CGContext对缩放图片的剪裁

本例子类似于微信头像上传前对上传图片的剪裁功能，利用CGContext的CTM属性对坐标系的宽高进行了缩放倍数，再对图片进行剪裁。该例子还使用了UIImageView对图片的显示，UIScrollView对图片的缩放，移动。

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor blackColor];
    //设置剪裁按钮
    UIButton* crop = [UIButton buttonWithType:UIButtonTypeCustom];
    crop.frame = CGRectMake(0, 0, 60, 30);
    [crop setTitle:@"crop" forState:UIControlStateNormal];
    [crop setTitleColor:[UIColor redColor] forState:UIControlStateNormal];
    crop.backgroundColor = [UIColor grayColor];
    crop.alpha = 0.5;
    [crop addTarget:self action:@selector(crop:) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:crop];
    //创建一个UIImageView用于显示需要剪裁的图片
    imageView = [[UIImageView alloc]initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, 400)];
    imageView.image = [UIImage imageNamed:@"test.png"];
    //创建一个UIScrollView用于对图片进行缩放效果
    scrollView = [[UIScrollView alloc]initWithFrame:CGRectMake(0, 50, self.view.frame.size.width, 400)];
    scrollView.delegate = self;
    scrollView.contentSize = self.view.frame.size;
    //设置UIScrollView有水平拉动反弹效果
    scrollView.alwaysBounceHorizontal = YES;
    //设置UIScrollView有垂直拉动反弹效果
    scrollView.alwaysBounceVertical = YES;
    scrollView.backgroundColor = [UIColor blackColor];
    //设置能给图片最小的放大倍率
    scrollView.minimumZoomScale = 1.f;
    //设置能给图片最大的放大倍率为2倍
    scrollView.maximumZoomScale = 2.f;
    [scrollView addSubview:imageView];
    [self.view addSubview:scrollView];
    
    //创建一个截取图片的边框
    CALayer* cropLayer = [CALayer layer];
    cropLayer.borderColor = [[UIColor grayColor]CGColor];
    cropLayer.borderWidth = 3.f;
    cropLayer.anchorPoint = CGPointMake(0, 0);
    cropLayer.position = CGPointMake(0, 50);
    cropLayer.bounds = CGRectMake(0, 0, self.view.frame.size.width, 320);
    [self.view.layer addSublayer:cropLayer];
    
}
//UIScrollView缩放回调事件
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView{
    return imageView;
}
//剪裁按钮触发的事件
-(void)crop:(id)sender{
    NSLog(@"transform:%@",NSStringFromCGAffineTransform(imageView.transform));
    NSLog(@"coordinate:%@",NSStringFromCGRect(imageView.frame));
    NSLog(@"offset:%@",NSStringFromCGPoint(scrollView.contentOffset));
    //把剪裁后的图片保存到相册中
    UIImageWriteToSavedPhotosAlbum([UIImage imageWithCGImage:[self cropImage]], self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
}
//剪裁图片方法
-(CGImageRef)cropImage{
    //创建context，用于产出处理后的图片
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(imageView.frame.size.width, imageView.frame.size.height), NO, 0);
    CGContextRef context = UIGraphicsGetCurrentContext();
    //改变context的CTM属性，根据UIImageView被放大的几何变换修改context的坐标系宽高倍率
    CGContextScaleCTM(context, scrollView.zoomScale, scrollView.zoomScale);
    //把图片绘制到context中，还要根据UIScrollView对图片的偏移量
    [imageView.image drawInRect:CGRectMake(-scrollView.contentOffset.x, -scrollView.contentOffset.y,imageView.frame.size.width, imageView.frame.size.height)];
    UIImage* image= UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    NSLog(@"crop:%@",NSStringFromCGSize(image.size));
    //把缩放生成后的图片进行剪裁，剪裁的宽高要同时根据缩放的比率
    CGImageRef corp = CGImageCreateWithImageInRect(image.CGImage, CGRectMake(0, 0, self.view.frame.size.width*scrollView.zoomScale, 320*scrollView.zoomScale));
    return corp;
}
//保存图片成功后的回调
- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error
  contextInfo:(void *)contextInfo{
    
    NSLog(@"saved..");
}

效果如下：