Called when the user begins a pan or scale gesture on the widget.
At the time this is called, the TransformationController will not have changed due to this interaction.
Will be called even if the interaction is disabled with panEnabled or scaleEnabled for both touch gestures and mouse interactions.
A GestureDetector wrapping the InteractiveViewer will not respond to GestureDetector.onScaleStart, GestureDetector.onScaleUpdate, and GestureDetector.onScaleEnd. Use onInteractionStart, onInteractionUpdate, and onInteractionEnd to respond to those gestures.
The coordinates provided in the details'
localFocalPoint are normal Flutter event coordinates, not
InteractiveViewer scene coordinates. See
TransformationController.toScene for how to convert these coordinates to
scene coordinates relative to the child.
- onInteractionUpdate, which handles an update to the same interaction.
- onInteractionEnd, which handles the end of the same interaction.
final GestureScaleStartCallback? onInteractionStart;