Update Focus after 0.5 seconds

Author: toshi0383

AutoScrollCollectionViewController/ViewController.swift

@@ -6,6 +6,7 @@
 //  Copyright © 2016 toshi0383. All rights reserved.
 //
 
+import Async
 import UIKit
 
 class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource {
@@ -15,14 +16,39 @@ class ViewController: UIViewController, UICollectionViewDelegate, UICollectionVi
             collectionView.dataSource = self
         }
     }
+    private var focusUpdateTask: Async?
+    private var _currentIndex: Int = 0
+    private func updateCurrentIndex(_ index: Int) {
+        _currentIndex = index
+        collectionView.scrollToItem(at: currentIndexPath, at: .centeredVertically, animated: true)
+        // This is also okay.
+        //let point = CGPoint(x: 0, y: 290 + 540*index)
+        //collectionView.setContentOffset(point, animated: true)
+        updateFocusWithDelay()
+    }
+    func updateFocusWithDelay() {
+        focusUpdateTask?.cancel()
+        focusUpdateTask = Async.main(after: 0.5) {
+            self.setNeedsFocusUpdate()
+        }
+    }
+    private var currentIndexPath: IndexPath {
+        return IndexPath(item: _currentIndex, section: 0)
+    }
+
     override func viewDidLoad() {
         super.viewDidLoad()
     }
 
     override func viewDidAppear(_ animated: Bool) {
         super.viewDidAppear(animated)
-        let indexPath = IndexPath(item: 5, section: 0)
-        collectionView.scrollToItem(at: indexPath, at: .centeredVertically, animated: true)
+        updateCurrentIndex(5)
+    }
+
+    // MARK: UIFocusEnvironment
+    override var preferredFocusEnvironments: [UIFocusEnvironment] {
+        let cell = collectionView?.cellForItem(at: currentIndexPath)
+        return [cell].flatMap{$0}
     }
 
     // MARK: Delegate & DataSource
@@ -36,6 +62,9 @@ class ViewController: UIViewController, UICollectionViewDelegate, UICollectionVi
     func numberOfSections(in collectionView: UICollectionView) -> Int {
         return 1
     }
+    func collectionView(_ collectionView: UICollectionView, didUpdateFocusIn context: UICollectionViewFocusUpdateContext, with coordinator: UIFocusAnimationCoordinator) {
+        _currentIndex = context.nextFocusedIndexPath?.row ?? 0
+    }
 }
 
 class Cell: UICollectionViewCell {