图片任意截屏的实现

功能：任意一张图片，通过触摸屏幕，可以剪切任意大小的子图片。
思路：1.用手势+重绘图片实现；
2.手势控制截图的区域，重绘设置裁剪区域和生成图片。
主要代码如下(这是手势出发的方法实现)：

- (void)panGestureAction:(UIPanGestureRecognizer *)pan
{
    CGPoint currentPoint = [pan locationInView:self.view];
    if (pan.state == UIGestureRecognizerStateBegan)
    {
        self.startPoint = currentPoint;
    }else if (pan.state == UIGestureRecognizerStateChanged)
    {
        CGPoint point = [pan locationInView:self.view];
        CGFloat width = fabs(self.startPoint.x - point.x);
        CGFloat height = fabs(self.startPoint.y - point.y);
        self.coverView.frame = CGRectMake(self.startPoint.x, self.startPoint.y, width, height);
    }else if (pan.state == UIGestureRecognizerStateEnded){
        //1.生成一个与原来图片一样大的图片
        UIGraphicsBeginImageContextWithOptions(self.baseImageView.bounds.size, NO, 1);
        //2.设置剪裁区
        UIBezierPath *clipath = [UIBezierPath bezierPathWithRect:self.coverView.frame];
        [clipath addClip];
        //3.渲染当前图片
        CGContextRef ctx = UIGraphicsGetCurrentContext();
        [self.baseImageView.layer renderInContext:ctx];
        //4.获取新图片
        UIImage *currentImage = UIGraphicsGetImageFromCurrentImageContext();
        //5.关闭上下文
        UIGraphicsEndImageContext();
        //6.重新设置图片
        self.baseImageView.image = currentImage;
        //7.移除浮层
        [self.coverView removeFromSuperview];
       self.coverView = nil;
    }
}