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：

1 // With a UIImage, the API user can use -[UIImage CGImage] to get a CGImageRef, and cast -[UIImage imageOrientation] to ALAssetOrientation.
2 - (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");
3 
4 // The API user will have to specify the orientation key in the metadata dictionary to preserve the orientation of the image
5 - (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");
6 
7 // If there is a conflict between the metadata in the image data and the metadata dictionary, the image data metadata values will be overwritten
8 - (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`

1 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）。

 1 typedef void (^completion_t)(id result);
 2 
 3 
 4 - (void) writeImages:(NSMutableArray*)images
 5           completion:(completion_t)completionHandler {
 6     if ([images count] == 0) {
 7         if (completionHandler) {
 8             // Signal completion to the call-site. Use an appropriate result,
 9             // instead of @"finished" possibly pass an array of URLs and NSErrors
10             // generated below  in "handle URL or error".
11             completionHandler(@"images are all saved.");
12         }
13         return;
14     }
15     UIImage* image = [images firstObject];
16     [assetsLibrary writeImageToSavedPhotosAlbum:image.CGImage
17                                     orientation:ALAssetOrientationUp
18                                 completionBlock:^(NSURL *assetURL, NSError *error){
19          // Caution: check the execution context - it may be any thread,
20          // possibly use dispatch_async to dispatch to the main thread or
21          // any other queue.
22          // handle URL or error
23          if (error) {
24              NSLog(@"error = %@", [error localizedDescription]);
25              isImagesSavedFailed = true;
26              return;
27          }
28          [images removeObjectAtIndex:0];
29          // next image:
30          [self writeImages:images completion:completionHandler];
31      }];
32 }

3. 调用示例

 1 NSMuatableArray *saveImages;//保存到相册的图片
 2 [self writeImages:saveImages completion:^(id result) {
 3       NSLog(@"Result: %@", result);
 4       [hud stopLoading];
 5        if (isImagesSavedFailed) {
 6             //handle failed.
 7        }else{
 8             //handle success.
 9         }
10 }];

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。