ALAssetsLibrary没有从Iphone中的相册中检索图像？

Question

我正在尝试从iphone（xcode 4.2）中的相册中获取所有图像/照片缩略图，遗憾的是我没有从中获取任何价值。我做了以下代码......

#import <UIKit/UIKit.h>
#import <AssetsLibrary/AssetsLibrary.h>
@interface MyImgPickerViewController : UIViewController<UITableViewDataSource,UITableViewDelegate>
{
    IBOutlet UITableView *tableView;
    IBOutlet UIActivityIndicatorView *activity;
    NSMutableArray *assets;
}
@property(nonatomic,retain) IBOutlet UITableView *tableView;
@property(nonatomic,retain) IBOutlet UIActivityIndicatorView *activity;
@end

导入“MyImgPickerViewController.h”

    @implementation MyImgPickerViewController
        @synthesize title,tableView,activity;
        - (void)viewDidLoad
        {
            [super viewDidLoad];
            [activity startAnimating];
            //Define Blocks here;
            assets =[[NSMutableArray alloc]init];
            ALAssetsLibrary *library=[[ALAssetsLibrary alloc]init];
            void (^assetEnumerator)( ALAsset *, NSUInteger , BOOL *) = ^(ALAsset *result, NSUInteger index, BOOL *stop)
            {
                    if(result != NULL)
                    {
                            NSLog(@"See Asset: %@", result);
                            [assets addObject:result];

                    }
            };    
            void (^assetGroupEnumerator)( ALAssetsGroup *, BOOL *) =  ^(ALAssetsGroup *group, BOOL *stop)
            {
                if(group != nil)
                    {
                            [group enumerateAssetsUsingBlock:assetEnumerator];
                    }      
                        [self.tableView reloadData];
                    [self.activity stopAnimating];
                    [self.activity setHidden:YES];
            };    
            void(^ErrorBlock)(NSError*)=^(NSError *error)
            {
                 NSLog(@"Failure");
            };    


                [library enumerateGroupsWithTypes:ALAssetsGroupAlbum
                                       usingBlock:assetGroupEnumerator
                                     failureBlock: ErrorBlock
                 ];    
        }
    -(NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return [assets count];
}
-(NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

        - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

            static NSString *CellIdentifier = @"Cell";

            UITableViewCell *cell = [self.tableView dequeueReusableCellWithIdentifier:CellIdentifier];
            if (cell == nil) {
                cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
            }

            //ALAsset *asset = [assets objectAtIndex:indexPath.row];
            NSLog(@"......%@",assets);
            [cell.imageView setImage:[UIImage imageWithCGImage:[[assets objectAtIndex:indexPath.row ]thumbnail]]];
            [cell.textLabel setText:[NSString stringWithFormat:@"Photo %d", indexPath.row+1]];

            return cell;
        }

如果我正在运行此应用程序我没有获得任何资产，我找不到我错误的地方...... 有没有可用于ALAssetsLibrary的文档，除了类referance