dispose method
Called by the delegate when it is no longer sending events to this object.
Implementation
@mustCallSuper
void dispose() {
// TODO(polina-c): stop duplicating code across disposables
// https://github.com/flutter/flutter/issues/137435
if (kFlutterMemoryAllocationsEnabled) {
FlutterMemoryAllocations.instance.dispatchObjectDisposed(object: this);
}
_lastDetails = null;
onDragCanceled?.call();
}