title: How to change a UIView zPosition?
body: <p>I dont understand how to change the zPosition of a view, i try this but nothing happens :</p>
  
  <pre><code>- (void)viewDidLoad {
      [super viewDidLoad];
  
      UIView *view = [[[UIView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)] autorelease];
      view.backgroundColor = [UIColor blackColor];
      view.layer.zPosition = -400;
      [self.view addSubview:view];
  }
  </code></pre>
  
  <p>In the CALayer doc it doesn't say anything should be done before using this property. Does anyone knows what i'm missing ?</p>
  
  <p>Thanks,
  Vincent.</p>