ios通过SDWebImage实现图片加载时的渐变效果

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30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：

1. kcatransitionfade  //交叉淡化过渡                    
2. kcatransitionmovein  //移动覆盖原图                    
3. kcatransitionpush  //新视图将旧视图推出去                    
4. kcatransitionreveal  //底部显出来
   

因为我们的需求是渐变嘛，所以就使用kcatransitionfade

注意啦

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。

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