BDDROneFingerZoomGestureRecognizerUIGestureRecognizer子类，用于启用单指缩放手势。许可证: MITObjective CBDDRScrollViewAdditionsUIScrollView类别，用于居中内容，启用附加缩放手势并添加动画属性获取器。许可证: MITObjective C
