title: How do you only show a shadow outside of the fill of a UIView?
body: <p>I have a UIView with a translucent fill and a drop shadow. Since the fill is translucent, I can see the shadow behind the fill.  </p>
  
  <pre><code>- (id)init
  {
      self = [super init];
      if (self) {
          self.backgroundColor = [UIColor colorWithWhite:1.0 alpha:0.8];
          self.layer.shadowColor = [UIColor blackColor].CGColor;
          self.layer.shadowOffset = CGSizeMake(0.0, 0.0);
          self.layer.shadowOpacity = 0.5;
          self.layer.shadowRadius = 2.0;
          self.layer.cornerRadius = 3.0;
      }
      return self;
  }
  </code></pre>
  
  <p>I do not like this behavior. I cannot see anything behind the view because the fill's opacity + the shadow's opacity > 100%. How do I make it like CSS where the shadow is only drawn outside of a box's border?</p>
  
  <pre><code>.someStyle {
     background: white;
     opacity: 0.8;
     box-shadow: 0 0 1em rgba(0,0,0,0.5);
  }
  </code></pre>