title: Adding autolayout constraint programmatically turns uiview to black
body: <p>I need a view controller without xib. There should be a webview with fully filled to the view. For that I've added the following code to loadView method.</p>
  
  <pre><code>- (void)loadView {
      CGRect applicationFrame = [[UIScreen mainScreen] applicationFrame];
      UIView *view = [[UIView alloc] initWithFrame:applicationFrame];
  
      view.translatesAutoresizingMaskIntoConstraints = NO;
      [view setBackgroundColor:[UIColor greenColor]];
      [self setView:view];
  //    //create webview
      self.webview = [[UIWebView alloc] initWithFrame:CGRectZero];
      self.webview.translatesAutoresizingMaskIntoConstraints = NO;
      [view addSubview:self.webview];
      [self.webview setBackgroundColor:[UIColor orangeColor]];
      [self.webview setDelegate:self];
  
      NSDictionary *viewBindings = NSDictionaryOfVariableBindings(view,_webview);
  //    //add constraints
      [view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[_webview]|" options:0 metrics:nil views:viewBindings]];
      [view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[_webview]|" options:0 metrics:nil views:viewBindings]];
  
  }
  </code></pre>
  
  <p>But this turns the entire view to black. If I coment <code>[view addConstraints:...</code> method calls its showing a green view. What's wrong with my code?</p>