Objective-C图片切圆角优化

iOS开发中我们会经常对图片进行切圆角操作，常见的做法是这样的：

myImgView.layer.cornerRadius = 25;
myImgView.layer.masksToBounds = YES;

 

其中masksToBounds(CALayer)表示视图的图层上的子图层,如果超出父图层的部分就截取掉；还有clipsToBounds(UIView)，是指视图上的子视图,如果超出父视图的部分就截取掉。
在iOS9.0之前这样设置会触发离屏渲染，比较消耗性能。比如当一个页面上有十几头像这样设置了圆角，尤其是tableView中，大量操作会明显的感到页面的轻微卡顿。
GitHub上有不少开源的第三方，其中基本都是用了Core Graphics绘制圆角。其实只要在你的UIImageView的分类中加一个方法即可：

+ (UIImage *)imageWithRoundCorner:(UIImage *)sourceImage cornerRadius:(CGFloat)cornerRadius size:(CGSize)size{
    CGFloat scale = [UIScreen mainScreen].scale;
    UIGraphicsBeginImageContextWithOptions(size, NO, scale);
    CGRect bounds = CGRectMake(0, 0, size.width, size.height);

    UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:bounds cornerRadius:cornerRadius];
    [path addClip];
    [sourceImage drawInRect:bounds];

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

 

也可以和SDWebImage相结合：

- (void)setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
    [self sd_cancelCurrentImageLoad];

    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            self.image = [self _vvr_processImage:placeholder];
        });
    }

    if (url) {
        __weak __typeof(self)wself = self;
        id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            if (!wself) return;
            dispatch_main_sync_safe(^{
                if (!wself) return;
                if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock)
                {
                    completedBlock([self _vvr_processImage:image], error, cacheType, url);
                    return;
                }
                else if (image) {
                    wself.image = [self _vvr_processImage:image];
                    [wself setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        wself.image = [self _vvr_processImage:placeholder];
                        [wself setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
    } else {
        dispatch_main_async_safe(^{
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}