Unable to enumerate groups/results while using ALAssetsLibrary in static library

Question

This question is kind of solved, but one more request remaining at the bottom of this question. And I will choose an answer after all.

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I am creating a static library that is using AssetsLibrary framework. While I am using it in a project to test whether is work. It return nothing while I call the enumerateGroupsWithTypes:usingBlock:failureBlock: method of the instance of AssetsLibrary.

I have trying -

1. to set breakpoints to see how it running this method. Turns out it did not go into the block that passing to usingBlock:, which is ALAssetsLibraryGroupsEnumerationResultsBlock, and either the failureBlock:. So that I got nothing.

2. to add the same enumerating code to the project I mentioned at the beginning to try to calling method of AssetsLibrary. it worked perfectly fine.

3. to test whether it is block by the main thread, then run it in the main thread. Got the same result as before.

For this issue, I have found an answer of other question that is talking about using media in the static library: https://stackoverflow.com/a/15331319/1583560, and I am not sure I have run into the same situation, is the media he/she mentions including accessing AssetsLibrary, I guess no here.

Hope someone can point some directions of this, thank you :)

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Update

This is the code I used -

[[[ALAssetsLibrary alloc] init] enumerateGroupsWithTypes:ALAssetsGroupAll
                                              usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
                                                  NSLog(@"%s",__PRETTY_FUNCTION__);
                                              } failureBlock:^(NSError *error) {
                                                  NSLog(@"%s",__PRETTY_FUNCTION__);
                                              }];

Code in the static library is the same as in the test project, the only difference is I make a model class to access AssetsLibrary in the static library.

To be clear, here are few changes I have made in the static library -

• Change Build Product Path in Target > Build Settings to $(PROJECT_DIR)/../build
• Moving required header file to Project section in Target > Build Phases > Copy Headers
• Set Skip Install to YES in Target > Build Settings

Environment related -

• OS X 10.9.1
• Xcode 5.0.2
• Standard architectures (including 64-bit) (both static library and the project)
• ARC

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More details

Here is my propose to make a assets model for easy accessing in this static library.

• Having a group array to store all the albums, which is ALAssetsGroup here, in devices.
• Enumerating albums out at the init of this model and storing into the group array.
• Enumerating photos, which is ALAssets result, by the group given while needed.

And this model using singleton pattern.

BTW, I have tried to observe the ALAssetsLibraryChangedNotification in this static library. It's not working, too. Are there any potential obstructs at the front of AssetsLibrary?

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Update

I have find out that I enumerate the groups while init the model I created. And there are threads make blocks not work. If I trigger the enumerate after the init complete, will work perfectly. And I also found the way to know when it is done enumerating (cf. https://stackoverflow.com/a/13309871/1583560) to get the group array which I stored.

Further, I still cannot find the document, Apple's, that addressing the threading of block, why it will not been call while init, yet. If someone could point out for me, I will appreciate it :).

Answer 1

This is not about "unable to enumerate assets on devices", because of the async of enumerating ALAssetsGroup.

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The Async

According to apple's document about enumerateGroupsWithTypes:usingBlock:failureBlock: of ALAssetsLibrary, this is an async method, will not get full data stored in array after it run.

This method is asynchronous. When groups are enumerated, the user may be asked to confirm the application's access to the data; the method, though, returns immediately. You should perform whatever work you want with the assets in enumerationBlock.

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Completion Notification

I do want to know when it is done. so that I have found an answer for knowing when the progress is done, and execute a block (cf. Create my own completion blocks in iOS). Even though this is not about the notification, but still gave me a hint to work it out.

postNotification: while it reach the end of the enumeration

[_assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos  
                              usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
                                  if(group){
                                      // add into the array you are going to use
                                  }
                                  else
                                  {
                                      // the last one will be nil, 
                                      // it also means the enumeration is done
                                      // post notification then
                                      [[NSNotificationCenter defaultCenter] postNotificationName:kEnumerateGroupCompleteNotification]; 
                                  }
                              }
                            failureBlock:nil]; // leave nil here for make subject out

addObserver:selector:name:object: to add observer for reloading data

First, using the empty NSArray, which is the retained array we are using at previous step, in the ~DataSource protocol of UICollectionView or UITableView.

Second, adding an observer to the UIViewController, using @selector pass to this method to trigger the instance's reloadData to get the complete array just enumerating out. And the data will be shown on the view.

// In a UIViewController's implementation 

- (void)viewDidLoad{
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(reloadTableView:)
                                                 name:kEnumerateGroupCompleteNotification
                                               object:nil];
}

-(void)reloadTableView:(NSNotification*) notification {

    // reloadData after receive the notification
    [_tableView reloadData];

}

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Conclusion

This is the way how I achieve my requirement. I have addressed ALAssetsGroup only above, but it will be the same in ALAssets's enumeration.

It is the same way if you want to get ALAssetsLibraryChangedNotification and reload data.

And a great thank for Ramshad's answer, I have accessing ALAssetsLibrary as your suggestion in my static library now.

Answer 2

Add the following method to your class:

+ (ALAssetsLibrary *)defaultAssetsLibrary
{
    static dispatch_once_t pred = 0;
    static ALAssetsLibrary *library = nil;
    dispatch_once(&pred, ^{
        library = [[ALAssetsLibrary alloc] init];
    });
    return library;
}

Then modify your code as

ALAssetsLibrary *assetsLibrary = [ViewController defaultAssetsLibrary];
[assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop)
 {
    [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop)
     {
        if(result)
        {
           NSLog(@"Success");
        }
     }];

 } failureBlock:^(NSError *error)
 {
        NSLog(@"Error loading images %@", error);
 }];