Qt Quick Examples - Item Variable Refresh Rate

This example demonstrates using the live property of a Item Layers to get a variable and independent refresh rate for Quick items. This can provide a significant boost to performance if specific parts of a UI does not need to be re-rendered every frame, but still require internal updates. Drawing parts that are static most of the time or animations that does not have to run smoothly at all times is a waste of resources. This property gives the user control over when the item is drawn.

The example uses FrameAnimation to control how often the item is rendered. Useful when you only want to e.g. every second frame. Using something like a timer or other mechanics to toggle the flag is also valid.

FrameAnimation {
  id: frameAnimation
  property real fps: smoothFrameTime > 0 ? (1.0 / smoothFrameTime) : 0
  running: true
}

In the above snippet, the fps property is only there for visual demonstration purposes.

The item requires layer.enabled to be true, and then the item will be rendered depending on the state of layer.live as shown in the below snippet.

Item {
    id: qt_logo
    width: 230
    height: 230
    anchors.fill: parent
    anchors.topMargin: 125
    layer.enabled: true
    layer.live: slider.value > 0 && frameAnimation.currentFrame % slider.value == 0

Using modulo operator and a slider to set the value, we can dynamically change how often the Qt logo is rendered.

Example project @ code.qt.io