How to write toga dummy backend code

[proneon267]

In the dummy backend, we need to write the same backend functions as on other backends. So, how does one write a:

• Function that returns a value:
      Directly return the value with return?
• Function that returns an object:
      I have no idea, how to do this.
• Function that gets the value from an attribute:
      Using self._get_value()?
• Function that sets the value to an attribute:
      Using self._set_value()?
• Function that does not return anything:
      Using self._action()?

Also it would be really helpful, if someone could provide an example usage of accomplishing these tasks in the dummy backend.

[freakboy3742]

I'm a little confused about these questions. The dummy backend isn't some magical special language - it's Python code. So - how do you write a function that returns a value? Like this:

def some_function(...):
    return some_value

The behavior of the dummy backend is design to be the same as a "normal" GUI backend, but without actually creating a GUI or having a dependency on a GUI framework. It's a mock - a simulation of what a "real" backend would do. To that end:

• If the backend has persistent state, use _set_value() to save that state, and _get_value() to retrieve that state
• If the backend does some action that doesn't have state, use _action to record that the action occurred.

As an example - when you give a widget focus, that's an action - so focus() invokes _action(). When you set a widget as enabled, that's stateful (because you might want to query the enabled/disabled state of the widget later), so you use _set_value() and _get_value().

As for examples... the entire dummy backend is full of examples.

If you need help with a specific task, you'll need to ask specific question.

[proneon267]

Yes, I need help with a specific task:
I have a backend function called get_primary_screen() that returns a reference to the primary screen as screen._impl. How should I simulate this in the dummy backend?

[freakboy3742]

By pretending.

Dummy doesn't actually have screens - But it needs to pretend as if it actually has screens. Provide a dummy implementation of Screen as if it were a normal GUI backend like Cocoa or GTK; but instead of calling a native API to retrieve details about a specific, real screen, or to change properties of an actual screen - fake it, and return something plausible, or log the fact that you did something with an _action. Pretend that the dummy has 2 screens (that should be enough for testing purposes); and pick one of them as the primary.

[proneon267]

Thanks. I will try this out and let you know.