title: How do I (successfully) set a maskView to a UIView?
body: <p>I'm making a camera-based app and would like to make a frosted overlay on the camera preview, with a rounded rectangle cut out from the frosted view.</p>
  
  <p>I've been trying to achieve this with the maskLayer of UIView, introduced with iOS 8 and have been wildly unsuccessful.</p>
  
  <p>Here's what my code currently looks like:</p>
  
  <pre><code>// Blur the preview
  UIBlurEffect *effect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleLight];
  UIView *blurView = [[UIVisualEffectView alloc] initWithEffect:effect];
  blurView.translatesAutoresizingMaskIntoConstraints = NO;
  [self.view insertSubview:blurView aboveSubview:imagePicker.view];
  
  // Make blur view fill screen
  {
      [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-0-[blurView]-0-|"
                                                                        options:0
                                                                        metrics:nil
                                                                          views:NSDictionaryOfVariableBindings(blurView)]];
      [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-0-[blurView]-0-|"
                                                                        options:0
                                                                        metrics:nil
                                                                          views:NSDictionaryOfVariableBindings(blurView)]];
  }
  
  // Add a rounded of transparency to the blurView  **not working**
  UIView *mask = [[UIView alloc] initWithFrame:CGRectMake(50.f, 50.f, 50.f, 50.f)];
  mask.alpha = 1.0f;
  mask.backgroundColor = [UIColor redColor];
  [blurView setMaskView:mask]; // Remove this line, and everything is frosted.  Keeping this line and nothing is frosted. 
  </code></pre>
  
  <p>As my comments indicate, adding the mask removes the frosted view completely (changing the opacity of the mask does nothing).  I know that this won't create the effect I'm looking for (it should only be frosted in that rectangle), but I can't get the maskView to work in any capacity.</p>