Should pointer event bubble up when only a tap is enabled?

[axelpale]

Consider a grid of tappable items. Users cannot move the view because all pointer events are caught by the tap recognisers.

A solution is to let the events propagate up in DOM when the interaction mode is set to allow only taps. Is there downsides to this?

• Two nested elements with tap recognizers on both, would together fire two independent tap events. To distinguish the two, the tap event should carry information whether the element was the first in the bubbling hierarchy.
• In the case of a grid full of items, a programmer could be tempted to make every item touchable. However, would it be better to recognise taps globally on the view and make individual items touchable only when needed?

[axelpale]

It is possible that the temptation of making every item touchable is the most robust way. Tapping items when multiple users are interacting with the space could otherwise become problematic. Also, finding the topmost item given a coordinate in space is not a trivial task, but it would be needed if only the container captured everything.

[axelpale]

A simple approach is to provide a new mode property: propagate. This boolean, when true, would disable ev.stopPropagation() calls, causing the events to bubble up. The programmer could then control when to bubble. Changing the propagation behaviour would be as simple as updating the mode.

On the other hand, the approach opens a possibility for a non-trivial error. If a node and its ancestor can both be transformed, bubbling the events would cause the node to be double transformed.

Another approach would be to always propagate when the mode enables only tap. This probably leads to even more weird bugs because the behaviour is hidden inside taach. Also, tapping with a little pinch could activate the native pinch zoom, and that is not something to expect.

Therefore it seems that the propagate property is our best choice. It is probably simple to implement, only adds to current API, and is explicit for the programmer.