类似微信头像选择裁剪功能 ，设置UIImagePickerController的allowsEditing属性为YES即可实现，但如何自己实现此功能呢？如何实现类似系统联系人圆形头像裁剪选取呢？

如图：

Simulator Screen Shot 00.png

图片要支持缩放和拖拽，首先想到scrollView，具体实现方法如下：

//CropImageController.h
@protocol CropImageDelegate <NSObject>
- (void)cropImageDidFinishedWithImage:(UIImage *)image;
@end

@interface CropImageController : UIViewController

@property (nonatomic, weak) id <CropImageDelegate> delegate;
//圆形裁剪，默认NO;
@property (nonatomic, assign) BOOL ovalClip;
- (instancetype)initWithImage:(UIImage *)originalImage delegate:(id)delegate;
@end

在viewDidLoad初始化scrollView,设置frame为裁剪框大小，若需要超出边界线内容仍然可以显示，设置其layer属性masksToBounds即可

- (void)viewDidLoad {
    [super viewDidLoad];
// Do any additional setup after loading the view.
    self.view.backgroundColor = [UIColor blackColor];
    self.automaticallyAdjustsScrollViewInsets = NO;
    CGFloat height = (ScreenHeight - ScreenWidth)/2.0;
    _scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, height ,ScreenWidth,ScreenWidth)];
    _scrollView.bouncesZoom = YES;
    _scrollView.minimumZoomScale = 1;
    _scrollView.maximumZoomScale = 3;
    _scrollView.zoomScale = 1;
    _scrollView.delegate = self;
    _scrollView.layer.masksToBounds = NO;
    _scrollView.showsHorizontalScrollIndicator = NO;
    _scrollView.showsVerticalScrollIndicator = NO;
    _scrollView.layer.borderWidth = 1.5;
    _scrollView.layer.borderColor = [UIColor whiteColor].CGColor;
      if (_ovalClip) {
        _scrollView.layer.cornerRadius = ScreenWidth/2.0;
    }
    self.view.layer.masksToBounds = YES;
    if (_originalImage) {
        _imageView = [[UIImageView alloc] initWithImage:_originalImage];
        CGFloat img_width = ScreenWidth;
        CGFloat img_height = _originalImage.size.height * (img_width/_originalImage.size.width);
        CGFloat img_y= (img_height - self.view.bounds.size.width)/2.0;
        _imageView.frame = CGRectMake(0,0, img_width, img_height);
        _imageView.userInteractionEnabled = YES;
        [_scrollView addSubview:_imageView];
        
        
        _scrollView.contentSize = CGSizeMake(img_width, img_height);
        _scrollView.contentOffset = CGPointMake(0, img_y);
        [self.view addSubview:_scrollView];
    }
    [self userInterface];
}

设置self.view中间镂空效果：

- (void)userInterface {
    
    CGRect cropframe = _scrollView.frame;
    UIBezierPath * path = [UIBezierPath bezierPathWithRoundedRect:self.view.bounds cornerRadius:0];
    UIBezierPath * cropPath = [UIBezierPath bezierPathWithRoundedRect:cropframe cornerRadius:0];
    if (_ovalClip) {
        cropPath = [UIBezierPath bezierPathWithOvalInRect:cropframe];
    }
    [path appendPath:cropPath];
    
    CAShapeLayer * layer = [[CAShapeLayer alloc] init];
    layer.fillColor = [UIColor colorWithRed:.0 green:.0 blue:.0 alpha:0.5].CGColor;
    //填充规则
    layer.fillRule=kCAFillRuleEvenOdd;
    layer.path = path.CGPath;
    [self.view.layer addSublayer:layer];
}

UIScrollViewDelegate,缩放和滑动过程中调整内容视图大小：

#pragma mark -- UIScrollViewDelegate
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView {
    return self.imageView;
}
- (void)scrollViewDidZoom:(UIScrollView *)scrollView {
    //调整位置
    [self centerContent];
}
- (void)centerContent {
    CGRect imageViewFrame = _imageView.frame;
    
    CGRect scrollBounds = CGRectMake(0, 0, ScreenWidth, ScreenWidth);
    if (imageViewFrame.size.height > scrollBounds.size.height) {
        imageViewFrame.origin.y = 0.0f;
    }else {
        imageViewFrame.origin.y = (scrollBounds.size.height - imageViewFrame.size.height) / 2.0;
    }
    if (imageViewFrame.size.width < scrollBounds.size.width) {
        imageViewFrame.origin.x = (scrollBounds.size.width - imageViewFrame.size.width) /2.0;
    }else {
        imageViewFrame.origin.x = 0.0f;
    }
    _imageView.frame = imageViewFrame;
}

调整位置确定选取图片时候，可根据当前ScrollView的contentOffset和缩放倍数确定具体裁剪位置：

- (UIImage *)cropImage {
    CGPoint offset = _scrollView.contentOffset;
    //图片缩放比例
    CGFloat zoom = _imageView.frame.size.width/_originalImage.size.width;
    //视网膜屏设备像素相关
    zoom = zoom / [UIScreen mainScreen].scale;
    
    CGFloat width = _scrollView.frame.size.width;
    CGFloat height = _scrollView.frame.size.height;
    if (_imageView.frame.size.height < _scrollView.frame.size.height) {//太胖了,取中间部分
        offset = CGPointMake(offset.x + (width - _imageView.frame.size.height)/2.0, 0);
        width = height = _imageView.frame.size.height;
    }
    
    CGRect rec = CGRectMake(offset.x/zoom, offset.y/zoom,width/zoom,height/zoom);
    CGImageRef imageRef =CGImageCreateWithImageInRect([_originalImage CGImage],rec);
    UIImage * image = [[UIImage alloc]initWithCGImage:imageRef];
    CGImageRelease(imageRef);
    if (_ovalClip) {
        image = [image ovalClip];
    }
    return image;
}

UIImage分类(图片指定尺寸缩放和圆形裁剪)：

//UIImage+Crop.m
- (UIImage *)resizeImageWithSize:(CGSize)newSize {
    CGFloat newWidth = newSize.width;
    CGFloat newHeight = newSize.height;
    float width  = self.size.width;
    float height = self.size.height;
    if (width != newWidth || height != newHeight) {
        UIGraphicsBeginImageContextWithOptions(CGSizeMake(newWidth, newHeight), YES, [UIScreen mainScreen].scale);
        [self drawInRect:CGRectMake(0, 0, newWidth, newHeight)];
        
        UIImage *resized = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        return resized;
    }
    return self;
}
- (UIImage *)ovalClip {

    CGSize size = self.size;
    UIGraphicsBeginImageContextWithOptions(size, NO, [UIScreen mainScreen].scale);
    UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(0, 0, self.size.width, self.size.height)];
    [path addClip];
    [self drawAtPoint:CGPointZero];
    UIImage * image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return image;
}

注：由于对图片进行裁剪，先将原始图片等比缩放到指定裁剪宽度。
源码下载
CropImage