title: How to create MutableProperty<T> from MutableProperty<U> in Swift ReactiveCocoa 4
body: <p>Is there any easy way to create MutableProperty from MutableProperty in Swift ReactiveCocoa 4?</p>
  
  <p>I have a case that, and I want an way to create classB with classA, in which I need to setup statusB with statusA, but how to do this? </p>
  
  <pre><code>class ClassA {
    var statusA = MutableProperty&lt;T&gt;
  }
  
  class ClassB {
      var statusB = MutableProperty&lt;U&gt;
  
      func getStatusB(from StatusA: T) -&gt; U {
          // .. assume this is implemented.
      }
  
      init(statusB: U) {
          //...
      }
  
      convenience init(from classA: ClassA) {
          self.statusB = // here how to setup this value from classA's statusA with getStatusB(from:)?
      }
  }
  </code></pre>