UIPanGestureRecognizer是UIGestureRecognizer类的一个扩展类，其扩展类有UITapGestureRecognizer，UIPinchGestureRecognizer，UIRotationGestureRecognizer，UISwipeGestureRecognizer，UIPanGestureRecognizer，UILongPressGestureRecognizer。
借助这些类，可以实现UIView对象的一些操作如对象放大缩小，移动，旋转，滑动，轻击等。再也不用去重写UIView的touchBegin等方法来实现这些功能。
知识点：
UIGestureRecognizer是一个定义基本手势的抽象类，具体什么手势，在以下子类中包含：
1、拍击UITapGestureRecognizer (任意次数的拍击)
2、向里或向外捏UIPinchGestureRecognizer (用于缩放)
3、摇动或者拖拽UIPanGestureRecognizer (拖动)
4、擦碰UISwipeGestureRecognizer (以任意方向)
5、旋转UIRotationGestureRecognizer (手指朝相反方向移动)
6、长按UILongPressGestureRecognizer (长按)
这些操作的目的都是用来修改UIView对象的frame,center,bounds属性，还有一个Transform属性。
我写了一个例子，在UIView和UITableView上分别添加UIPanGestureRecognizer，实现两个对象在手指按住对象于屏幕中拖动的效果。
声明一个UIPanGestureRecognizer对象，添加到UIView对象上去。UIView类有这样的方法用来动态添加和删除UIPanGestureRecognizer对象。
UIPanGestureRecognizer *panRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handlePan:)];
[testPanView addGestureRecognizer:panRecognizer];
UIView管理手势识别器的方法有：
– addGestureRecognizer:
– removeGestureRecognizer:
gestureRecognizers property
– gestureRecognizerShouldBegin:
我在viewDidAppear:方法中，动态添加视图和手势识别器。然后，实现识别器需要操作的两个方法，用来移动视图对象。
在这两个方法中最终的方法是这个 CGPoint translatedPoint = [recognizer translationInView:self.view];
每一次拖动操作状态，都会获取到translatedPoint，从开始到结束。它是一个绝对值，可以看着在”self.view“对应的坐标体系中，拖动的视图对象center的移动开始和结束的点差。
最简单的处理过程是这样：
CGPoint translatedPoint = [recognizer translationInView:self.view];
CGFloat x = recognizer.view.center.x + translatedPoint.x;
CGFloat y = recognizer.view.center.y + translatedPoint.y;
recognizer.view.center = CGPointMake(x, y);
[recognizer setTranslation:CGPointMake(0, 0) inView:self.view];
首先获取到移动点的值，然后算一下视图的center值，相加一下，就得到在self.view坐标体系中，视图该移动到那个center上，一次结束就清零一次。
因为拖动操作持续进行，所以，这个过程会持续执行。
稍微复杂点的处理过程，会捕获到拖动开始，移动，结束等几个状态下的translatedPoint的值。然后做一下逻辑处理，如视图不能溢出self.view的坐标系中，如在结束时会根据方向自动滑动到某个位置。可以在handlePan2：方法中找到这些逻辑的实现代码。
- (void)viewDidAppear:(BOOL)animated
{
NSLog(@" viewDidAppear is at %@.", [NSDate date]);
UIImage *image = [UIImage imageNamed:@"5.jpg"];
testPanView = [[UIView alloc] initWithFrame:CGRectMake(18, 11, 100, 100)];
UIImageView *imageview = [[UIImageView alloc] initWithFrame:[testPanView frame]];
[imageview setImage:image];
[testPanView addSubview:imageview];
UIPanGestureRecognizer *panRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handlePan:)];
[panRecognizer setMinimumNumberOfTouches:1];
[panRecognizer setMaximumNumberOfTouches:1];
[panRecognizer setDelegate:self];
[testPanView addGestureRecognizer:panRecognizer];
[self.view addSubview:testPanView];
testPanTableView = [[UITableView alloc] initWithFrame:CGRectMake(118, 121, 100, 100) style:UITableViewStylePlain];
UIPanGestureRecognizer *panRecognizer2 = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handlePan2:)];
[panRecognizer2 setMinimumNumberOfTouches:1];
[panRecognizer2 setMaximumNumberOfTouches:1];
[panRecognizer2 setDelegate:self];
[testPanTableView addGestureRecognizer:panRecognizer2];
[self.view addSubview:testPanTableView];
}
- (void)handlePan:(UIPanGestureRecognizer *)recognizer
{
CGPoint translatedPoint = [recognizer translationInView:self.view];
NSLog(@"gesture translatedPoint is %@", NSStringFromCGPoint(translatedPoint));
CGFloat x = recognizer.view.center.x + translatedPoint.x;
CGFloat y = recognizer.view.center.y + translatedPoint.y;
recognizer.view.center = CGPointMake(x, y);
NSLog(@"pan gesture testPanView moving is %@,%@", NSStringFromCGPoint(recognizer.view.center), NSStringFromCGRect(recognizer.view.frame));
[recognizer setTranslation:CGPointMake(0, 0) inView:self.view];
}
- (void)handlePan2:(UIPanGestureRecognizer *)recognizer
{
// NSLog(@"gesture translatedPoint xxoo xxoo");
CGPoint translatedPoint = [recognizer translationInView:self.view];
if ([(UIPanGestureRecognizer *)recognizer state] == UIGestureRecognizerStateBegan) {
firstX = recognizer.view.center.x;
firstY = recognizer.view.center.y;
NSLog(@"self.view bounds is %@", NSStringFromCGRect(self.view.bounds));
NSLog(@"pan gesture testPanView begin is %@,%@", NSStringFromCGPoint([recognizer view].center), NSStringFromCGRect([recognizer view].frame));
}
if ([(UIPanGestureRecognizer *)recognizer state] == UIGestureRecognizerStateChanged) {
CGFloat x = firstX + translatedPoint.x;
CGFloat y = firstX + translatedPoint.y;
if (x < recognizer.view.width / 2.0) {
x = recognizer.view.width / 2.0;
} else if (x + recognizer.view.width / 2.0 > self.view.width) {
x = self.view.width - recognizer.view.width / 2.0;
}
if (y < recognizer.view.height / 2.0) {
y = recognizer.view.height / 2.0;
} else if (y + recognizer.view.height / 2.0 > self.view.height) {
