图片擦除
// 获取当前点
CGPoint curP = [pan locationInView:self.view];
// 获取擦除的矩形范围
CGFloat wh = 100;
CGFloat x = curP.x - wh * 0.5;
CGFloat y = curP.y - wh * 0.5;
CGRect rect = CGRectMake(x, y, wh, wh);
// 开启上下文
UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, NO, 0);
CGContextRef ctx = UIGraphicsGetCurrentContext();
// 控件的layer渲染上去
[_imageView.layer renderInContext:ctx];
// 擦除图片
CGContextClearRect(ctx, rect);
// 生成一张图片
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
_imageView.image = image;
// 关闭上下文
UIGraphicsEndImageContext();
