fix(push): Incorrect handling of simple alert (#3502)

Fixes #3435.

Simple alerts on iOS take the shape (see [docs](https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CreatingtheNotificationPayload.html)):

```json
{
  "aps": {
    "alert": "Message"
   }
}
```

This case was not being handled in the deserializer (only the case where `alert` was a Map).

packages/amplify_core/lib/src/types/notifications/push/push_notification_message.dart

@@ -21,57 +21,74 @@ class PushNotificationMessage
   });
 
   factory PushNotificationMessage.fromJson(Map<Object?, Object?> json) {
-    String? title;
-    String? body;
-    String? imageUrl;
-    String? deeplinkUrl;
-    String? goToUrl;
-    ApnsPlatformOptions? apnsOptions;
-    FcmPlatformOptions? fcmOptions;
-
-    final data = (json['data'] as Map<Object?, Object?>?) ?? {};
-    final aps = json['aps'] as Map<Object?, Object?>?;
-    if (aps != null) {
-      final alert = aps['alert'] as Map<Object?, Object?>?;
-      if (alert != null) {
-        title = alert['title'] as String?;
-        body = alert['body'] as String?;
-        imageUrl = data['media-url'] as String?;
-
-        if (data['pinpoint'] != null) {
-          final pinpointData = data['pinpoint'] as Map<Object?, Object?>;
-          deeplinkUrl = pinpointData['deeplink'] as String?;
-          goToUrl = deeplinkUrl;
-        }
-        apnsOptions = ApnsPlatformOptions(subtitle: aps['subtitle'] as String?);
-      }
-    } else {
-      final action = json['action'] as Map<Object?, Object?>?;
-      title = json['title'] as String?;
-      body = json['body'] as String?;
-      imageUrl = json['imageUrl'] as String?;
-      if (action != null) {
-        deeplinkUrl = action['deeplink'] as String?;
-        goToUrl = action['url'] as String?;
-      }
-      final fcmOptionsMap = json['fcmOptions'] as Map<Object?, Object?>?;
-      if (fcmOptionsMap != null) {
-        fcmOptions = FcmPlatformOptions(
-          channelId: fcmOptionsMap['channelId'] as String?,
-          messageId: fcmOptionsMap['messageId'] as String?,
+    final data = (json['data'] as Map<String, Object?>?) ?? const {};
+    switch (json) {
+      // `aps` dictionary references:
+      // - https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification
+      // - https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CreatingtheNotificationPayload.html
+      case {'aps': final Map<String, Object?> aps}:
+        final title = switch (aps) {
+          {'alert': final String alert} => alert,
+          {'alert': {'title': final String title}} => title,
+          _ => null,
+        };
+        final body = switch (aps) {
+          {'alert': {'body': final String body}} => body,
+          _ => null,
+        };
+        final imageUrl = data['media-url'] as String?;
+        final deeplinkUrl = switch (data) {
+          {'pinpoint': {'deeplink': final String deeplink}} => deeplink,
+          _ => null,
+        };
+        final apnsOptions = ApnsPlatformOptions(
+          subtitle: switch (aps) {
+            {'alert': {'subtitle': final String subtitle}} => subtitle,
+            _ => null,
+          },
+        );
+        return PushNotificationMessage(
+          title: title,
+          body: body,
+          imageUrl: imageUrl,
+          deeplinkUrl: deeplinkUrl,
+          goToUrl: deeplinkUrl,
+          apnsOptions: apnsOptions,
+          data: data,
+        );
+      default:
+        final title = json['title'] as String?;
+        final body = json['body'] as String?;
+        final imageUrl = json['imageUrl'] as String?;
+        final (deeplinkUrl, goToUrl) = switch (json['action']) {
+          {'deeplink': final String deeplink, 'url': final String url} => (
+              deeplink,
+              url
+            ),
+          {'deeplink': final String deeplink} => (deeplink, null),
+          {'url': final String url} => (null, url),
+          _ => (null, null),
+        };
+        final fcmOptions = FcmPlatformOptions(
+          channelId: switch (json['fcmOptions']) {
+            {'channelId': final String channelId} => channelId,
+            _ => null,
+          },
+          messageId: switch (json['fcmOptions']) {
+            {'messageId': final String messageId} => messageId,
+            _ => null,
+          },
+        );
+        return PushNotificationMessage(
+          title: title,
+          body: body,
+          imageUrl: imageUrl,
+          deeplinkUrl: deeplinkUrl,
+          goToUrl: goToUrl,
+          fcmOptions: fcmOptions,
+          data: data,
         );
-      }
     }
-    return PushNotificationMessage(
-      title: title,
-      body: body,
-      imageUrl: imageUrl,
-      deeplinkUrl: deeplinkUrl,
-      goToUrl: goToUrl,
-      apnsOptions: apnsOptions,
-      fcmOptions: fcmOptions,
-      data: data,
-    );
   }
 
   final String? title;
@@ -81,7 +98,7 @@ class PushNotificationMessage
   final String? goToUrl;
   final FcmPlatformOptions? fcmOptions;
   final ApnsPlatformOptions? apnsOptions;
-  final Map<Object?, Object?> data;
+  final Map<String, Object?> data;
 
   @override
   String get runtimeTypeName => 'PushNotificationMessage';

packages/notifications/push/amplify_push_notifications/test/push_notification_message_test.dart

@@ -20,6 +20,10 @@ void main() {
         'PINPOINT.NOTIFICATION',
       );
 
+      final parsedSimpleiOSMessage =
+          PushNotificationMessage.fromJson(simpleAlertiOSMessage);
+      expect(parsedSimpleiOSMessage.title, 'Hello, world');
+
       final parsediOSMessage =
           PushNotificationMessage.fromJson(standardiOSMessage);
       expect(parsediOSMessage.title, 'TITTLE');

packages/notifications/push/amplify_push_notifications/test/test_data/fake_notification_messges.dart

@@ -60,6 +60,12 @@ const standardiOSMessage = {
   },
 };
 
+const simpleAlertiOSMessage = {
+  'aps': {
+    'alert': 'Hello, world',
+  },
+};
+
 const imageUrliOSMessage = {
   'data': {
     'media-url': 'TEST_URL',