Question

I am using this code to retrieve all images form my device but its not returning an result

- (void)viewDidLoad
{
    [super viewDidLoad];
    void (^assetEnumerator)(ALAsset *, NSUInteger, BOOL *) = ^(ALAsset *result, NSUInteger index, BOOL *stop) {
        if(result != NULL) {
            NSLog(@"See Asset: %@", result);
            [_assets addObject:result];
            // Here storing the asset's image URL's in NSMutable array urlStoreArr
            NSURL *url = [[result defaultRepresentation] url];
            [_urlStoreArr addObject:url];
        }
    };

    void (^assetGroupEnumerator)(ALAssetsGroup *, BOOL *) =  ^(ALAssetsGroup *group, BOOL *stop)
    {
        if(group != nil)
        {
            [group enumerateAssetsUsingBlock:assetEnumerator];
        }
    };

    _urlStoreArr = [[NSMutableArray alloc] init];
    _assets = [[NSMutableArray alloc] init];
    _library = [[ALAssetsLibrary alloc] init];

    [_library enumerateGroupsWithTypes:ALAssetsGroupAlbum
                           usingBlock:assetGroupEnumerator
                         failureBlock: ^(NSError *error) {
                             NSLog(@"Failure");
                         }];



    [self UploadImagesToServer];

}

-(void) UploadImagesToServer
{
    for (int i=0; i<[_urlStoreArr count]; i++)
    {
        // To get the each image URL here...

        NSString *str = [_urlStoreArr objectAtIndex:i];
        NSLog(@"str: %@",str);

        // Need to upload the images to my server..
    }
}

Answer 1

You are using the _urlStoreArr before it is initialized.

When you are defining blocks, they take the current local value of variables that you use in the block.

So, in the assetEnumerator block you are using _urlStoreArr but you aren’t initialising it until later in the code.

I’m assuming that _urlStoreArr is an iVar, since it has a leading underscore. If you are using ARC, then the iVar is initialised to nil, so your assetEnumerator block is sending a message to nil. This is legal in Objective-C, but it just returns nil.

You have two options to fix this.

1. Move the declaration of _urlStoreArr = [[NSMutableArray alloc] init]; above the definition of the assetEnumerator block
2. The preferred way – don’t use the iVar directly, use property syntax. i.e. rather than [_urlStoreArr addObject:url]; try [self.urlStoreArr addObject:url];. This works because when using property access, you aren’t using the current value of _urlStoreArr at the time of definition, you are using the value at the time the block runs, which is after the iVar has been initialised.