calculateMove method
计算移动距离
Implementation
void calculateMove(Offset localPosition) {
/// 相对位置
Offset offset = localPosition - Offset(widget.areaSize.width / 2, widget.areaSize.height / 2) - backgroundOffset;
/// 更新摇杆位置 活动范围控制在Size之内
Offset handle = Offset.fromDirection(offset.direction, min(widget.size.width / 2, offset.distance));
setState(() {
handleOffset = handle;
});
/// [-pi,pi]
updateDirection(offset.direction);
}