• 业务逻辑：为需要切图的view添加pan手势，定义一个需要截取范围视图的成员属性，定义一个成员变量用来记录手指的开始点，在pan方法中，根据手势的状态，获取矩形框的开始起点和结束点，然后根据起点和终点，获取截取范围，然后开启位图上下文，设置裁剪区域，把内容渲染到上下文，获取新的图片，结束上下文，最后删除截取范围视图，并值为nil，移除手势

@interface ViewController ()
// 开始起点属性
@property (nonatomic, assign) CGPoint startP;
// 截取范围视图
@property (nonatomic, weak) UIView *clipView;
@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@end

@implementation ViewController

- (UIView *)clipView
{
    if (!_clipView) {
        UIView *clipView = [[UIView alloc] init];
        clipView.backgroundColor = [UIColor blackColor];
        clipView.alpha = 0.5;
        _clipView = clipView;
        [self.view addSubview:_clipView];
    }
    
    return _clipView;
}

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    [self addGesture];
}
- (void)addGesture
{
    // 添加pan手势
    UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(pan:)];
    [self.view addGestureRecognizer:pan];
}
#pragma mark - pan手势
- (void)pan:(UIPanGestureRecognizer *)pan
{
    CGPoint endP = CGPointZero;
    // 获取矩形框的开始起点
    if (pan.state == UIGestureRecognizerStateBegan) {
        // 获取一开始触摸点
        self.startP = [pan locationInView:self.view];
    }else if (pan.state == UIGestureRecognizerStateChanged)
    {
        // 获得结束那一时刻的点
        endP = [pan locationInView:self.view];
        // 根据起点和终点，获取截取范围
        CGFloat w = endP.x - self.startP.x;
        CGFloat h = endP.y - self.startP.y;
        
        CGRect clipRect = CGRectMake(self.startP.x, self.startP.y, w, h);
        
        self.clipView.frame = clipRect;
        
    }else if (pan.state == UIGestureRecognizerStateEnded)
    {
        // 开启上下文
        UIGraphicsBeginImageContextWithOptions(self.imageView.bounds.size, NO, 0);
        
        // 设置裁剪区域
        UIBezierPath *path = [UIBezierPath bezierPathWithRect:self.clipView.frame];
        [path addClip];
        
        // 把内容渲染到上下文
        [self.imageView.layer renderInContext:UIGraphicsGetCurrentContext()];
        
        // 获取新的图片
        self.imageView.image = UIGraphicsGetImageFromCurrentImageContext();
        
        // 结束上下文
        UIGraphicsEndImageContext();
        
        // 删除clipView视图
        [self.clipView removeFromSuperview];
        self.clipView = nil;
        
        [self.view removeGestureRecognizer:pan];
    }
}