title: Wait for assetForURL blocks to be completed
body: <p>I would like to wait this code to be executed before to continue but as these blocks are called assynchronously I don't know how to do???</p>
  
  <pre><code>NSURL *asseturl;
  NSMutableArray *tmpListAsset = [[NSMutableArray alloc] init];
  
  ALAssetsLibrary *library = [[[ALAssetsLibrary alloc] init] autorelease];
  NSMutableArray *objectsToRemove = [[NSMutableArray alloc] init];
  for (NSDictionary *dico in assetsList) {
      asseturl = [NSURL URLWithString:[dico objectForKey:@"assetUrl"]];
      NSLog(@"asset url %@", asseturl);
      // Try to load asset at mediaURL
      [library assetForURL:asseturl resultBlock:^(ALAsset *asset) {
          // If asset doesn't exists
          if (!asset){
              [objectsToRemove addObject:dico];
          }else{
              [tmpListAsset addObject:[asseturl absoluteString]];
              NSLog(@"tmpListAsset : %@", tmpListAsset);
          }
      } failureBlock:^(NSError *error) {
          // Type your code here for failure (when user doesn't allow location in your app)
      }];
  }
  </code></pre>