title: Vue change object in array and trigger reactivity
body: <p>How can I trigger an update when altering part of an object found by index in an array?</p>
  
  <p>The docs show how to alter the value of an array:</p>
  
  <pre class="lang-js prettyprint-override"><code>Vue.set(example1.items, indexOfItem, newValue)
  </code></pre>
  
  <p>or</p>
  
  <pre class="lang-js prettyprint-override"><code>example1.items.splice(indexOfItem, 1, newValue)
  </code></pre>
  
  <p>But how to alter the value of a property on an object in an array without changing the rest of the object?</p>
  
  <p>The following works to update the property, but Vue doesn't react to the change until something else triggers an update.</p>
  
  <pre class="lang-js prettyprint-override"><code>example1.items[indexOfItem].some_object_property = false
  </code></pre>