如何确定 QML 组件当前是否在屏幕上可见

Question

I have a set of controls which have bindings to frequently changing data values. The data comes from a limited hardware-bus. Therefore, it would be better to disable the binding while the control is not visible on screen. The item's visible -property doesn't help in this case. So, how to determine if an Item-based QML widget is currently visible on screen (and not hidden by an overlay or currently outside the visible area)?

Source: https://forum.qt.io/topic/54116/how-to-check-if-a-item-is-currently-visible-on-screen

I have almost the same problem. Hoping someone here has a solution.

Answer 1

Here's what I would try to get working:

First, I presume there is a ScrollView or Flickable in play here? If so, then hook to signals like Flickable::movementEnded().

Second, when that signal fires, use Item::mapToItem() to check if each of your Item's visible rectangle (based on x, y, width, height) intersects your window's contentItem rectangle. Set the result as a boolean on each of your items and make sure the data retrieval is disabled when it is false (using && or a tertiary JS expression).

Or if more convenient, remove the binding when false and reapply it with Qt.binding() when true.