notif: On Android handle notification taps via Pigeon API

android/app/src/main/kotlin/com/zulip/flutter/MainActivity.kt

@@ -1,20 +1,27 @@
 package com.zulip.flutter
 
 import android.content.Intent
+import com.zulip.flutter.notifications.NotificationTapEventListener
+import com.zulip.flutter.notifications.NotificationTapEventsStreamHandler
 import io.flutter.embedding.android.FlutterActivity
 import io.flutter.embedding.engine.FlutterEngine
 
 class MainActivity : FlutterActivity() {
   private var androidIntentEventListener: AndroidIntentEventListener? = null
+  private var notificationTapEventListener: NotificationTapEventListener? = null
 
   override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
     super.configureFlutterEngine(flutterEngine)
 
     androidIntentEventListener = AndroidIntentEventListener()
     AndroidIntentEventsStreamHandler.register(
-      flutterEngine.dartExecutor.binaryMessenger,
-      androidIntentEventListener!!
+      flutterEngine.dartExecutor.binaryMessenger, androidIntentEventListener!!
     )
+    notificationTapEventListener = NotificationTapEventListener()
+    NotificationTapEventsStreamHandler.register(
+      flutterEngine.dartExecutor.binaryMessenger, notificationTapEventListener!!
+    )
+
     maybeHandleIntent(intent)
   }
 
@@ -35,6 +42,17 @@ class MainActivity : FlutterActivity() {
         return true
       }
 
+      Intent.ACTION_VIEW -> {
+        if (notificationTapEventListener!!.maybeHandleViewNotif(intent)) {
+          // Notification tapped
+          return true
+        }
+
+        // Let Flutter handle other intents, in particular the web-auth intents
+        // have ACTION_VIEW, scheme "zulip", and authority "login".
+        return false
+      }
+
       // For other intents, let Flutter handle it.
       else -> return false
     }

android/app/src/main/kotlin/com/zulip/flutter/notifications/NotificationTapEventListener.kt

@@ -0,0 +1,47 @@
+package com.zulip.flutter.notifications
+
+import android.content.Intent
+import android.net.Uri
+
+class NotificationTapEventListener : NotificationTapEventsStreamHandler() {
+  private var eventSink: PigeonEventSink<NotificationTapEvent>? = null
+  private val buffer = mutableListOf<NotificationTapEvent>()
+
+  override fun onListen(p0: Any?, sink: PigeonEventSink<NotificationTapEvent>) {
+    eventSink = sink
+    if (buffer.isNotEmpty()) {
+      buffer.forEach { sink.success(it) }
+      buffer.clear()
+    }
+  }
+
+  private fun onNotificationTapEvent(dataUrl: Uri) {
+    val event = AndroidNotificationTapEvent(dataUrl.toString())
+    if (eventSink != null) {
+      eventSink!!.success(event)
+    } else {
+      buffer.add(event)
+    }
+  }
+
+  /**
+   * Recognize if the ACTION_VIEW intent came from tapping a notification; handle it if so
+   *
+   * If the intent is recognized, sends a notification tap event via
+   * the Pigeon event stream to the Dart layer and returns true.
+   * Else does nothing and returns false.
+   *
+   * Do not call if `intent.action` is not ACTION_VIEW.
+   */
+  fun maybeHandleViewNotif(intent: Intent): Boolean {
+    assert(intent.action == Intent.ACTION_VIEW)
+
+    val url = intent.data
+    if (url?.scheme == "zulip" && url.authority == "notification") {
+      onNotificationTapEvent(url)
+      return true
+    }
+
+    return false
+  }
+}