I wrote a application that take pictures from PhotoLibray. When I run the app, it succeed, and then I select a picture from the library, and I set the allowEditing property to YES, but when the edit screen shows, no edit tool show, I cannot edit the picture. I used xcode6, the class is UIImagePickerController. Code is below,

```
- (IBAction)takePicture:(id)sender
{
    UIImagePickerController *imagePicker = [[UIImagePickerController alloc] init];

    // If the device ahs a camera, take a picture, otherwise,
    // just pick from the photo library
    if ([UIImagePickerController isSourceTypeAvailable:UIImagePickerControllerSourceTypeCamera]) {
        imagePicker.sourceType = UIImagePickerControllerSourceTypeCamera;
    } else {
        imagePicker.allowsEditing = YES;
        imagePicker.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
    }

    imagePicker.delegate = self;

    // Place image picker on the screen
    [self presentViewController:imagePicker animated:YES completion:nil];
}

```

When the edit screen shows, there are 2 buttons, one is "Cancel" and the other is "choose". I press the left mouse button and drag it, but nothing shows, I can only move the picture and when I release the left mouse, the picture return back to the old place.  

Why I cannot edit or crop the picture?