关于离屏渲染，参考这篇，说的不要太详细：http://ios.xueliedu.com/a/iospx/2016/0605/22671_5.html

因为现在app圆角的东西已经太多了，为了60帧的信仰，配合SDWebImage做了些修改，实际上是先将image处理后再做的缓存。所以当url相同的时候去缓存池读出来的图片是先做过剪裁的，也有其相应的弊端。

//剪裁方法
- (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;
}

//添加在UIImageView+WebCache
- (void)sd_setRoundImageWithURL:(NSURL *)url cornerRadius:(CGFloat)cornerRadius placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
    
    [self sd_cancelCurrentImageLoad];
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            self.image = [self imageWithRoundCorner:placeholder cornerRadius:cornerRadius size:placeholder.size];
        });
    }
    
    if (url) {
        
        // check if activityView is enabled or not
        if ([self showActivityIndicatorView]) {
            [self addActivityIndicator];
        }
        
        __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) {
            [wself removeActivityIndicator];
            if (!wself) return;
            dispatch_main_sync_safe(^{
                if (!wself) return;
                if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock)
                {
                    completedBlock( [self imageWithRoundCorner:image cornerRadius:cornerRadius size:placeholder.size], error, cacheType, url);
                    return;
                }
                else if (image) {
                    wself.image = [self imageWithRoundCorner:image cornerRadius:cornerRadius size:placeholder.size];
                    [wself setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        wself.image = [self imageWithRoundCorner:placeholder cornerRadius:cornerRadius size:placeholder.size];
                        
                        [wself setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    //这时候读的是缓存里的，无需再剪裁
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
    } else {
        dispatch_main_async_safe(^{
            [self removeActivityIndicator];
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}

//添加在UIButton+WebCache
- (void)sd_setRoundImageWithURL:(NSURL *)url forState:(UIControlState)state placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options completed:(SDWebImageCompletionBlock)completedBlock {
    
    [self setImage:[self imageWithRoundCorner:placeholder cornerRadius:placeholder.size.width size:placeholder.size] forState:state];
    [self sd_cancelImageLoadForState:state];
    
    if (!url) {
        [self.imageURLStorage removeObjectForKey:@(state)];
        
        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);
            }
        });
        
        return;
    }
    
    self.imageURLStorage[@(state)] = url;
    
    __weak __typeof(self)wself = self;
    id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:nil completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
        if (!wself) return;
        dispatch_main_sync_safe(^{
            __strong UIButton *sself = wself;
            if (!sself) return;
            if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock)
            {
                completedBlock([self imageWithRoundCorner:placeholder cornerRadius:placeholder.size.width size:placeholder.size], error, cacheType, url);
                return;
            }
            else if (image) {
                [sself setImage:[self imageWithRoundCorner:placeholder cornerRadius:placeholder.size.width size:placeholder.size] forState:state];
            }
            if (completedBlock && finished) {
                completedBlock([self imageWithRoundCorner:placeholder cornerRadius:placeholder.size.width size:placeholder.size], error, cacheType, url);
            }
        });
    }];
    [self sd_setImageLoadOperation:operation forState:state];
}