Add support for delaying widget destruction from an onBeforeDestroy handler

[colin-aarts]

Having a mechanism for delaying destruction of a widget from a widget's onBeforeDestroy (or similar) callback would be useful for a number of cases, such as easily running teardown animations, without having to micro-manage that from outside. E.g, based on Glitter discussion:

onBeforeDestroy(event) {
    var destroy = event.delayDestroy();
    this.el.addEventListener('transitionend', function() {
        destroy();
    });
    /* animate el */
}

[CestDiego]

This would be very helpful to have

[patrick-steele-idem]

This is actually a feature of Marko v4, but I'm realizing it is not documented anywhere. Marko v4 supports on-attach and on-detach events for any DOM elements that can be used to animate in and animate out DOM elements from the DOM. You can see it in action here: http://markojs.com/try-online/?file=%2Ftodomvc%2Fcomponents%2Ftodomvc-todo-item%2Fdemo.marko&gist=

Calling event.preventDefault() in the on-detach event handler prevents an element from being removed from the DOM. After the animation has been completed you should call event.detach(); to complete the removal of the DOM element out of the DOM.

I'll open an issue on the main marko repo to document this feature.