iOS 多张图片保存到相册问题（add multiple images to photo album）

　　不知道朋友们有木有做过多图保存到系统的相册这个需求，我在用`UIImageWriteToSavedPhotosAlbum`保存图片时候，在代理回调方`didFinishSavingWithError`中有些图片会出现这样的错误：

　　

　　

　　原因上面写的很清楚，在同时保存多张图的时候，系统资源有限，来不及处理全部图片，容易出现写入错误。如果每次保存的时候一张保存完再保存另一张，就不会出现这个错误了。

　　其实管理相册的是`ALAssetsLibrary`这个类，苹果官方对它的描述是这样的：An instance of ALAssetsLibrary provides access to the videos and photos that are under the control of the Photos application. `ALAssetsLibrary`中提供了保存到相册的API：

 // With a UIImage, the API user can use -[UIImage CGImage] to get a CGImageRef, and cast -[UIImage imageOrientation] to ALAssetOrientation.
 - (void)writeImageToSavedPhotosAlbum:(CGImageRef)imageRef orientation:(ALAssetOrientation)orientation completionBlock:(ALAssetsLibraryWriteImageCompletionBlock)completionBlock NS_DEPRECATED_IOS(4_0, 9_0, "Use creationRequestForAssetFromImage: on PHAssetChangeRequest from the Photos framework to create a new asset instead");

 // The API user will have to specify the orientation key in the metadata dictionary to preserve the orientation of the image
 - (void)writeImageToSavedPhotosAlbum:(CGImageRef)imageRef metadata:(NSDictionary *)metadata completionBlock:(ALAssetsLibraryWriteImageCompletionBlock)completionBlock NS_DEPRECATED_IOS(4_1, 9_0, "Use creationRequestForAssetFromImage: on PHAssetChangeRequest from the Photos framework to create a new asset instead");

 // If there is a conflict between the metadata in the image data and the metadata dictionary, the image data metadata values will be overwritten
 - (void)writeImageDataToSavedPhotosAlbum:(NSData *)imageData metadata:(NSDictionary *)metadata completionBlock:(ALAssetsLibraryWriteImageCompletionBlock)completionBlock NS_DEPRECATED_IOS(4_1, 9_0, "Use creationRequestForAssetFromImageData: on PHAssetChangeRequest from the Photos framework to create a new asset instead");

后面两个需要提供图片的metadata, 我用的是第一个方法。下面上码~

1. 初始化一个全局的`ALAssetsLibrary`

ALAssetsLibrary *assetsLibrary = [[ALAssetsLibrary alloc] init];

2.封装一个方法，把需要保存的图片放在一个数组中，每次取第一张，调用`writeImageToSavedPhotosAlbum`保存，如果保存成功，则将其移出数组，再保存下一张，如果有错误，我这边是用一个bool变量`isImagesSavedFailed`来记录（我这边需要图片全部保存成功）。

 typedef void (^completion_t)(id result);

 - (void) writeImages:(NSMutableArray*)images
           completion:(completion_t)completionHandler {
     if ([images count] == ) {
         if (completionHandler) {
             // Signal completion to the call-site. Use an appropriate result,
             // instead of @"finished" possibly pass an array of URLs and NSErrors
             // generated below  in "handle URL or error".
             completionHandler(@"images are all saved.");
         }
         return;
     }
     UIImage* image = [images firstObject];
     [assetsLibrary writeImageToSavedPhotosAlbum:image.CGImage
                                     orientation:ALAssetOrientationUp
                                 completionBlock:^(NSURL *assetURL, NSError *error){
          // Caution: check the execution context - it may be any thread,
          // possibly use dispatch_async to dispatch to the main thread or
          // any other queue.
          // handle URL or error
          if (error) {
              NSLog(@"error = %@", [error localizedDescription]);
              isImagesSavedFailed = true;
              return;
          }
          [images removeObjectAtIndex:];
          // next image:
          [self writeImages:images completion:completionHandler];
      }];
 }

3. 调用示例

 NSMuatableArray *saveImages;//保存到相册的图片
 [self writeImages:saveImages completion:^(id result) {
       NSLog(@"Result: %@", result);
       [hud stopLoading];
        if (isImagesSavedFailed) {
             //handle failed.
        }else{
             //handle success.
         }
 }];

参考链接 ：http://stackoverflow.com/questions/20662908/ios-programming-using-threads-to-add-multiple-images-to-library

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好久木有写博客了，有很多东西木有记录，还是记录下印象深一点。