Support conditional binding for listeners like v-touch-pan

[gruppler]

Is your feature request related to a problem? Please describe.
I want to conditionally capture swiping, but currently there doesn't seem to be a way to do this. To be clear, I don't want to capture/prevent touch-starts unless my handler is enabled, because this interferes with scrolling.

Describe the solution you'd like
If a handler is null, silently abort and unbind the listener. If the handler changes from null to a function, rebind the listener.

Describe alternatives you've considered
I can't think of any other way to accomplish this. Currently, I'm using the following code, which works but produces errors ("ctx.handler is not a function"):
v-touch-pan.prevent.mouse="board3D ? rotateBoard : null"

Additional context
Vue 2.6 allows for conditional event binding.

[metalsadman]

@gruppler because the directive expects a handler, you can just supply a noop function. ie. v-touch-pan.prevent.mouse="condition ? yourHandler : () => {}"

https://codepen.io/metalsadman/pen/ExaRKyJ?editable=true&editors=111

[gruppler]

The problem with that solution is it still captures the event even when inactive, preventing scrolling.

[rstoenescu]

Starting with future "quasar" v1.7.2, if the handler is a Function, it'll do something with it, otherwise it's gonna be equivalent to not having the touch directive(s) attached.

So this will be perfectly valid:
v-touch-pan.prevent.mouse="condition ? yourHandler : void 0"

Again, if handler is not a Function, then it won't interfere with ANY events.

[gruppler]

Thank you so much!