7.通过SDWebImage实现图片加载时的渐变效果

回头望，伴你走，从来未曾幸福过
恨太多，没结果，往事重提是折磨

先上效果图：
这些图片是在我限制了网速的情况下加载的：

实现效果

思路解析

想到渐变属性的时候，自然而然的想起CATransition这个类
先看整体的实现代码：

首先找到UIImageView+WebCache.m这个文件中的- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock这个函数（大约在44行处）

修改成这个样子

- (void)sd_setImageWithURL:(NSURL *)url 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 = placeholder;
        });
    }
    
    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(image, error, cacheType, url);
                    return;
                }
                else if (image) {
                    CATransition *animation = [CATransition animation];
                    animation.duration = .85f;
                    animation.type = kCATransitionFade;
                    animation.removedOnCompletion = YES;
                    [wself.layer addAnimation:animation forKey:@"transition"];
                    wself.image = image;
                    [wself setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        wself.image = placeholder;
                        [wself setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        [self.layer removeAnimationForKey:@"transition"];
        [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);
            }
        });
    }
}

在大约72行处添加

                    CATransition *animation = [CATransition animation];
                    animation.duration = .85f;
                    animation.type = kCATransitionFade;
                    animation.removedOnCompletion = YES;
                    [wself.layer addAnimation:animation forKey:@"transition"];

不需要过多解释kCATransitionFade意思是 交叉淡化过渡
这个 type 还有几个兄弟：

kCATransitionFade              //交叉淡化过渡                     
kCATransitionMoveIn            //移动覆盖原图                     
kCATransitionPush              //新视图将旧视图推出去                     
kCATransitionReveal            //底部显出来 

因为我们的需求是渐变嘛，所以就使用kCATransitionFade

<small>注意啦</small>
一定要在下载图片的这个Block结束后，把animation去掉[self.layer removeAnimationForKey:@"transition"];。
为什么呢，如果你不去掉，在cell复用的时候，会出现加载重复的情况呢。/坏笑 不信的话，你别加呀。

好心一早放开我，从头努力也坎坷，通通不要好过
为何唱着这首歌，为怨恨而分手，问你是否原谅我