IOS 判斷相冊是否允許訪問

ALAssetsLibrary *library = [[ALAssetsLibrary alloc]init];
    [library writeImageToSavedPhotosAlbum:p_w_picpath.CGImage orientation:p_w_picpath.p_w_picpathOrientation completionBlock:^(NSURL *asSetUrl,NSError *error){
        if (error) {
            UIAlertView *alert = [[UIAlertView alloc]initWithTitle:@"保存圖片失敗"
                                                           message:@"請打開 設置-隱私-照片 來進行設置"
                                                          delegate:nil
                                                 cancelButtonTitle:@"確定"
                                                 otherButtonTitles:nil, nil];
            [alert show];
            [alert release];
        }else{
            UIAlertView *alert = [[UIAlertView alloc]initWithTitle:@"保存圖片成功"
                                                           message:nil
                                                          delegate:nil
                                                 cancelButtonTitle:@"確定"
                                                 otherButtonTitles:nil, nil];
            [alert show];
            [alert release];
        }

    }];