加载图片的时候通常会有渐渐出现的效果, 下面就是需要加的代码(SDWebImage第三方"sd_setImageWithURL: placeholderImage:" 中添加warning部分 代码):

- (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) {
                    wself.image = image;
                    
                    








#warning  图片渐渐出现的效果.
                    wself.alpha = 0;
                    if (cacheType != SDImageCacheTypeMemory) {
                        
                        [UIView animateWithDuration:1 animations:^{
                            wself.alpha = 1;
                        }];
                    }
                    else{
                        
                        wself.alpha = 1;
                    }
                    
                    











                    wself.alpha = 0;
                    
                    if (cacheType != SDImageCacheTypeMemory) {
                        
                        [UIView animateWithDuration:1 animations:^{
                            wself.alpha = 1;
                        }];
                    }
                    else{
                        
                        wself.alpha = 1;
                    }
                    
                    
                    [wself setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        wself.image = placeholder;
                        [wself setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
    } else {
        dispatch_main_async_safe(^{
            [self removeActivityIndicator];
            NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
            if (completedBlock) {
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}