onDrag method
Implementation
void onDrag(double dx, double dy) {
var newHeight = draggedHeight.value + dy;
var newWidth = draggedWidth.value + dx;
draggedHeight.value = getTop(newHeight + draggedTop.value > imageHeight.value ? draggedHeight.value : newHeight > 0 ? newHeight : 0);
draggedWidth.value = getLeft(newWidth + draggedLeft.value > imageWidth.value ? draggedWidth.value : newWidth > 0 ? newWidth : 0);
}