Event handling of child widgets

Event handling of child widgets like e.g. `event is consumed' in Android.
Logic of which child widget should receive the mouse event.

https://stackoverflow.com/questions/3756383/what-is-meaning-of-boolean-value-returned-from-an-event-handling-method-in-andro

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Beware of one thing - vast majority of today's GUIs (including Android) do not do it well - namely they build upon the premise, that event will be consumed just once and therefore by default do send the event to just one (usually the first). That's though an anti-pattern and events should not be discardable by default (i.e. consuming them should not discard them).

This anti-pattern emerged at the very beginning of GUIs probably due to performance reasons and low needs.

So to wrap up - please no consuming, no discarding. Just deliver everything to every subscriber (and allow any subscriber to change the order in which the events shall be delivered to already subscribed subscribers including their removal from the list) and disallow any discarding of any event (thus forcing the programmer manually notify the places which shall not any more react to the event handled at some other place).

This is actually a widely known architecture in distributed systems (because it scales well and makes all non-standard dependencies and interconnections explicit unlike e.g. the event hell in HTML).

Btw, it can also be viewed as broadcasting with some pruning (explicit/manual or implicit/heuristical).

In this case, clicking btn fires both Button's on_click function and Dropdown's on_selected function.