title: RxSwift observing changes in array
body: <p>Let say we have array of InvoiceDataModel</p>
  
  <pre><code>private let invoices Variable&lt;[InvoiceDataModel]&gt; = Variable([])
  
  class InvoiceDataModel { 
      let id: Variable&lt;Int&gt;
      var entity: Variable&lt;InvoiceDto&gt;
      var isSelected: Variable&lt;Bool&gt; 
  }
  </code></pre>
  
  <p>On tap on checkbox I am changing value of isSelected.
  What I want to achieve is to react on isSelect change to:</p>
  
  <ul>
  <li>calculate total amount of selected items (each entity has <code>var amount: Double</code>)</li>
  <li>detect if all items in collection are selected</li>
  </ul>
  
  <p>Is it possible to observe whole array and react on single property from element change?
  Not sure how am I supposed to achieve this.</p>
  
  <p>Probably my approach to this case is totally wrong. However I am not sure how am I supposed to operate here in a different way.  </p>