title: How to reload data in section in UICollectionView when using UICollectionViewDiffableDataSource?
body: <p>I am using <code>UICollectionViewDiffableDataSource</code> for my collectionView's data source. I have 3 sections in my CollectionView:</p>
  <pre class="lang-swift prettyprint-override"><code>enum Section {
      case section1
      case section2
      case section3
  }
  </code></pre>
  <p>Initially, I am appending these 3 sections to the collectionView using the following code:</p>
  <pre class="lang-swift prettyprint-override"><code>var snapshot = self.diffableDataSource.snapshot()
  snapshot.appendSections([.section1, .section2, .section3])
  self.diffableDataSource.apply(snapshot)
  </code></pre>
  <p>I then append items to the sections using the following code:</p>
  <pre class="lang-swift prettyprint-override"><code>var snapshot = self.diffableDataSource.snapshot()
  snapshot.appendItems([myItems], toSection: .section1)
  self.diffableDataSource.apply(snapshot)
  </code></pre>
  <p>My problem is that I cannot figure out how to reload a section in the collection view with a new set of items without appending them to the current ones. The methods available to the <code>snapshot</code> only allow for items to be appended to the section, but I need the items of the section to be replaced. I have tried deleting the section, appending it back then appending the new set of items:</p>
  <pre class="lang-swift prettyprint-override"><code>snapshot.deleteSections([.section1])
  snapshot.appendSections([.section1])
  snapshot.appendItems([myItems], toSection: .section1)
  </code></pre>
  <p>This just removes the section but does not load the new items. I am looking for a way to simply refresh the section with the new items, similarly to how you would call <code>collectionView.reloadData()</code> when using the normal UICollectionViewDataSource.</p>