feat: display quickly even with many open windows (closes #171)

BREAKING CHANGE: the window thumbnails are now updated *after* the UI is shown. AltTab will first display its window, with the first 3 thumbnails up-to-date, then asynchronously update the rest of the thumbnails one-by-one. This improves the experience of users with lots of windows open.

src/api-wrappers/HelperExtensions.swift

@@ -41,7 +41,7 @@ extension NSView {
     }
 }
 
-extension Array {
+extension Collection {
     // forEach with each iteration run concurrently on the global queue
     func forEachAsync(fn: @escaping (Element) -> Void) {
         let group = DispatchGroup()

src/logic/Window.swift

@@ -42,6 +42,7 @@ class Window {
         self.position = position
         self.title = bestEffortTitle(axTitle)
         self.isTabbed = false
+        refreshThumbnail()
         debugPrint("Adding window", cgWindowId, title ?? "nil", application.runningApplication.bundleIdentifier ?? "nil")
         observeEvents()
     }

src/logic/Windows.swift

@@ -5,6 +5,8 @@ class Windows {
     static var list = [Window]()
     static var previousFocusedWindowIndex = Int(0)
     static var focusedWindowIndex = Int(0)
+    // the first few thumbnails are the most commonly looked at; we pay special attention to them
+    static let criticalFirstThumbnails = 3
 
     static func updateFocusedWindowIndex(_ newIndex: Int) {
         previousFocusedWindowIndex = focusedWindowIndex
@@ -84,11 +86,35 @@ class Windows {
         Windows.list = sortedTuples.map { $0.1 }
     }
 
-    static func refreshAllThumbnails() {
+    static func refreshFirstFewThumbnailsSync() {
         list.filter { $0.shouldShowTheUser }
+            .prefix(criticalFirstThumbnails)
             .forEachAsync { window in window.refreshThumbnail() }
     }
 
+    static func refreshThumbnailsAsync(_ screen: NSScreen, _ currentIndex: Int = criticalFirstThumbnails) {
+        guard App.app.appIsBeingUsed else { return }
+        BackgroundWork.mainQueueConcurrentWorkQueue.async {
+            if currentIndex < list.count {
+                let window = list[currentIndex]
+                if window.shouldShowTheUser {
+                    window.refreshThumbnail()
+                    DispatchQueue.main.async {
+                        let view = ThumbnailsView.recycledViews[currentIndex]
+                        if view.thumbnail.image != window.thumbnail {
+                            view.thumbnail.image = window.thumbnail
+                            let (thumbnailWidth, thumbnailHeight) = ThumbnailView.thumbnailSize(window.thumbnail, screen)
+                            let thumbnailSize = NSSize(width: thumbnailWidth.rounded(), height: thumbnailHeight.rounded())
+                            view.thumbnail.image?.size = thumbnailSize
+                            view.thumbnail.frame.size = thumbnailSize
+                        }
+                    }
+                }
+                refreshThumbnailsAsync(screen, currentIndex + 1)
+            }
+        }
+    }
+
     static func refreshWhichWindowsToShowTheUser(_ screen: NSScreen) {
         Windows.list.forEach { (window: Window) in
             refreshIfWindowShouldBeShownToTheUser(window, screen)

src/ui/App.swift

@@ -228,21 +228,23 @@ class App: AppCenterApplication, NSApplicationDelegate {
             Windows.updateFocusedWindowIndex(0)
             Windows.cycleFocusedWindowIndex(1)
             DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Preferences.windowDisplayDelay) { () -> () in
-                self.rebuildUi()
+                self.rebuildUi(screen)
             }
         } else {
             cycleSelection(.leading)
         }
     }
 
-    func rebuildUi() {
+    func rebuildUi(_ screen: NSScreen = Screen.preferred()) {
         guard appIsBeingUsed else { return }
-        Windows.refreshAllThumbnails()
+        Windows.refreshFirstFewThumbnailsSync()
         guard appIsBeingUsed else { return }
         Applications.refreshBadges()
         guard appIsBeingUsed else { return }
         refreshOpenUi()
         guard appIsBeingUsed else { return }
         thumbnailsPanel.show()
+        guard appIsBeingUsed else { return }
+        Windows.refreshThumbnailsAsync(screen)
     }
 }