图片的读取

 通常的实现是使用UIImagePickerController控件，而后实现UIImagePickerControllerDelegate里面的两个回调方法。 

 - (void)button2Pressed:(id)sender 
{
    UIImagePickerController* vc = [[UIImagePickerController alloc] init];
    vc.sourceType = UIImagePickerControllerSourceTypeSavedPhotosAlbum;
    vc.delegate = self;
    [self presentViewController:vc animated:YES completion:nil];
    [vc release];
}

#pragma mark - UIImagePickerControllerDelegate
- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
    // 图片在这里
    UIImage* image = [info objectForKey:UIImagePickerControllerOriginalImage];
    [self dismissViewControllerAnimated:YES completion:nil];
}

- (void)imagePickerControllerDidCancel:(UIImagePickerController *)picker
{
    [self dismissViewControllerAnimated:YES completion:nil];
}

 使用新的方法，首先作如下两点： 1.引入头文件：#import (尖括号)AssetsLibrary/AssetsLibrary.h(尖括号) 2.在工程中引入库：AssetsLibrary.framework 具体代码以下： 

// 打印信息，仅作演示
- (void)printALAssetInfo:(ALAsset*)asset
{
    //取图片的url
    NSString *photoURL=[NSString stringWithFormat:@"%@",asset.defaultRepresentation.url];
    NSLog(@"photoURL:%@", photoURL);
    // 取图片
    UIImage* photo = [UIImage imageWithCGImage:asset.defaultRepresentation.fullScreenImage];
    NSLog(@"PHOTO:%@", photo);
    NSLog(@"photoSize:%@", NSStringFromCGSize(photo.size));
    // 取图片缩图图
    UIImage* photoThumbnail = [UIImage imageWithCGImage:asset.thumbnail];
    NSLog(@"PHOTO2:%@", photoThumbnail);
    NSLog(@"photoSize2:%@", NSStringFromCGSize(photoThumbnail.size));
}

-(void)loadImageFromPhotoLibrary
{
    // 为了防止界面卡住，能够异步执行
    dispatch_async(dispatch_get_main_queue(), ^{
        // 获取相册每一个组里的具体照片
        ALAssetsGroupEnumerationResultsBlock groupEnumerAtion = ^(ALAsset *result, NSUInteger index, BOOL *stop){
            if (result!=nil) {
                // 检查是不是照片，还多是视频或其它的
                // 因此这里咱们还能类举出枚举视频的方法。。。
                if ([[result valueForProperty:ALAssetPropertyType] isEqualToString:ALAssetTypePhoto]) {
                    [self printALAssetInfo:result];
                }
            }
        };
        //获取相册的组
        ALAssetsLibraryGroupsEnumerationResultsBlock groupsEnumeration = ^(ALAssetsGroup* group, BOOL* stop){
            if (group!=nil) {
                NSString *groupInfo=[NSString stringWithFormat:@"%@",group];
                NSLog(@"GROUP INFO:%@",groupInfo);

                [group enumerateAssetsUsingBlock:groupEnumerAtion];
            }
        };

        ALAssetsLibraryAccessFailureBlock failureblock = ^(NSError *error){
            // 相册访问失败的回调，能够打印一下失败缘由
            NSLog(@"相册访问失败，ERROR:%@", [error localizedDescription]);
        };

        ALAssetsLibrary* library = [[ALAssetsLibrary alloc] init];
        [library enumerateGroupsWithTypes:ALAssetsGroupAll
                               usingBlock:groupsEnumeration
                             failureBlock:failureblock];
        [library release];     
    }); 

}

// 同上面的原理，咱们再作一个根据URL取图片及缩略图的方法
- (void)loadImageForURL:(NSURL*)photoUrl
{
    ALAssetsLibrary *assetLibrary=[[ALAssetsLibrary alloc] init];
    [assetLibrary assetForURL:photoUrl
                  resultBlock:^(ALAsset *asset)
     {
         [self printALAssetInfo:asset];
    }
                 failureBlock:^(NSError *error)
     {
        NSLog(@"error=%@",error);
    }];
}
原文链接：http://ilovedev.github.io/blog/2013/11/03/bu-shi-yong-uiimagepickercontrollershi-xian-du-qu-xiang-ce-tu-pian-de-fang-fa/