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 ;-)
[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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