Qt5 QML, why are onHeightChanged and onWidthChanged called without change? - qml

Question

import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Controls.Styles 1.4
import QtQuick.Layouts 1.2
ApplicationWindow
{
visible: true
width: 640
height: 480
property int lastW: 0
property int lastH: 0
function doSomething()
{
if (lastW == width && lastH == height)
console.log("width & height same as last time")
lastW = width;
lastH = height;
}
onHeightChanged: doSomething();
onWidthChanged: doSomething();
}
Why is doSomething called with no change in width and height (except for once at the start). When i resize the window, i get the console log message.
running windows 8.1

Answer 1

doSomething runs every time width or height of the ApplicationWindow changes. The window may change size in both dimensions simultaneously. If in one moment size change from 100x100 to 101x101 then both signals widthChanged and heightChanged will be emitted for width=101 and height=101. That is why console.log("width & height same as last time") is being executed despite the fact that at the first glance this should never happen.
To comment on doSomething being run on the start. For me doSomething never fires unless I resize window. If for you it does fire when application starts it may be because for a short moment the ApplicationWindow has some initial size (for example 0x0) and just after that it changes size to 640x480 and doSomething runs.
In some rare cases what I have written above may not be valid. You can try to resize ApplicationWindow in one dimension only and still sometimes changed signal will occur twice for the same value. My guess is that in those cases value has changed so fast that while triggering changed twice in QML you read only the second value.
I suspect that it works like this:
width=100 then quickly changes value to 101 fires changed, changes to 102 and again changed is fired. After that the QML signals are being executed. Now you receive two changed signals but in both you read value 102.