UIImagePickerController不调用代理

  
关键代码如下：

@implementation PublishAdViewController
 {
        UIImagePickerController *picker_library;
 }   

   picker_library = [[UIImagePickerController alloc] init];
    picker_library.delegate = self;
    picker_library.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
    picker_library.allowsEditing = YES;
    [self presentViewController:picker_library animated:YES completion:nil];

协议，代理都添加了，可是选完图片后压根不走-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info代理方法，这是为什么啊，急！！！！！