• 業務邏輯：為需要切圖的view添加pan手勢，定義一個需要截取范圍視圖的成員屬性，定義一個成員變量用來記錄手指的開始點，在pan方法中，根據手勢的狀態，獲取矩形框的開始起點和結束點，然后根據起點和終點，獲取截取范圍，然后開啟位圖上下文，設置裁剪區域，把內容渲染到上下文，獲取新的圖片，結束上下文，最后刪除截取范圍視圖，并值為nil，移除手勢

@interface ViewController ()
// 開始起點屬性
@property (nonatomic, assign) CGPoint startP;
// 截取范圍視圖
@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手勢
    UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(pan:)];
    [self.view addGestureRecognizer:pan];
}
#pragma mark - pan手勢
- (void)pan:(UIPanGestureRecognizer *)pan
{
    CGPoint endP = CGPointZero;
    // 獲取矩形框的開始起點
    if (pan.state == UIGestureRecognizerStateBegan) {
        // 獲取一開始觸摸點
        self.startP = [pan locationInView:self.view];
    }else if (pan.state == UIGestureRecognizerStateChanged)
    {
        // 獲得結束那一時刻的點
        endP = [pan locationInView:self.view];
        // 根據起點和終點，獲取截取范圍
        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)
    {
        // 開啟上下文
        UIGraphicsBeginImageContextWithOptions(self.imageView.bounds.size, NO, 0);
        
        // 設置裁剪區域
        UIBezierPath *path = [UIBezierPath bezierPathWithRect:self.clipView.frame];
        [path addClip];
        
        // 把內容渲染到上下文
        [self.imageView.layer renderInContext:UIGraphicsGetCurrentContext()];
        
        // 獲取新的圖片
        self.imageView.image = UIGraphicsGetImageFromCurrentImageContext();
        
        // 結束上下文
        UIGraphicsEndImageContext();
        
        // 刪除clipView視圖
        [self.clipView removeFromSuperview];
        self.clipView = nil;
        
        [self.view removeGestureRecognizer:pan];
    }
}