iOS如何实现一个可以在屏幕中自由移动的按钮

#import "CrossBtnVC.h"

@interface CrossBtnVC ()
{
 CGPoint beginPoint;
 CGFloat rightMargin;
 CGFloat leftMargin;
 CGFloat topMargin;
 CGFloat bottomMargin;
 CGMutablePathRef pathRef;
}
@property (nonatomic,strong) UIButton *crossBtn;//聊天移动
@end

@implementation CrossBtnVC

- (void)viewDidLoad {
 [super viewDidLoad];
 self.view.backgroundColor = [UIColor whiteColor];

 _crossBtn = [UIButton buttonWithType:UIButtonTypeCustom];
 [_crossBtn setImage:[UIImage imageNamed:@"移动聊天"] forState:UIControlStateNormal];
 _crossBtn.frame = CGRectMake(UI_View_Width-54*UI_Width_Scale, UI_View_Height-103, 40, 40);
 [self.view addSubview:_crossBtn];
 [_crossBtn addTarget:self action:@selector(btnAction:) forControlEvents:UIControlEventTouchUpInside];
 UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc]initWithTarget:self action:@selector(handlePan:)];
 [_crossBtn addGestureRecognizer:pan];

 rightMargin = [UIScreen mainScreen].bounds.size.width-30;
 leftMargin = 30;
 bottomMargin = [UIScreen mainScreen].bounds.size.height-30-50;
 topMargin = 30+64;

 pathRef=CGPathCreateMutable();
 CGPathMoveToPoint(pathRef, NULL, leftMargin, topMargin);
 CGPathAddLineToPoint(pathRef, NULL, rightMargin, topMargin);
 CGPathAddLineToPoint(pathRef, NULL, rightMargin, bottomMargin);
 CGPathAddLineToPoint(pathRef, NULL, leftMargin, bottomMargin);
 CGPathAddLineToPoint(pathRef, NULL, leftMargin, topMargin);
 CGPathCloseSubpath(pathRef);
}

#pragma mark - 事件
- (void)btnAction:(UIButton*)sender{

}

#pragma mark - 手势
- (void)handlePan:(UIPanGestureRecognizer *)pan
{
 if (pan.state == UIGestureRecognizerStateBegan) {

  beginPoint = [pan locationInView:self.view];
 }else if (pan.state == UIGestureRecognizerStateChanged){

  CGPoint nowPoint = [pan locationInView:self.view];

  float offsetX = nowPoint.x - beginPoint.x;
  float offsetY = nowPoint.y - beginPoint.y;
  CGPoint centerPoint = CGPointMake(beginPoint.x + offsetX, beginPoint.y + offsetY);

  if (CGPathContainsPoint(pathRef, NULL, centerPoint, NO))
  {
   _crossBtn.center = centerPoint;
  }else{
   if (centerPoint.y>bottomMargin)
   {
    if (centerPoint.x<rightMargin&&centerPoint.x>leftMargin) {
     _crossBtn.center = CGPointMake(beginPoint.x + offsetX, bottomMargin);
    }
   }
   else if (centerPoint.y<topMargin)
   {
    if (centerPoint.x<rightMargin&&centerPoint.x>leftMargin) {
     _crossBtn.center = CGPointMake(beginPoint.x + offsetX, topMargin);
    }
   }
   else if (centerPoint.x>rightMargin)
   {
    _crossBtn.center = CGPointMake(rightMargin, beginPoint.y + offsetY);
   }
   else if (centerPoint.x<leftMargin)
   {
    _crossBtn.center = CGPointMake(leftMargin, beginPoint.y + offsetY);
   }
  }
 }else if (pan.state == UIGestureRecognizerStateEnded || pan.state == UIGestureRecognizerStateFailed){
 }
}
@end