macOS: use NSDockTilePlugIn to update app icons

macos/DockTilePlugIn/DockTilePlugin.swift

@@ -0,0 +1,115 @@
+import AppKit
+
+/// This class lives as long as the app is in the Dock.
+/// If the user pins the app to the Dock, it will not be deallocated.
+/// Be careful when storing state in this class.
+class DockTilePlugin: NSObject, NSDockTilePlugIn {
+    private let pluginBundle = Bundle(for: DockTilePlugin.self)
+    #if DEBUG
+    private let ghosttyUserDefaults = UserDefaults(suiteName: "com.mitchellh.ghostty.debug")
+    #else
+    private let ghosttyUserDefaults = UserDefaults(suiteName: "com.mitchellh.ghostty")
+    #endif
+
+    private var iconChangeObserver: Any?
+
+    func setDockTile(_ dockTile: NSDockTile?) {
+        guard let dockTile, let ghosttyUserDefaults else {
+            iconChangeObserver = nil
+            return
+        }
+        // Try to restore the previous icon on launch.
+        iconDidChange(ghosttyUserDefaults.appIcon, dockTile: dockTile)
+
+        iconChangeObserver = DistributedNotificationCenter.default().publisher(for: Ghostty.Notification.ghosttyIconDidChange)
+            .map { [weak self] _ in
+                self?.ghosttyUserDefaults?.appIcon
+            }
+            .receive(on: DispatchQueue.global())
+            .sink { [weak self] newIcon in
+                guard let self else { return }
+                iconDidChange(newIcon, dockTile: dockTile)
+            }
+    }
+
+    func getGhosttyAppPath() -> String {
+        var url = pluginBundle.bundleURL
+        // Remove "/Contents/PlugIns/DockTilePlugIn.bundle" from the bundle URL to reach Ghostty.app.
+        while url.lastPathComponent != "Ghostty.app", !url.lastPathComponent.isEmpty {
+            url.deleteLastPathComponent()
+        }
+        return url.path
+    }
+
+    func iconDidChange(_ newIcon: Ghostty.CustomAppIcon?, dockTile: NSDockTile) {
+        guard let appIcon = newIcon?.image(in: pluginBundle) else {
+            resetIcon(dockTile: dockTile)
+            return
+        }
+        let appBundlePath = getGhosttyAppPath()
+        NSWorkspace.shared.setIcon(appIcon, forFile: appBundlePath)
+        NSWorkspace.shared.noteFileSystemChanged(appBundlePath)
+
+        dockTile.setIcon(appIcon)
+    }
+
+    func resetIcon(dockTile: NSDockTile) {
+        let appBundlePath = getGhosttyAppPath()
+        let appIcon: NSImage
+        if #available(macOS 26.0, *) {
+            // Reset to the default (glassy) icon.
+            NSWorkspace.shared.setIcon(nil, forFile: appBundlePath)
+            #if DEBUG
+            // Use the `Blueprint` icon to
+            // distinguish Debug from Release builds.
+            appIcon = pluginBundle.image(forResource: "BlueprintImage")!
+            #else
+            // Get the composed icon from the app bundle.
+            if let iconRep = NSWorkspace.shared.icon(forFile: appBundlePath).bestRepresentation(for: CGRect(origin: .zero, size: dockTile.size), context: nil, hints: nil) {
+                appIcon = NSImage(size: dockTile.size)
+                appIcon.addRepresentation(iconRep)
+            } else {
+                // If something unexpected happens on macOS 26,
+                // fall back to a bundled icon.
+                appIcon = pluginBundle.image(forResource: "AppIconImage")!
+            }
+            #endif
+        } else {
+            // Use the bundled icon to keep the corner radius
+            // consistent with other apps.
+            appIcon = pluginBundle.image(forResource: "AppIconImage")!
+            NSWorkspace.shared.setIcon(appIcon, forFile: appBundlePath)
+        }
+        NSWorkspace.shared.noteFileSystemChanged(appBundlePath)
+        dockTile.setIcon(appIcon)
+    }
+}
+
+private extension NSDockTile {
+    func setIcon(_ newIcon: NSImage) {
+        // Update the Dock tile on the main thread.
+        DispatchQueue.main.async {
+            let iconView = NSImageView(frame: CGRect(origin: .zero, size: self.size))
+            iconView.wantsLayer = true
+            iconView.image = newIcon
+            self.contentView = iconView
+            self.display()
+        }
+    }
+}
+
+extension NSDockTile: @unchecked @retroactive Sendable {}
+
+#if DEBUG
+private extension NSAlert {
+    static func notify(_ message: String, image: NSImage?) {
+        DispatchQueue.main.async {
+            let alert = NSAlert()
+            alert.messageText = message
+            alert.icon = image
+            _ = alert.runModal()
+        }
+    }
+}
+#endif
+

macos/Ghostty-Info.plist

@@ -2,6 +2,8 @@
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
+	<key>NSDockTilePlugIn</key>
+	<string>DockTilePlugin.plugin</string>
 	<key>CFBundleDocumentTypes</key>
 	<array>
 		<dict>