VoiceOver: improve description of gap cell, add actions to specify direction

Tusker/Screens/Timeline/TimelineGapCollectionViewCell.swift

@@ -15,6 +15,8 @@ class TimelineGapCollectionViewCell: UICollectionViewCell {
     private let indicator = UIActivityIndicatorView(style: .medium)
     private let chevronView = AnimatingChevronView()
     
+    var fillGap: ((TimelineGapDirection) async -> Void)?
+    
     override var isHighlighted: Bool {
         didSet {
             backgroundColor = isHighlighted ? .systemFill : .systemGroupedBackground
@@ -100,6 +102,36 @@ class TimelineGapCollectionViewCell: UICollectionViewCell {
         }
     }
     
+    // MARK: Accessibility
+    
+    override var isAccessibilityElement: Bool {
+        get { true }
+        set {}
+    }
+    
+    override var accessibilityLabel: String? {
+        get {
+            "Load \(direction.accessibilityLabel)"
+        }
+        set {}
+    }
+    
+    override var accessibilityCustomActions: [UIAccessibilityCustomAction]? {
+        get {
+            [TimelineGapDirection.below, .above].map { dir in
+                UIAccessibilityCustomAction(name: "Load \(dir.accessibilityLabel)") { [unowned self] _ in
+                    Task {
+                        showsIndicator = true
+                        await fillGap?(dir)
+                        showsIndicator = false
+                    }
+                    return true
+                }
+            }
+        }
+        set {}
+    }
+    
 }
 
 private class AnimatingChevronView: UIView {

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -137,8 +137,11 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         }
         let zeroHeightCell = UICollectionView.CellRegistration<ZeroHeightCollectionViewCell, Void> { _, _, _ in
         }
-        let gapCell = UICollectionView.CellRegistration<TimelineGapCollectionViewCell, Void> { cell, indexPath, _ in
+        let gapCell = UICollectionView.CellRegistration<TimelineGapCollectionViewCell, Void> { [unowned self] cell, indexPath, _ in
             cell.showsIndicator = false
+            cell.fillGap = { [unowned self] direction in
+                await self.controller.fillGap(in: direction)
+            }
         }
         let timelineDescriptionCell = UICollectionView.CellRegistration<PublicTimelineDescriptionCollectionViewCell, Item> { [unowned self] cell, indexPath, item in
             guard case .public(let local) = timeline else {

Tusker/TimelineLikeController.swift

@@ -379,4 +379,13 @@ enum TimelineGapDirection {
     case below
     /// Fill in above the gap. I.e., statuses that are immediately older than the status above the gap.
     case above
+    
+    var accessibilityLabel: String {
+        switch self {
+        case .below:
+            return "Newer"
+        case .above:
+            return "Older"
+        }
+    }
 }