NumberAnimation does not take into account property change until stopped - qt

Question

I have a QML application with 2 rectangles: a large rectangle and a small rectangle inside the large one.
The small rectangle is animated and moves inside the large rectangle.
The animation is done by combining 2 NumberAnimation in a SequentialAnimation.
It works well, except that the to property of one of the NumberAnimation can change.
I would except the change of value to be applied immediately.
However, it is not taken into account until the animations are fully stopped and restarted.
Calling stop()/start() or restart() does not do anything.
I need to wait for the animation to actually finish and then start it again.
This can be demonstrated with the following QML code:
import QtQuick
import QtQuick.Controls
import QtQuick.Layouts
Window {
width: 640
height: 480
visible: true
RowLayout {
Rectangle {
id: topRect
width: 400
height: 400
border {
color: "red"
width: 2
}
Rectangle {
id: animatedRectangle
width: 100
height:100
color: "blue"
}
SequentialAnimation{
id: animation
loops: Animation.Infinite
running: cbAnimate.checked
alwaysRunToEnd: true
NumberAnimation {
id: forwardAnimation
target: animatedRectangle
property: "x"
to: sbWidth.value
duration: 2000
}
NumberAnimation {
id: backwardAnimation
target: animatedRectangle
property: "x"
to: 0
duration: 2000
}
}
}
ColumnLayout {
CheckBox {
id: cbAnimate
text: "Animate"
}
SpinBox {
id: sbWidth
value: 300
to: 400
}
SpinBox {
value: forwardAnimation.to
to: 999
}
}
}
}
Start the animation with the checkbox
Change the value of to with sbWidth
See in the other SpinBox that the value of to was changed
Observe that the animation is still using the old value
Stop the animation, wait for the Rectangle to stop moving, Start the animation
Observe that the animation is using the value set in step 2
Isn't there a way to make the animation use the new value of to immediately?
This behavior is particularly painful when a QML element is animated by default and the to value depends on the geometry of Items, as during the creation of the QML scene Qt will create and then resize Items. Meaning that animation started at creation time won't get their values updated during the resize.

Answer 1

TLDR
In this particular case the best solution is to use the workaround suggedted by #stephen-quan: animate a proxy property property real animatedX between 0.0 and 1.0. And then bind the property I want to animate to this animated property and do the extra computation in this binding: x: animatedX * sbWidth.value. Eliminating the need of changing the to property of the animation.
Details
The issue of animations not taking property change into account until restarted is a very old issue. It has been reported numerous times:
PropertyAnimation ignores from/to changes
Changing an Animation
duration has no effect
Since Qt 6.4, the state has slightly improved. Quoting Qt documentation:
Since Qt 6.4, it is possible to set the from, to, duration, and easing properties on a top-level animation while it is running. The animation will take the changes into account on the next loop.
However, it still does not affect the current loop and requires the animation to be top-level. So even with this improvement, I still need to animate a proxy property, ensuring changes are taken into account in real-time.

Answer 2

I made various changes to your sample.
I introduced from: 0 to your first NumberAnimation. This ensures that whenever you stop/start the animation, it will reset. Also, removing alwaysRunToEnd helps with that.
I introduced a new property double val: 0 property which will range from 0.0 to 1.0. This is what I used NumberAnimation on instead of x. The advantage is, we know that the NumberAnimation will happily move from 0.0 to 1.0 and back to 0.0 consistently.
Then, I introduced a formula linking val to x and takes into account of sbWidth.value.
To make it easier to change sbWidth.value I changed it from a SpinBox to a Slider.
import QtQuick
import QtQuick.Controls
import QtQuick.Layouts
Page {
RowLayout {
Rectangle {
id: topRect
width: 400
height: 400
border {
color: "red"
width: 2
}
Rectangle {
id: animatedRectangle
property double val: 0.0
x: val * sbWidth.value
width: 100
height:100
color: "blue"
Label {
anchors.centerIn: parent
text: parent.x.toFixed(2)
color: "white"
}
}
SequentialAnimation{
id: animation
loops: Animation.Infinite
running: cbAnimate.checked
//alwaysRunToEnd: true
NumberAnimation {
id: forwardAnimation
target: animatedRectangle
property: "val"
from: 0
to: 1.0
duration: 2000
}
NumberAnimation {
id: backwardAnimation
target: animatedRectangle
property: "val"
to: 0
duration: 2000
}
}
}
ColumnLayout {
CheckBox {
id: cbAnimate
text: "Animate"
}
Slider {
id: sbWidth
value: 300
from: 100
to: 400
}
Text {
text: sbWidth.value.toFixed(2)
}
}
}
}
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