[Development] Lar's QtCore 6 talk

Lars Knoll lars.knoll at qt.io
Mon May 18 10:03:25 CEST 2020

Hi Jason,

> On 16 May 2020, at 22:27, Jason H <jhihn at gmx.com> wrote:
> https://www.youtube.com/watch?v=cHrrR3KhvUk
> Thanks for the insight into what is coming up. I had a minor panic attack though, which seemed to be shared with the first person to ask a question. If the property bindings are lazy evaluated, then all my QML code will be broken. I actively exploit the existing binding behavior of immediately evaluating dependent bindings.  I hope I am misunderstanding this, but I think this should continue to be the behavior.  I also I don't want to have to choose or specify.
> However there may happy automatic middle ground. If the bindings are a terminal leaf node, then they can be lazy evaluated. However any binding that is in the middle of a binding chain must be immediately evaluated. I think this will satisfy everyone.
> Given:
> Item { id: item; property int x2: parent.x*2 }
> If x2 is not used anywhere, it can be lazy evaluated. But the moment there is a onX2Changed or Item { x: item.x2 } then it must be immediately evaluated.  For something like QML Items and x it is probably not that interesting of an example.
> But let's now assume there is QObject-derived class bing used in the QML engine and it declares property x which is not required to be immediately updated, then as long as nothing is connected to the xChanged signal, then all those updates can be deferred[1]. But the moment something does connect to it, then it can no longer be lazy-evaluated.
> I don't know if this is what Lars meant? But I think it could work without giving me panic attacks ;-)

No need for panic attacks :)

Lazy binding evaluation means that we will do this where possible behind the scenes. The new property system works this way by default. This is possible, as long as the property is only used in binding expressions.

If you however have change handlers connected to the property, we will of course need to evaluate the binding immediately (as we can’t know the possible side effects of the change handler).

If you’re interested in the details, have a look at qproperty.h/cpp in the dev branch (look for the QPropertyChangeHandler class and the ObserverNotifiesChangeHandler flag in the implementation).


> [1] If you need a less abstract example, think of a QObject-derived class that processes video frames and uses OpenCV to find a target, and it emits the x,y position of the target.  If nothing is connected x() never needs to be evaluated, but if I connect a CrossHair Item and bind to the x() and y() properties, then the binding has to be evaluated immediately to update the position in the UI.
> I do have another concern that perhaps the lazy bindings might make it harder to find binding loops?
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