title: MVVM and where to keep displayed fields/properties
body: <p>I am learning MVVM by building a simple WPF calculator app.</p>
  
  <p>In this simple app, there is one view (Main window), one ViewModel, and one Model (the calculation engine).</p>
  
  <p>My calculator displays the current value of all previous operations, and the operations that got to that value.</p>
  
  <p>For example a display might look like this:</p>
  
  <p>1 * 2 * 3</p>
  
  <pre><code>    6
  </code></pre>
  
  <p>I am wondering if these should be saved as Properties on the ViewModel, Model, or both?</p>
  
  <p>More general -- should the ViewModel contain only properties that exist on the Model, or this is not mandatory?</p>
  
  <p>In this case i would save a CurrentValue and DisplayValue properties, just wondering which architecture layer should these belong to.</p>