問題：
拍照後，點選選擇照片在-(void)imagePickerController:(UIImagePickerController*)picker didFinishPickingMediaWithInfo:(NSDictionary *)info 的代理方法中獲取了相簿的Alasset後，發現為空

解決辦法：

-(void)imagePickerController:(UIImagePickerController*)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
    NSString *type = [info objectForKey:UIImagePickerControllerMediaType];
    if
 
 ([type isEqualToString:@"public.image"])
    {
        //先把圖片轉成NSData
        UIImage* image = [info objectForKey:@"UIImagePickerControllerOriginalImage"];
     ALAssetsLibrary *library  = [[ALAssetsLibrary alloc] init];
        [library writeImageToSavedPhotosAlbum:[image CGImage] orientation:(ALAssetOrientation)[image imageOrientation] completionBlock:^(NSURL
 
 *assetURL, NSError *error){
            if (error) {
                // TODO: error handling
            } else {
                [library  assetForURL:assetURL resultBlock:^(ALAsset *asset) {
                    [self.selectPhotos addObject:asset];
                    [_imageCollection reloadData];
                } failureBlock:^(NSError
 
 *error) {

                }];
            }
        }];
    }
}