would bind event handlers in Blaze or jQuery.

The issue is that using event maps is error prone. Your selector can match too much, or too little, especially when component's template is changed in the future. Furthermore, event maps invert the relationship between templates and components. Observe how components provide template helpers to templates and how you define in the template itself how those template helpers are mapped to the template. Logic stays in the component, but where exactly the template helper is used is defined in the template. If you are modifying the template, you can easily see how you have to move template helpers around to keep the template working. With event maps this is inverted. You often have to define custom CSS classes to allow event handlers to be bound to the DOM element and later on is unclear how those CSS classes are connected to the event handlers. If you are modifying the template you have to be careful to maintain correct mapping of event handlers.

Blaze Components provide an alternative approach. You can define event handlers as component's methods and they are available to you to bind them to DOM elements in the template directly. The recommended approach is to name event handlers based on which element they are handling and not what they are doing:

<template name="ExampleComponent">
  <button onClick={{onButtonClick}}>Click me</button>
</template>
class ExampleComponent extends BlazeComponent {
  onCreated() {
    super.onCreated();
    this.counter = new ReactiveField(0);
  }

  onButtonClick(event) {
    this.counter(this.counter() + 1);
    this.callFirstWith(this, 'onButtonClick', event);
  }

  customHelper() {
    if (this.counter() > 10) {
      return "Too many times";
    }
    else if (this.counter() === 10) {
      return "Just enough";
    }
    else {
      return "Click more";
    }
  }
}

Blaze Components will recognize as an event binding attribute any element attribute beginning with on followed by a event name starting with a capital letter. In these examples, onClick is recognized as a binding for the click event, but onclick would be passed on as it is as an attribute to the DOM. onKeyup would be recognized as a binding for the keyup event, and so on.

Moreover, observe the callFirstWith call. This allows mixins to hook into the same event. Only the first resolved event handler with a given name is called and any other event handlers with the same name are not called automatically for events bound in templates.

While it is recommended that you define only one event handler per event per element and then do multiple actions inside the component's code, you can still use the following syntax to bind multiple event handlers:

<template name="ExampleComponent">
  <button onClick="{{onClick1}} {{onClick2}}">Click me</button>
</template>

If you do not provide the name of the event handler, the name of the event is used instead. This will call onClick method on the component for every click event on the button:

<template name="ExampleComponent">
  <button onClick>Click me</button>
</template>

Furthermore, when using a template helper to return a dynamic set of attributes, if any of those attributes are named like an event, the value will be bound as a event handler (or multiple of those if the value is an array):

<template name="ExampleComponent">
  <button {{attrs}}>Click me</button>
</template>
class ExampleComponent extends BlazeComponent {
  /* ... */
  
  attrs() {
    return {
      onClick: this.onButtonClick,
      title: "Button title"
    }
  }
}

You can also pass custom arguments to the event handlers:

<template name="ExampleComponent">
  <button onClick={{onButtonClick 'first'}}>Click me</button>
  <button onClick={{onButtonClick 'second'}}>Click me</button>
</template>
onButtonClick(event, argument) {
  this.counter(this.counter() + 1);
  this.callFirstWith(this, 'onButtonClick', event, argument);
}

Those arguments can even be reactive.

Component-based block helpers

You can use Blaze Components to define block helpers as well.

Example:

<template name="TableWrapperBlockComponent">
  <table>
    <thead>
      <tr>
        <th>Name</th>
        <th>Email</th>
      </tr>
    </thead>
    <tbody>
      {{> Template.contentBlock}}
    </tbody>
    <tfoot>
      <tr>
        <td colspan="2">
          {{> Template.elseBlock}}
        </td>
      </tr>
    </tfoot>
  </table>
</template>
{{#TableWrapperBlockComponent}}
  {{#each customers}}
    <td>{{name}}</td>
    <td>{{email}}</td>
  {{/each}}
{{else}}
  <p class="copyright">Content available under the CC0 license.</p>
{{/TableWrapperBlockComponent}}

You can use Template.contentBlock and Template.elseBlock to define "content" and "else" inclusion points.

You can modify just block helpers data context by passing it in the tag:

<template name="TableWrapperBlockComponent">
  <table class="{{color}}">
...
{{#TableWrapperBlockComponent color='red'}}
...

Notice that block helper's data context is available only inside a block helper's template, but data context where it is used (one with customers) stays the same. On the other hand, wrapping template content with a block helper component does change what currentComponent returns: it returns the block helper component instance. In a way you can still access the block helper's data context through its component's instance (and data).

When working with block helper components it is important to remember: block helper's data context is hidden when traversing the tree of data contexts, but the block helper's component is available in the component tree as the parent of any components included inside the block helper's content, both the content of the block helper's component itself, and the content wrapped with the block helper.

You can also pass arguments to a component:

{{#TableWrapperBlockComponent args displayCopyright=false}}
...

For when to use a data context and when arguments the same rule of thumb from the Passing arguments section applies.

Blaze provides up to two inclusion points in block helpers. If you need more you should probably not use a component as a block helper but move the logic to the component's method, returning a rendered Blaze Component instance or template which provides any content you want. You can provide content (possibly as Blaze Components themselves) to the component through your component arguments or data context.

Example:

<template name="CaseComponent">
  {{> renderCase}}
</template>

<template name="useCaseTemplate">
  {{> CaseComponent args left='LeftComponent' middle='MiddleComponent' right='RightComponent'}}
</template>
class CaseComponent extends BlazeComponent {
  constructor(kwargs) {
    super();

    this.cases = kwargs.hash;
  }

  renderCase() {
    let caseComponent = this.cases[this.data().case];
    if (!caseComponent) return null;
    return BlazeComponent.getComponent(caseComponent).renderComponent(this.currentComponent());
  }
}

CaseComponent.register('CaseComponent');

If you use CaseComponent now in the {case: 'left'} data context, a LeftComponent component will be rendered. If you want to control in which data context LeftComponent is rendered, you can specify data context as {{> renderCase dataContext}}.

Example above is using renderComponent method which is not yet public.

Animations

Blaze Components provide low-level DOM manipulation hooks you can use to hook into insertion, move, or removal of DOM elements. Primarily you can use this to animate manipulation of DOM elements, but at the end you have to make sure you do the requested DOM manipulation correctly because Blaze will expect it done.

Hooks are called only when DOM elements themselves are manipulated and not when their attributes change.

A common pattern of using the hooks is to do the DOM manipulation as requested immediately, to the final state, and then only visually instantaneously revert to the initial state and then animate back to the final state. For example, to animate move of a DOM element you can first move the DOM element to the new position, and then use CSS translation to visually move it back to the previous position and then animate it slowly to the new position. The DOM element itself stays in the new position all the time in the DOM, only visually is being translated to the old position and animated.

One way for animating is to modify CSS, like toggling a CSS class which enables animations. Another common way is to use a library like Velocity.

Animations are best provided as reusable mixins. But for performance reasons the default implementation of insertDOMElement, moveDOMElement, and removeDOMElement just performs the manipulation and does not try to call mixins. So for components where you want to enable mixin animations for, you should extend those methods with something like:

insertDOMElement(parent, node, before, next) {
  next = next || () => {
    super.insertDOMElement(parent, node, before);
    return true;
  }

  if (!this.callFirstWith(this, 'insertDOMElement', parent, node, before, next)) {
    return next();
  }

  // It has been handled.
  return true;
}

moveDOMElement(parent, node, before, next) {
  next = next || () => {
    super.moveDOMElement(parent, node, before);
    return true;
  }

  if (!this.callFirstWith(this, 'moveDOMElement', parent, node, before, next)) {
    return next();
  }

  // It has been handled.
  return true;
}

removeDOMElement(parent, node, next) {
  next = next || () => {
    super.removeDOMElement(parent, node);
    return true;
  }

  if (!this.callFirstWith(this, 'removeDOMElement', parent, node, next)) {
    return next();
  }

  // It has been handled.
  return true;
}

Your method should return true if it has handled insertion/move/removal of the DOM element.

See Momentum Meteor package for more information on how to use these hooks to animate DOM elements.

Mixins

Blaze Components are designed around the composition over inheritance paradigm. JavaScript is a single-inheritance language and instead of Blaze Components trying to force fake multiple-inheritance onto a language, it offers a set of utility methods which allow the component to interact with its mixins and mixins with the component. The code becomes more verbose because of the use of methods instead of overloading, overriding or extending the existing elements of the language or objects, but we believe that results are easier to read, understand, and maintain.

Each mixin becomes its own JavaScript object with its own state, but they share a life-cycle with the component. Most commonly mixin is an instance of a provided mixin class.

A contrived example to showcase various features of mixins:

class MyComponent extends BlazeComponent {
  mixins() {
    return [FirstMixin, new SecondMixin('foobar')];
  }

  alternativeName() {
    return this.callFirstWith(null, 'templateHelper');
  }

  values() {
    return 'a' + (this.callFirstWith(this, 'values') || '');
  }
}

MyComponent.register('MyComponent');

class FirstMixinBase extends BlazeComponent {
  templateHelper() {
    return "42";
  }

  extendedHelper() {
    return 1;
  }

  onClick() {
    if (this.values() !== this.valuesPrediction) throw new Error();
  }
}

class FirstMixin extends FirstMixinBase {
  extendedHelper() {
    return super.extendedHelper() + 2;
  }

  values() {
    return 'b' + (this.mixinParent().callFirstWith(this, 'values') || '');
  }

  dataContext() {
    return EJSON.stringify(this.data());
  }

  events() {
    return super.events().concat({
      'click': this.onClick
    });
  }

  onCreated() {
    super.onCreated();
    this.valuesPrediction = 'bc';
  }
}

class SecondMixin {
  constructor(name) {
    super();

    this.name = name;
  }

  mixinParent(mixinParent) {
    if (mixinParent) this._mixinParent = mixinParent;
    return this._mixinParent;
  }

  values() {
    return 'c' + (this.mixinParent().callFirstWith(this, 'values') || '');
  }
}
<template name="MyComponent">
  <p>alternativeName: {{alternativeName}}</p>
  <p>values: {{values}}</p>
  <p>templateHelper: {{templateHelper}}</p>
  <p>extendedHelper: {{extendedHelper}}</p>
  <p>name: {{name}}</p>
  <p>dataContext: {{dataContext}}</p>
</template>

When this component is rendered using the {top: '42'} as a data context, it is rendered as:

<p>alternativeName: 42</p>
<p>values: abc</p>
<p>templateHelper: 42</p>
<p>extendedHelper: 3</p>
<p>name: foobar</p>
<p>dataContext: {"top":"42"}</p>

We can visualize class structure and mixins.

Example mixins visualization

Full lines represent JavaScript inheritance. Dashed lines represent mixins relationships based on the order of mixins specified.

Let's dissect the example.

As we can see all methods become template helpers and they are searched for in the normal order, first the component, then mixins. On the diagram from left to right. First implementation found is called. If the implementation wants to continue with the traversal it can do it by itself, probably using callFirstWith.

mixins() {
  return [FirstMixin, new SecondMixin('foobar')];
}

We can see that mixins can be also already made instances. And that mixins do not have to extend BlazeComponent. You get some methods for free, but you can use whatever you want to provide your features.

alternativeName() {
  return this.callFirstWith(null, 'templateHelper');
}

Wa call callFirstWith with null which makes it traverse the whole structure, the component and all mixins, when searching for the first implementation of templateHelper.

This allows us to not assume much about where the templateHelper is implemented. But be careful, if templateHelper would do the same back, calling the alternativeName on the whole structure, you might get into an inifinite loop.

On the diagram of our example, this starts traversal on MyComponent, checking for the templateHelper on its instance through JavaScript inheritance. Afterwards it moves to FirstMixin, looking at its instance and its inheritance parent, where it finds it.

values() {
  return 'a' + (this.callFirstWith(this, 'values') || '');
}

values method is passing this to callFirstWith, signaling that only mixins after the component should be traversed.

This is a general pattern for traversal which all values methods in this example use. Similar to how you would use super call in inheritance. values methods add their own letter to the result and ask later mixins for possible more content.

Calling callFirstWith in this way traverses the structure from the left to the right on the diagram of our example, one implementation of values after another. First, values method from MyComponent component is found. This method calls callFirstWith which continues searching on FirstMixin, where it is found again. That method calls callFirstWith, which now finds values again, this time on SecondMixin. Call from the SecondMixin does not find any more implementations. The result is thus:

'a' + ('b' + ('c' + ''))
onClick() {
  if (this.values() !== this.valuesPrediction) throw new Error();
}

Event handlers (and all other methods) have this bound to the mixin instance, not the component. Here we can see how the event handler can access values and valuesPrediction on mixin's instance and how normal JavaScript inheritance works between FirstMixinBase and FirstMixin.

Event handlers are independent from other mixins and the component's event handlers. They are attached to DOM in the normal traversal order, first the component's, then mixins'.

To control how events are propagated between the component and mixins you can use event object methods like stopPropagation and stopImmediatePropagation.

extendedHelper() {
  return super.extendedHelper() + 2;
}

You can use normal JavaScript inheritance in your mixins. On the diagram of our example super traverses upwards.

dataContext() {
  return EJSON.stringify(this.data());
}

The method to access the data context used for the component automatically first finds the mixin's component and then accesses its data context. All Blaze Components methods which can operate only on components and not mixins automatically assure that they are called on the mixin's component. For methods which can operate on both components and mixins this is not done automatically. You should first call component to get an instance of the mixin's component and then call the method on it.

onCreated() {
  super.onCreated();
  this.valuesPrediction = 'bc';
}

Mixin's life-cycle matches that of the component and mixin's life-cycle hooks are automatically called by Blaze Components when the component is created, rendered, and destroyed. this is bound to the mixin instance.

mixinParent(mixinParent) {
  if (mixinParent) this._mixinParent = mixinParent;
  return this._mixinParent;
}

Because SecondMixin does not extend BlazeComponent we have to provide the mixinParent method ourselves. It is called by the Blaze Components as a setter to tell the mixin what its component instance is.

mixinParent is a good place to add any dependencies to the component your mixin might need. Extend it and your additional logic.

Example:

mixinParent(mixinParent) {
  if (mixinParent) mixinParent.requireMixin(DependencyMixin);
  return super.mixinParent(mixinParent);
}

Don't forget to call super.

See the tutorial for a more real example of mixins.

Namespaces

As your project grows and you are using more and more components, especially from 3rd party packages, flat structure of components (and templates) might lead to interference. To address this issue Blaze Components provide multi-level namespacing, with . character as a separator.

Example:

class Buttons {}

Buttons.Red = class Red extends BlazeComponent {}

Buttons.Red.register('Buttons.Red');

Buttons.Blue = class Blue extends BlazeComponent {}

Buttons.Blue.register('Buttons.Blue');
{{> Buttons.Red}}

You do not have to export Buttons from your package for components to be available in templates throughout your project. The registry of components is shared between all packages and the project. Even if you need to access a component's class in your code, you can use BlazeComponent.getComponent('Buttons.Red') to access it.

Sometimes you want some non-component logic to be available together with your components. You can export one symbol and nest components under it like in the example above, having access to both non-component logic through that symbol, and components through Blaze Components registry.

On the other hand, you do not even have to register components to use them. But then you have to get them into templates through some other means, for example, using some other component's template helper (method). Remember though that Meteor has a global namespace for all template names, so probably you want to use namespaced template names in large projects or packages you publish even if you are not registering components.

Let's imagine thar your package exports Buttons class above. Then you could do:

<template name="OtherComponent">
  {{> renderButton}}
</template>
class OtherComponent extends BlazeComponent {
  renderButton() {
    return Buttons.Red.renderComponent(this.currentComponent());
  }
}

OtherComponent.register('OtherComponent');

If you would leave your components registered, you could still do:

renderButton() {
  return BlazeComponent.getComponent('Buttons.Red').renderComponent(this.currentComponent());
}

You do not even have to create a namespacing class in your code like we did in the example above. It does make the code more readable and uniform, though.

How exactly you structure your code and components depends on various factors. Blaze Components provide multiple ways to keep your components structured, tidy, and reusable.

Example above is using renderComponent method which is not yet public.

Server side rendering

Blaze Components support rendering on the server side as well. You can use renderComponentToHTML method to render the Blaze Component to a HTML string. For Blaze Components to work well on the server side they have to use pure Blaze reactivity to render its content, and do not manipulate rendered DOM in the onRendered life-cycle hook (the hook is not even called when using renderComponentToHTML). Such Blaze Components are recommended anyway: use instance-bound reactive variables to keep the component's state and manipulate those variables to change how the component is rendered by Blaze. For animations and other special effects, use Blaze Components' support for animations (animations are not called when rendering to a HTML string, too).

Because it is possible to use Blaze Components on both client and server side it is important to be conscious to which target(s) you add HTML template file and Blaze Component code file for your Blaze Component. In general it is not a problem if you add files to the server side even if you do not plan to use Blaze Components on the server side, but be careful to not expose server side only components to your clients.

As a reminder, put files into the client directory in your app if you want them to be available only on the client side, into the server directory if you want them to be available only on the server side, and elsewhere (like lib directory) if you want them to be available both on the client and server side. For packages, use architecture argument to your addFiles calls to control where are added files available, 'client', 'server', and ['client', 'server'], respectivelly.

Use with existing classes

Blaze Components are designed to work with existing class hierarchies. There are no restrictions on class constructor, for example. In fact, Blaze Components can be seen simply as an API a class or object has to provide to be compatible with the system. The easiest way to bootstrap your class hierarchy is to copy default implementations from BlazeComponent to your class.

Example:

for (let property in BlazeComponent) {
  if (!(property in YourBaseClass)) continue;
  YourBaseClass[property] = BlazeComponent[property];
}

for (let property in BlazeComponent.prototype) {
  if (!(property in YourBaseClass.prototype)) continue;
  YourBaseClass.prototype[property] = BlazeComponent.prototype[property];
}

Reference

Class methods

static register(componentName, [componentClass])

Registers a new component with the name componentName. This makes it available in templates and elsewhere in the component system under that name, and assigns the name to the component. If componentClass argument is omitted, class on which register is called is used.

static getComponent(componentName)

Retrieves the component class with componentName name. If such component does not exist, null is returned.

static getComponentForElement(domElement)

Returns a component instance used to render a particular DOM element, if it was rendered using Blaze Components. Otherwise null.

static currentData([path], [equalsFunc])

This is a complementary class method to the currentData instance method. Use it when you do not have a component instance available.

static currentComponent()

This is a complementary class method to the currentComponent instance method. Use it when you do not have a component instance available.

static componentName([componentName])

When called without a componentName argument it returns the component name.

When called with a componentName argument it sets the component name.

Setting the component name yourself is needed and required only for unregistered classes because register sets the component name automatically otherwise. All component should have a component name associated with them.

static extendComponent([constructor], methods)

A helper method to extend a component into a new component when using vanilla JavaScript. It configures prototype-based inheritance and assigns properties and values from methods to the prototype of the new component. It accepts an optional constructor function to be used instead of a default one which just calls the constructor of the parent component.

Inside a method you can use this.constructor to access the class. Parent class prototype is available at Object.getPrototypeOf(this.constructor).prototype. You can use it to do super calls.

Example (in vanilla JavaScript):

var OurComponent = MyComponent.extendComponent({
  values: function () {
    return '>>>' + Object.getPrototypeOf(OurComponent).prototype.values.call(this) + '<<<';
  }
});

In ES2015 and CoffeeScript you do not have to use Object.getPrototypeOf(this.constructor).prototype but can use languages' super.

static renderComponent([parentComponent])

This is a complementary class method to the renderComponent instance method. It automatically instantiates the component before calling renderComponent on it.

Despite being documented, renderComponent method is not yet considered public and is subject to change.

static renderComponentToHTML([parentComponent], [parentView], [data])

This is a complementary class method to the renderComponentToHTML instance method. It automatically instantiates the component before calling renderComponentToHTML on it.

Instance methods

Event handlers

events()

Extend this method and return event hooks for the component's DOM content. Method should return an array of event maps, where an event map is an object where the properties specify a set of events to handle, and the values are the handlers for those events. The property can be in one of several forms:

  • eventtype – Matches a particular type of event, such as click.
  • eventtype selector – Matches a particular type of event, but only when it appears on an element that matches a certain CSS selector.
  • event1, event2 – To handle more than one type of event with the same function, use a comma-separated list.

The handler function receives one argument, a jQuery event object, and optional extra arguments for custom events. The common pattern is to simply pass methods as event handlers to allow subclasses to extend the event handlers logic through inheritance.

By default the method returns preexisting event handlers defined on the template returned from the template method call.

Example:

events() {
  return super.events().concat({
    'click': this.onClick,
    'click .accept': this.onAccept,
    'click .accept, focus .accept, keypress': this.onMultiple
  });
}
   
// Fires when any element is clicked.
onClick(event) {
}

// Fires when any element with the "accept" class is clicked.
onAccept(event) {
}

// Fires when 'accept' is clicked or focused, or a key is pressed.
onMultiple(event) {
}

Blaze Components make sure that event handlers are called bound with the component itself in this. This means you can normally access data context and the component itself in the event handler.

When extending this method make sure to not forget about possible ancestor event handlers you can get through the super call. Concatenate additional event handlers in subclasses and/or modify ancestor event handlers before returning them all. This is also the reason why the method has to return an array. There might be multiple handlers for the same event specification (eg., multiple handlers to handle click event) and we do not want them to be clobbered.

Returned values from event handlers are ignored. To control how events are propagated you can use event object methods like stopPropagation and stopImmediatePropagation.

It is a good practice to always call parent class method using super.

When mixins provide event handlers, they are attached in order of mixins, with the component first.

For more information about event maps, event handling, and event object, see Blaze documentation and jQuery documentation.

DOM content

template()

Extend this method and return the name of a Blaze template or template object itself. By default it returns the component name.

Template content will be used to render component's DOM content.

All component methods (properties, more precisely) are available in the template as template helpers. Template helpers are bound to the component itself in this.

It is recommended that you use templates only to specify DOM content and that you do not define any template helpers, event handlers, and life-cycle hooks on the template itself but instead define it at the component level. But to support wrapping existing templates into Blaze Components so that you can extend or override their behavior, Blaze Components reuse template helpers, event handlers, and life-cycle hooks from the template in a backwards compatible way. Preexisting template helpers are searched if the component does not have a property with the requested helper name. Preexisting event handlers are returned from the default events method. Default life-cycle hooks methods call template's preexisting life-cycle hooks as well.

You can include other templates (to keep individual templates manageable) and components.

Convention is to name component templates the same as components, which are named the same as their classes. And because components are classes, they start with an upper-case letter, TitleCase.

When mixins provide template method, the component's default implementation first calls them and only if mixins do not provide a template, component name is used instead.

See Spacebars documentation for more information about the template language.

Access to rendered content

$(selector)

Finds all DOM elements matching the selector in the rendered content of the component, and returns them as a JQuery object.

The component serves as the document root for the selector. Only elements inside the component and its sub-components can match parts of the selector.

Wrapper around Blaze's $.

find(selector)

Finds one DOM element matching the selector in the rendered content of the component, or returns null if there are no such elements.

The component serves as the document root for the selector. Only elements inside the component and its sub-components can match parts of the selector.

Wrapper around Blaze's find.

findAll(selector)

Finds all DOM element matching the selector in the rendered content of the component. Returns an array.

The component serves as the document root for the selector. Only elements inside the component and its sub-components can match parts of the selector.

Wrapper around Blaze's findAll.

firstNode()

Returns the first top-level DOM node in the rendered content of the component.

The two nodes firstNode and lastNode indicate the extent of the rendered component in the DOM. The rendered component includes these nodes, their intervening siblings, and their descendents. These two nodes are siblings (they have the same parent), and lastNode comes after firstNode, or else they are the same node.

Wrapper around Blaze's firstNode.

lastNode()

Returns the last top-level DOM node in the rendered content of the component.

Wrapper around Blaze's lastNode.

Access to data context and components

data([path], [equalsFunc])

Returns current component-level data context. A reactive data source.

If path is provided, it returns only the value under the path. It uses data-lookup package to resolve the path. Moreover, this limits reactivity only to changes of that value and not the whole data context.

Use this to always get the top-level data context used to render the component.

currentData([path], [equalsFunc])

Returns current caller-level data context. A reactive data source.

In event handlers use currentData to get the data context at the place where the event originated (target context). In template helpers currentData returns the data context where a template helper was called. In life-cycle hooks onCreated, onRendered, and onDestroyed, it is the same as data. Inside a template accessing the method as a template helper currentData is the same as this.

If path is provided, it returns only the value under the path. It uses data-lookup package to resolve the path. Moreover, this limits reactivity only to changes of that value and not the whole data context.

When path is provided, current lexical scope is consulted first, before path is looked up in the data context. This allows one to access lexical scope values from the component. currentData('foo.bar') thus becomes the same as {{foo.bar}}, even when foo is defined through the lexical scope. For example, to access current iteration index, you can do:

<template name="ListComponent">
  <ul>
    {{#each list}}
      <li class="{{oddOrEven}}">{{item}}</li>
    {{/each}}
  </ul>
</template>
class ListComponent extends BlazeComponent {
  oddOrEven() {
    // @index is zero-based, so "odd" and "even" are swapped.
    return this.currentData('@index') % 2 === 0 ? 'odd' : 'even';
  }
}

ListComponent.register('ListComponent');

component()

Returns the component. Useful in templates and mixins to get a reference to the component.

currentComponent()

Similar to currentData, currentComponent returns current caller-level component.

In most cases the same as this in the code, but in event handlers it returns the component at the place where event originated (target component), and inside template content wrapped with a block helper component, it is the closest block helper component.

componentName()

This is a complementary instance method which calls componentName class method.

parentComponent()

Returns the component's parent component, if it exists, or null. A reactive data source.

The parent component is available only after the component has been created, and until is destroyed.

childComponents([nameOrComponent])

Returns an array of component's children components. A reactive data source. The order of children components in the array is arbitrary.

You can specify a component name, class, or instance to limit the resulting children to.

The children components are in the array only after they have been created, and until they are destroyed.

childComponentsWith(propertyOrMatcherOrFunction)

Returns an array of component's children components which match a propertyOrMatcherOrFunction predicate. A reactive data source. The order of children components in the array is arbitrary.

A propertyOrMatcherOrFunction predicate can be:

  • a property name string, in this case all children components which have a property with the given name (or their mixins do) are matched
  • a matcher object specifying mapping between property names and their values, in this case all children components which (or their mixins) have all properties from the matcher object equal to given values are matched (if a property is a function, it is called and its return value is compared instead)
  • a function which receives (child, parent) with this bound to parent, in this case all children components for which the function returns a true value are matched

Examples:

component.childComponentsWith('propertyName');
component.childComponentsWith({propertyName: 42});
component.childComponentsWith((child, parent) => child.propertyName === 42);

The children components are in the array only after they have been created, and until they are destroyed.

Life-cycle hooks

constructor([args...])

When a component is created, its constructor is first called. There are no restrictions on component's constructor and Blaze Components are designed to coexist with classes which require their own arguments when instantiated. To facilitate this, Blaze Components operate equally well with classes (which are automatically instantiated as needed) or already made instances. The real life-cycle of a Blaze Component starts after its instantiation.

When including a component in a template, you can pass arguments to a constructor by using the args keyword.

Example:

{{> ButtonComponent args 12 color='red'}}

Blaze Components will call ButtonComponent's constructor with arguments 12 and Spacebars.kw({color: 'red'}) when instantiating the component's class. Keyword arguments are wrapped into Spacebars.kw.

It is a good practice to always call parent class constructor using super.

After the component is instantiated, all its mixins are instantiated as well.

onCreated()

Extend this method to do any initialization of the component before it is rendered for the first time. This is a better place to do so than a class constructor because it does not depend on the component nature, mixins are already initialized, and most Blaze Components methods work as expected (component was not yet rendered, so DOM related methods do not yet work).

By default the method calls preexisting onCreated callbacks defined on the template returned from the template method call.

A recommended use is to initialize any reactive variables and subscriptions internal to the component.

Example:

class ButtonComponent extends BlazeComponent {
  onCreated() {
    super.onCreated();

    this.color = new ReactiveField("Red");

    $(window).on('message.buttonComponent', (event) => {
      let color = event.originalEvent.data && event.originalEvent.data.color;
      if (color) this.color(color);
    });
  }

  onDestroyed() {
    super.onDestroyed();

    $(window).off('.buttonComponent');
  }
}

ButtonComponent.register('ButtonComponent');
<template name="ButtonComponent">
  <button>{{color}}</button>
</template>

You can now use postMessage to send messages like {color: "Blue"} which would reactively change the label of the button.

It is a good practice to always call parent class method using super.

When mixins provide onCreated method, they are called after the component in mixins order automatically.

onRendered()

This method is called once when a component is rendered into DOM nodes and put into the document for the first time.

By default the method calls preexisting onRendered callbacks defined on the template returned from the template method call.

Because your component has been rendered, you can use DOM related methods to access component's DOM nodes.

This is the place where you can initialize 3rd party libraries to work with the DOM content as well. Keep in mind that interactions of a 3rd party library with Blaze controlled content might bring unintentional consequences so consider reimplementing the 3rd party library as a Blaze Component instead.

It is a good practice to always call parent class method using super.

When mixins provide onRendered method, they are called after the component in mixins order automatically.

onDestroyed()

This method is called when an occurrence of a component is taken off the page for any reason and not replaced with a re-rendering.

By default the method calls preexisting onDestroyed callbacks defined on the template returned from the template method call.

Here you can clean up or undo any external effects of onCreated or onRendered methods. See the example above.

It is a good practice to always call parent class method using super.

When mixins provide onDestroyed method, they are called after the component in mixins order automatically.

isCreated()

Returns true if the component is created, possibly rendered, but not (yet) destroyed. Otherwise false. A reactive data source.

isRendered()

Returns true if the component is rendered, but not (yet) destroyed. Otherwise false. A reactive data source.

isDestroyed()

Returns true if the component is destroyed. Otherwise false. If component was never created, it was also never destroyed so initially the value is false. A reactive data source.

Programmatic rendering

Blaze Components are most often automatically rendered by Blaze when included in templates. In that case Blaze takes care of the whole life-cycle of a component. Sometimes you might want to render a component programmatically by yourself. In that case you have to keep in mind that Blaze Components are first rendered to Blaze template, which are then rendered by Blaze to DOM.

renderComponent([parentComponent])

Renders a Blaze Component into a Blaze template.

In the case you want to render the Blaze template further to DOM, you can use that Blaze template as an input to Blaze.render or Blaze.renderWithData.

Alternatively, you can return that Blaze template from a Blaze Component method to have a dynamic include in the template:

class DynamicComponent extends BlazeComponent {
  renderButton(parentComponent) {
    parentComponent ||= this.currentComponent();
    return ButtonComponent.renderComponent(parentComponent);
  }
}

DynamicComponent.register('DynamicComponent');
<template name="DynamicComponent">
  {{> renderButton}}
</template>

Notice the use of currentComponent as the default parentComponent argument. This makes parentComponent be set to the correct value when renderButton method is used in a template include, but also allows calling renderButton from elsewhere in the code with a provided parentComponent.

Despite being documented, renderComponent method is not yet considered public and is subject to change.

removeComponent()

Removes rendered-to-DOM Blaze Component from DOM, destroys the underlying Blaze template, and destroys the Blaze Component.

You should call this method only if you have previously programmatically rendered the Blaze Component to DOM (using Blaze.render or Blaze.renderWithData). In all other cases Blaze Components will be removed from DOM and destroyed automatically by Blaze: you should not interfere with Blaze by removing rendered DOM by yourself.

renderComponentToHTML([parentComponent], [parentView], [data])

Renders a Blaze Component into a HTML string. If you pass data, it is used as a data context for rendering.

It works both on the client and server side. But you have to add your HTML template file and Blaze Component code file to the target (or targets) you want to call this method on.

When using this method to render a Blaze Component, life-cycle of a component is different: the component is created and destroyed, but not rendered (its onRendered life-cycle hook is also not called). This allows you to put into the onCreated life-cycle hook initialization of the component which does not depend on the fact that the component is being rendered to the DOM, and you put into the onRendered logic which needs access to the rendered DOM. Of course, the best is if you do not access DOM directly and do not use onRendered. Then such a component can work both on the client and server side the same.

Utilities

autorun(runFunc)

A version of Tracker.autorun that is stopped when the component is destroyed. You can use autorun from an onCreated or onRendered life-cycle hooks to reactively update the DOM or the component.

subscribe(name, [args...], [callbacks])

A version of Meteor.subscribe that is stopped when the component is destroyed. You can use subscribe from an onCreated life-cycle hook to specify which data publications this component depends on.

subscriptionsReady()

This method returns true when all of the subscriptions called with subscribe are ready. Same as with all other methods, you can use it as a template helper in the component's template.

Low-level DOM manipulation hooks

insertDOMElement(parent, node, before)

Every time Blaze wants to insert a new DOM element into the component's DOM content it calls this method. The default implementation is that if node has not yet been inserted, it simply inserts the node DOM element under the parent DOM element, as a sibling before the before DOM element, or as the last element if before is null.

You can extend this method if you want to insert the new DOM element in a different way, for example, by animating it. Make sure you do insert it correctly because Blaze will expect it to be there afterwards.

If you want to use mixins with the insertDOMElement method, you will have to extend the component's method to call them in the way you want.

moveDOMElement(parent, node, before)

Every time Blaze wants to move a DOM element to a new position between siblings it calls this method. The default implementation is that if node has not yet been moved, it simply moves the node DOM element before the before DOM element, or as the last element if before is null.

You can extend this method if you want to move the DOM element in a different way, for example, by animating it. Make sure you do move it correctly because Blaze will expect it to be there afterwards.

If you want to use mixins with the moveDOMElement method, you will have to extend the component's method to call them in the way you want.

removeDOMElement(parent, node)

Every time Blaze wants to remove a DOM element it calls this method. The default implementation is that if node has not yet been removed, it simply removes the node DOM element.

You can extend this method if you want to remove the DOM element in a different way, for example, by animating it. Make sure you do remove it correctly because Blaze will expect it to be removed afterwards.

If you want to use mixins with the removeDOMElement method, you will have to extend the component's method to call them in the way you want.

Mixins

mixins()

Extend this method and return mixins for the component. Mixins can be components themselves, or just classes or objects resembling them. No method is required for them, but methods will be called on them by Blaze Components if they do exist.

The mixins method should return an array of registered component names, mixin classes, or mixin instances. When component instance is created, all mixins' instances are created as well, if they were not already an instance. Life-cycle of mixin instances matches that of the component.

getMixin(nameOrMixin)

Returns the component's mixin instance for a given name, class, or instance. Returns null if mixin is not found.

You can use it to check if a given mixin is used by the component.

getFirstWith(afterComponentOrMixin, propertyOrMatcherOrFunction)

It searchers the component and its mixins in order to find the first which matches a propertyOrMatcherOrFunction predicate. If afterComponentOrMixin is null, it starts with the component itself. If afterComponentOrMixin is the component, it starts with the first mixin. Otherwise it starts with the mixin after afterComponentOrMixin.

Returns null if such component or mixin is not found.

A propertyOrMatcherOrFunction predicate can be:

  • a property name string, in this case the first component or mixin which has a property with the given name is matched
  • a matcher object specifying mapping between property names and their values, in this case the first component or mixin which has all properties from the matcher object equal to given values is matched (if a property is a function, it is called and its return value is compared instead)
  • a function which receives (mixin, component) with this bound to component, in this case the first component or mixin for which the function returns a true value is matched

callFirstWith(afterComponentOrMixin, propertyName, [args...])

It searchers the component and its mixins in order to find the first with a property propertyName, and if it is a function, calls it with args as arguments, otherwise returns the value of the property. If afterComponentOrMixin is null, it starts with the component itself. If afterComponentOrMixin is the component, it starts with the first mixin. Otherwise it starts with the mixin after afterComponentOrMixin.

Returns undefined if such component or mixin is not found.

mixinParent([mixinParent])

When called without a mixinParent argument it returns the mixin's parent. For a component instance's mixins it returns the component instance.

When called with a mixinParent argument it sets the mixin's parent.

To access the mixin's component, it is better to use component instead, which traverses multiple levels of mixins automatically.

Setting the mixin's parent is done automatically by calling this method when creating component's mixins. Extend (or provide) this method if you want to do any action when parent is set, for example, add dependency mixins to the parent using requireMixin. Make sure you call super as well.

requireMixin(nameOrMixin)

Adds a mixin after already added mixins. nameOrMixin can be a registered component name, mixin class, or mixin instance.

If mixin is already added to the component the method does nothing.

Use requireMixin to manually add additional mixins after a component was created. For example, to add dependencies required by automatically added mixins as a result of mixins.

Debugging

To help with debugging, BlazeComponentDebug class is available. It contains class methods which can help you introspect the current state of rendered components.

static dumpComponentSubtree(componentOrElement)

For a provided component instance or DOM element rendered by a component instance this class method prints to the browser web console the tree structure of component instances for which the provided component instance is an ancestor.

static dumpComponentTree(componentOrElement)

For a provided component instance or DOM element rendered by a component instance this class method prints to the browser web console the whole tree structure of component instances in which the provided component instance exists, from the root component instance down.

The provided component instance's name is underlined.

static dumpAllComponents()

Prints to the browser web console tree structures of all component instances currently rendered.

Related projects

  • meteor-template-extension – provides various ways of copying template helpers, event handlers and hooks between templates, allowing code reuse; a downside is that all copying has to be taken care by a developer, often again and again, which becomes problematic as codebase grows; moreover, without a clearly defined API community cannot build and share components
  • meteor-autoform – offers forms components through a sophisticated use of templates and template helpers but it is still hard to compose behaviors you want beyond defining additional input fields
  • Flow Components – Flow Components are closer to React in the API design while Blaze Components are keeping familiar concepts like data contexts and template helpers; Flow Components on the other hand still use template-based event handlers while Blaze Components make them class methods so it easier to extend or override them through inheritance; in general Blaze Components seems to be more OOP oriented; Flow Components are not yet officially released
  • ViewModel – instead of using event handlers to imperatively respond to user interactions, ViewModel provides a two-way data binding between the model and DOM elements, like form input elements, if data changes, form input element updates, if user modifies form input element, data is updated; alongside, ViewModel can be used as components; Blaze Components are in this way lower level and they do not prescribe the way you should be using data binding or event handlers, Blaze Components provide traditional DOM event handlers but you can build on top of it data binding in the way you want, when you want/need it; you can even create a mixin for that and reuse it across components; moreover, Blaze Components' main focus is on providing a good and extensible components system and its power shows exactly because you can build such features through opt-in mixins
  • Blaze Plus – Blaze Plus adds props and state management functionality to Blaze, allowing one to pass data from the top to the bottom. These ideas are inspired by React. A proposed way to limit rerenders in Blaze Components if one wants to pass data from the top to the bottom is to use computed field package.
  • TemplateController – Wrapping some of the same best practices Blaze Components do, but into only a thin layer without much extra features one can find in Blaze Components and without support for code reusability and sharing.