Does Qt cache ECMAScripted QML values?

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Hi.

This question might be a bit of a long shot, but.. I'm writing some GUI classes in Java that use a similar system to Qt where properties can be scripted. For example, I can set the position of a Widget to be based on the size of itself or a parent Widget, by exposing the Widget object(s) to the scripting language. Here's an example (JSON & JavaScript):

"offset" : {
    "x" : "-(me.getSize().getX() / 2)",
    "y" : "-(me.getSize().getY() / 2)"
},

"offset" : {
    "x" : "-(me.getSize().getX() / 2)",
    "y" : "-(me.getSize().getY() / 2)"
},

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I noticed that calling the script every frame is really slow - the FPS goes from ~3000 to ~150. I have thought about various ways to cache the values of the properties that are scripted, but the solution always involves having some kind of listeners that observe the values held by the objects that the scripts rely on, which I think will end up really messy.

So, I'm wondering if Qt does anything in terms of caching the result of scripted QML properties? I know C++ is different to Java in terms of performance, but I thought I'd ask anyway.

Cheers.

[wysota]

Values are just values, they are cached by definition. What Qt Quick does is that it monitors changes to properties that are part of expressions bound (not assigned, that's the crucial difference) to another property. So it recalculates them only when they really change. This is not inherent to ECMAScript but specifically to property binding features of Qt Quick.

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Values are just values, they are cached by definition. What Qt Quick does is that it monitors changes to properties that are part of expressions bound (not assigned, that's the crucial difference) to another property. So it recalculates them only when they really change. This is not inherent to ECMAScript but specifically to property binding features of Qt Quick.

Ah, yes, I should have said expressions not values. Can you share any insight on how it monitors the changes and how it determines which properties to monitor?

[wysota]

As far as I know (I might be wrong) each property has a Qt signal associated with it that fires each time the value changes. So it boils down to listening to those signals and reacting upon them (e.g. by requesting a recalculation next time the value is read).

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As far as I know (I might be wrong) each property has a Qt signal associated with it that fires each time the value changes. So it boils down to listening to those signals and reacting upon them (e.g. by requesting a recalculation next time the value is read).

Thank you, wysota.