notif ios: Handle opening of conversation on tap

ios/Runner/AppDelegate.swift

@@ -1,5 +1,6 @@
 import UIKit
 import Flutter
+import UserNotifications
 
 @main
 @objc class AppDelegate: FlutterAppDelegate {
@@ -8,6 +9,137 @@ import Flutter
     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
   ) -> Bool {
     GeneratedPluginRegistrant.register(with: self)
-    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
+    UNUserNotificationCenter.current().delegate = self
+
+    // Handle launch of application from the notification.
+    let controller = window.rootViewController as! FlutterViewController
+    if let remoteNotification = launchOptions?[.remoteNotification] as? [AnyHashable : Any],
+       let payload = ApnsPayload(fromJson: remoteNotification),
+       let routeUrl = internalRouteUrlFromNotification(payload: payload) {
+      // https://api.flutter.dev/flutter/dart-ui/PlatformDispatcher/defaultRouteName.html
+      // TODO(?) FlutterViewController.setInitialRoute is deprecated (warning visible in Xcode)
+      //   but the above doc mentions using it.
+      controller.setInitialRoute(routeUrl.absoluteString)
+    }
+
+    return super.application(
+      application,
+      didFinishLaunchingWithOptions: launchOptions
+    )
+  }
+
+  // Allow only `zulip://login` external urls.
+  override func application(_ application: UIApplication, open url: URL, options: [UIApplication.OpenURLOptionsKey : Any] = [:]) -> Bool {
+     if url.scheme == "zulip" && url.host == "login" {
+       return super.application(application, open: url, options: options)
+     }
+     return false
+   }
+
+  // Handle notification tap while the app is running.
+  override func userNotificationCenter(
+    _ center: UNUserNotificationCenter,
+    didReceive response: UNNotificationResponse,
+    withCompletionHandler completionHandler: @escaping () -> Void
+  ) {
+    if let payload = ApnsPayload(fromJson: response.notification.request.content.userInfo),
+       let routeUrl = internalRouteUrlFromNotification(payload: payload) {
+       let controller = window.rootViewController as! FlutterViewController
+      controller.pushRoute(routeUrl.absoluteString)
+      completionHandler()
+    }
+  }
+}
+
+// https://github.com/zulip/zulip/blob/aa8f47774f08b6fc5d947ae97cafefcf7dfb8bef/zerver/lib/push_notifications.py#L1087
+struct ApnsPayload {
+  enum Narrow {
+    case topicNarrow(channelId: Int, topic: String)
+    case dmNarrow(allRecipientIds: [Int])
+  }
+
+  let realmUrl: String
+  let userId: Int
+  let narrow: Narrow
+
+  init?(fromJson payload: [AnyHashable : Any]) {
+    guard let aps = payload["aps"] as? [String : Any],
+          let customData = aps["custom"] as? [String : Any],
+          let zulipData = customData["zulip"] as? [String : Any],
+          let realmUrl = (
+            zulipData["realm_url"] as? String ?? zulipData["realm_url"] as? String
+          ),
+          let userId = zulipData["user_id"] as? Int,
+          let senderId = zulipData["sender_id"] as? Int,
+          let recipientType = zulipData["recipient_type"] as? String
+    else {
+      return nil
+    }
+
+    var narrow: Narrow
+    switch recipientType {
+    case "stream":
+      guard let streamId = zulipData["stream_id"] as? Int,
+            let topic = zulipData["topic"] as? String else {
+        return nil
+      }
+
+      narrow = Narrow.topicNarrow(channelId: streamId, topic: topic)
+
+    case "private":
+      var allRecipientIds = Set<Int>()
+
+      if let pmUsersStr = zulipData["pm_users"] as? String {
+        for str in pmUsersStr.split(separator: ",") {
+          guard let recipientId = Int(str, radix: 10) else {
+            return nil
+          }
+          allRecipientIds.insert(recipientId)
+        }
+      } else {
+        allRecipientIds.formUnion([senderId, userId])
+      }
+
+      narrow = Narrow.dmNarrow(allRecipientIds: allRecipientIds.sorted(by: <))
+
+    default:
+      return nil
+    }
+
+    self.realmUrl = realmUrl
+    self.userId = userId
+    self.narrow = narrow
+  }
+}
+
+func internalRouteUrlFromNotification(payload: ApnsPayload) -> URL? {
+  var components = URLComponents()
+  components.scheme = "zulip"
+  components.host = "notification"
+
+  var queryItems = [
+    URLQueryItem(name: "realm_url", value: payload.realmUrl),
+    URLQueryItem(name: "user_id", value: String(payload.userId)),
+  ]
+
+  switch payload.narrow {
+  case .topicNarrow(channelId: let channelId, topic: let topic):
+    queryItems.append(contentsOf: [
+      URLQueryItem(name: "narrow_type", value: "topic"),
+      URLQueryItem(name: "channel_id", value: String(channelId)),
+      URLQueryItem(name: "topic", value: topic),
+    ])
+
+  case .dmNarrow(allRecipientIds: let allRecipientIds):
+    queryItems.append(
+      contentsOf: [
+        URLQueryItem(name: "narrow_type", value: "dm"),
+        URLQueryItem(
+          name: "all_recipient_ids",
+          value: allRecipientIds.map{ String($0) }.joined(separator: ",")),
+      ]
+    )
   }
+  components.queryItems = queryItems
+  return components.url
 }