ios programming: Using threads to add multiple images to library

Question

In Xcode, when I\'m trying to add more than 5 images to my library, it gives me the following error:

Error Domain=ALAssetsLibraryErrorDomain         


        

Answer 1

You may try to write all your images subsequently, instead of simultaneously. The following code utilizes ALAssetsLibrary, and implements an "asynchronous loop" which invokes a number of asynchronous methods in sequence.

typedef void (^completion_t)(id result);

- (void) writeImages:(NSMutableArray*)images 
          completion:(completion_t)completionHandler {
    if ([images count] == 0) {
        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(@"finished");  
        }
        return;
    }

    UIImage* image = [images firstObject];
    [images removeObjectAtIndex:0];

    [self.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
        ...
        // next image:
        [self writeImages:images completion:completionHandler];
    }];
}

Usage:

[foo writeImages:[foo.images mutableCopy] completion:^(id result){
    // Caution: check the execution context - it may be any thread
    NSLog(@"Result: %@", result);   
}];