notif: Use the same notification ID for every notification

Matching the `requestCode` is no longer necessary due to the removal
of `pakcage:flutter_local_notifications`, which required it.

lib/notifications/display.dart

@@ -1,10 +1,8 @@
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:http/http.dart' as http;
 import 'package:collection/collection.dart';
-import 'package:crypto/crypto.dart';
 import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart' hide Notification;
@@ -170,9 +168,7 @@ class NotificationDisplayManager {
       }).buildUrl();
 
     await _androidHost.notify(
-      // TODO the notification ID can be constant, instead of matching requestCode
-      //   (This is a legacy of `flutter_local_notifications`.)
-      id: notificationIdAsHashOf(conversationKey),
+      id: kNotificationId,
       tag: conversationKey,
       channelId: NotificationChannelManager.kChannelId,
       groupKey: groupKey,
@@ -215,7 +211,7 @@ class NotificationDisplayManager {
     );
 
     await _androidHost.notify(
-      id: notificationIdAsHashOf(groupKey),
+      id: kNotificationId,
       tag: groupKey,
       channelId: NotificationChannelManager.kChannelId,
       groupKey: groupKey,
@@ -300,11 +296,18 @@ class NotificationDisplayManager {
       // Even though we enable the `autoCancel` flag for summary notification
       // during creation, the summary notification doesn't get auto canceled if
       // child notifications are canceled programatically as done above.
-      await _androidHost.cancel(
-        tag: groupKey, id: notificationIdAsHashOf(groupKey));
+      await _androidHost.cancel(tag: groupKey, id: kNotificationId);
     }
   }
 
+  /// The constant numeric "ID" we use for all non-test notifications,
+  /// along with unique tags.
+  ///
+  /// Because we construct a unique string "tag" for each distinct
+  /// notification, and Android notifications are identified by the
+  /// pair (tag, ID), it's simplest to leave these numeric IDs all the same.
+  static const kNotificationId = 0x00C0FFEE;
+
   /// A key we use in [Notification.extras] for the [Message.id] of the
   /// latest Zulip message in the notification's conversation.
   ///
@@ -313,21 +316,6 @@ class NotificationDisplayManager {
   @visibleForTesting
   static const kExtraLastZulipMessageId = 'lastZulipMessageId';
 
-  /// A notification ID, derived as a hash of the given string key.
-  ///
-  /// The result fits in 31 bits, the size of a nonnegative Java `int`,
-  /// so that it can be used as an Android notification ID.  (It's possible
-  /// negative values would work too, which would add one bit.)
-  ///
-  /// This is a cryptographic hash, meaning that collisions are about as
-  /// unlikely as one could hope for given the size of the hash.
-  @visibleForTesting
-  static int notificationIdAsHashOf(String key) {
-    final bytes = sha256.convert(utf8.encode(key)).bytes;
-    return bytes[0] | (bytes[1] << 8) | (bytes[2] << 16)
-      | ((bytes[3] & 0x7f) << 24);
-  }
-
   static String _conversationKey(MessageFcmMessage data, String groupKey) {
     final conversation = switch (data.recipient) {
       FcmMessageChannelRecipient(:var streamId, :var topic) => 'stream:$streamId:$topic',

test/notifications/display_test.dart

@@ -223,8 +223,6 @@ void main() {
 
       final expectedTag = '${data.realmUrl}|${data.userId}|$expectedTagComponent';
       final expectedGroupKey = '${data.realmUrl}|${data.userId}';
-      final expectedId =
-        NotificationDisplayManager.notificationIdAsHashOf(expectedTag);
       const expectedPendingIntentFlags = PendingIntentFlag.immutable;
       const expectedIntentFlags = IntentFlag.activityClearTop | IntentFlag.activityNewTask;
       final expectedSelfUserKey = '${data.realmUrl}|${data.userId}';
@@ -256,7 +254,7 @@ void main() {
       check(testBinding.androidNotificationHost.takeNotifyCalls())
         .deepEquals(<Condition<Object?>>[
           (it) => it.isA<AndroidNotificationHostApiNotifyCall>()
-            ..id.equals(expectedId)
+            ..id.equals(NotificationDisplayManager.kNotificationId)
             ..tag.equals(expectedTag)
             ..channelId.equals(NotificationChannelManager.kChannelId)
             ..contentTitle.isNull()
@@ -288,7 +286,7 @@ void main() {
                 ..dataUrl.equals(expectedIntentDataUrl.toString())
                 ..flags.equals(expectedIntentFlags))),
           (it) => it.isA<AndroidNotificationHostApiNotifyCall>()
-            ..id.equals(NotificationDisplayManager.notificationIdAsHashOf(expectedGroupKey))
+            ..id.equals(NotificationDisplayManager.kNotificationId)
             ..tag.equals(expectedGroupKey)
             ..channelId.equals(NotificationChannelManager.kChannelId)
             ..contentTitle.isNull()
@@ -344,7 +342,7 @@ void main() {
       final expectedGroupKey = '${data.realmUrl}|${data.userId}';
       final expectedTag = '$expectedGroupKey|$tagComponent';
       return (it) => it.isA<StatusBarNotification>()
-        ..id.equals(NotificationDisplayManager.notificationIdAsHashOf(expectedTag))
+        ..id.equals(NotificationDisplayManager.kNotificationId)
         ..notification.which((it) => it
           ..group.equals(expectedGroupKey)
           ..extras.deepEquals(<String, String>{
@@ -355,7 +353,7 @@ void main() {
 
     Condition<Object?> conditionSummaryActiveNotif(String expectedGroupKey) {
       return (it) => it.isA<StatusBarNotification>()
-        ..id.equals(NotificationDisplayManager.notificationIdAsHashOf(expectedGroupKey))
+        ..id.equals(NotificationDisplayManager.kNotificationId)
         ..notification.which((it) => it
           ..group.equals(expectedGroupKey)
           ..extras.isEmpty())