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！