Culling items that are outside the visible area

问题:

From the docs:

The default renderer does not do any CPU-side viewport clipping nor occlusion detection. If something is not supposed to be visible, it should not be shown. Use Item::visible: false for items that should not be drawn. The primary reason for not adding such logic is that it adds additional cost which would also hurt applications that took care in behaving well.

So is there a trick to do it easily, without implementing it myself?

Note that in my case the items that are outside the visible area are there because they are in a ScrollView and they are not scrolled-to.

The reason I want culling is to reduce CPU usage for full-scene redraws.

回答1:

Here is a trivial example you can extend upon:

Window {
  visible: true
  width: 640
  height: 480

  Rectangle {
    anchors.centerIn: parent
    width: 200
    height: 200
    color: "yellow"

    Flickable {
      id: view
      anchors.fill: parent
      contentWidth: 200
      contentHeight: col.height
      property real span : contentY + height
      Column {
        id: col
        x: 90
        spacing: 2
        Repeater {
          model: 50
          delegate: Rectangle {
            width: 10
            height: 10
            color: inView ? "blue" : "red"
            property bool inView: y > view.contentY && y < view.span
          }
        }
      }
    }
  }
}

Obviously, a full-proof solution would also include the item's height in the calculation. You can also do the check in the x axis if necessary.

回答2:

To add to dtech's answer, I just learned that there are QML components, such as GridView and ListView, that do culling automatically.