A callback that is invoked whenever the devicePixelRatio, physicalSize, or padding values change, for example when the device is rotated or when the application is resized (e.g. when showing applications side-by-side on Android).
The framework invokes this callback in the same zone in which the callback was set.
Source
VoidCallback get onMetricsChanged => _onMetricsChanged;
Source
set onMetricsChanged(VoidCallback callback) { _onMetricsChanged = callback; _onMetricsChangedZone = Zone.current; }