title: How to set value without trigger KVO
body: <p>I use the following code to add KVO on object. </p>
  
  <pre><code>[self.model addObserver:self
               forKeyPath:@"userName"
                  options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
                  context:nil];
  </code></pre>
  
  <p>Now I want to set the userName like below. Yes, it will trigger KVO. </p>
  
  <pre><code>self.model.userName = @"testUser";
  </code></pre>
  
  <p>However, I want to set value without trigger KVO. How to do that? Are there any method like below allowing me to do that?</p>
  
  <pre><code>[self.model setValue:@"testUser" forKey:@"userName" isSilent:YES];
  </code></pre>