Cancel functionality for Widget animate method

[ishigo1987]

It will be great if you can add a cancel functionality for the widget animate method. It will be very helpful if we want to stop an infinite animation by example

[tbuschto]

Considering there is already internal api doing this it's only a question of how the public API would look like. I don't see any good way to integrate it in to the animate() method. So we can either make the internal Animation class public, or we can add a utility function like this:

cancelAnimation(widget).

Any thoughts @mpost?

[tbuschto]

Note to self: animate could also return an object with a cancel method that is an instance of a subclass of Promise so there is no breaking change but it's still convinient to tuse.

[tbuschto]

There are ambiguites with canceling animations:

• Should the promise still reject as it does when the widget is disposed? (current behavior: yes)
• What values should the animated properties contain? (current behavior: as though animation completed)
• What visual state should the widget stay in? (current behavior: unknown)

Note that here is currently no way for the animated properties to represent the visual state of an incomplete animation.

[cpetrov]

Should the promise still reject as it does when the widget is disposed? (current behavior: yes)

I would be in favor of resolving with a status instead of rejecting when the widget is disposed during a running animation or when the animation is manually aborted, since this is not exceptional behavior but is rather common.

E.g.:

... onSelect={() => componentChild.animate()} ...

... is a common pattern that would result into a console.error log due to an unhandled promise rejection in an event callback when componentChild (or often, its parent component) is disposed before the componentChild animation is finished.