Deprecate Region allowMissingEl

[paulfalgout]

#1984

allowMissingEl is currently missing from the v3 documentation.

However I think we should deprecate it. Maybe for removal in v5? I don't like it much as a solution. I also personally think future versions of regions should not accept selectors, but expect DOM elements passed in, and the view's should probably be doing the DOM querying prior to creation. But that's for another issue.

In any case, I don't think supporting allowMissingEl will make sense in the long term. In the meantime I might suggest using the ui hash to define region selectors and checking for their existence prior to showing, if this is an issue.

This to me handles an edge case, but makes the code less malleable.

[blikblum]

@paulfalgout do you think it can be removed in v4?

[paulfalgout]

I think it's a little early to remove it. There's a potential use case for it now as there's some odd places where errors could occur if you're rendering templates without every region available.

However if we can move to optimistic region instantiation or something like that, the flag will be unnecessary, or at least the predominate wisdom should be to only show a view in a region that exists 😸

[paulfalgout]

For removal in v5