这是我的代码,它在第四行,我得到错误：

- (IBAction)takePicture:(id)sender {
    UIImagePickerController *imagePicker = [[UIImagePickerController alloc] init];
    imagePicker.sourceType =  UIImagePickerControllerSourceTypeCamera;
    imagePicker.delegate = self;
    [self presentModalViewController:imagePicker animated:YES];
}

我收到以下警告：

Assigning to 'id<UINavigationControllerDelegate,UIImagePickerControllerDelegate>' from incompatible type 'todd_learningViewController *const __strong'

我不明白警告信息,不知道如何摆脱警告.

谢谢,

编辑：

我的头文件类似如下：

@interface todd_learningViewController : UIViewController <UIApplicationDelegate>

  - (IBAction)takePicture:(id)sender;

  @property (weak,nonatomic) IBOutlet UIImageView *pictureView;

  @end

我该怎么修改呢？