[Interest] Mouse event propagation in Qt Quick

[Mitch Curtis]

> -----Original Message-----
> From: J-P Nurmi
> Sent: Sunday, 16 October 2016 4:06 PM
> To: Mitch Curtis <mitch.curtis at qt.io>; Qt Project <interest at qt-project.org>
> Subject: Re: Mouse event propagation in Qt Quick
> 
> Hey Mitch,

Hey!

> 
> > - In example #2, why is Flickable happy to steal events that it doesn't do
> anything with? Shouldn't it see that it wasn't a "flick" and ignore the event, so
> that it goes to the next highest item in the stacking order (the mouse area)?
> 
> An interactive Flickable always accepts mouse press events, because it needs
> to become the "mouse grabber item" ie. the item that receives the
> consequent mouse move events. This way Flickable can detect drags and
> flicks.

I was about to reply to myself after realising exactly this, hahaha. It would be too late to propagate press events by the time it receives the final release...

> > - Why aren't the scroll bars blocked by the mouse area in example #3?
> 
> The MouseArea is under the ScrollBar, because its parent is under the
> ScrollBar. The MouseArea is a child of Flickable::contentItem, whereas
> ScrollBars are children of the Flickable they are attached to. ScrollBar and
> Flickable::contentItem are siblings, ScrollBar being higher in the stacking
> order.

Ahhhh. That makes perfect sense, thanks!

> > - Why does example #3 work if I remove "preventStealing: true"?
> 
> What do you mean? That's the exact use case "preventStealing" is meant for.
> :) When the MouseArea's preventStealing is true, Flickable honors it and
> won't be able to flick or drag since it's not allowed to steal events from the
> MouseArea. When preventStealing is false, Flickable's
> childMouseEventFilter() steals the press from the MouseArea when it
> detects a flick or drag.

Because of the other misunderstandings that I had, I thought that preventStealing was the only thing allowing the MouseArea to get events, which is why I thought it was strange that leaving it as false allowed the example to continue working. :D
 
> --
> J-P Nurmi
> 
[snip]
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