Offload Task JSON encoding and decoding to isolate

Moved JSON serialization and deserialization for `Task` and `TaskStatusUpdate` objects to a background isolate using `compute`. This change impacts `NativeDownloader`'s `initialize`, `allTasks`, and `initCallbackDispatcher` methods to prevent blocking the main thread during heavy JSON operations.

- Added static helper methods to `NativeDownloader` for isolate-safe JSON handling.
- Updated `methodCallHandler` and `allTasks` to use asynchronous `compute` calls.
- Updated `initCallbackDispatcher` to decode arguments and encode return values using `compute`.

Author: google-labs-jules[bot]

lib/src/native_downloader.dart

@@ -40,7 +40,7 @@ abstract base class NativeDownloader extends BaseDownloader {
       final args = call.arguments as List<dynamic>;
       var taskJsonString = args.first as String;
       final task = taskJsonString.isNotEmpty
-          ? Task.createFromJson(jsonDecode(taskJsonString))
+          ? await compute(_taskFromJson, taskJsonString)
           : DownloadTask(url: 'url');
       final message = (
         call.method,
@@ -184,8 +184,7 @@ abstract base class NativeDownloader extends BaseDownloader {
 
         // from ParallelDownloadTask
         case ('enqueueChild', String childTaskJsonString):
-          final childTask =
-              Task.createFromJson(jsonDecode(childTaskJsonString));
+          final childTask = await compute(_taskFromJson, childTaskJsonString);
           Future.delayed(const Duration(milliseconds: 100))
               .then((_) => FileDownloader().enqueue(childTask));
 
@@ -197,12 +196,8 @@ abstract base class NativeDownloader extends BaseDownloader {
 
         // from ParallelDownloadTask
         case ('pauseTasks', String listOfTasksJson):
-          final listOfTasks = List<DownloadTask>.from(jsonDecode(
-              listOfTasksJson,
-              reviver: (key, value) => switch (key) {
-                    int _ => Task.createFromJson(value as Map<String, dynamic>),
-                    _ => value
-                  }));
+          final listOfTasks =
+              await compute(_downloadTaskListFromJson, listOfTasksJson);
           Future.delayed(const Duration(milliseconds: 100)).then((_) async {
             for (final chunkTask in listOfTasks) {
               await FileDownloader().pause(chunkTask);
@@ -283,9 +278,7 @@ abstract base class NativeDownloader extends BaseDownloader {
     final result = await methodChannel.invokeMethod<List<dynamic>?>(
             'allTasks', allGroups ? null : group) ??
         [];
-    final tasks = result
-        .map((e) => Task.createFromJson(jsonDecode(e as String)))
-        .toList();
+    final tasks = await compute(_taskListFromListStrings, result);
     return [...retryAndPausedTasks, ...tasks];
   }
 
@@ -516,6 +509,46 @@ abstract base class NativeDownloader extends BaseDownloader {
     }
     return (configItem.$1, ''); // normal result
   }
+
+  /// Helper to create a Task from a JSON string, for use with [compute]
+  static Task _taskFromJson(String jsonString) {
+    return Task.createFromJson(jsonDecode(jsonString));
+  }
+
+  /// Helper to create a list of DownloadTasks from a JSON string, for use with [compute]
+  ///
+  /// Used for the 'pauseTasks' message
+  static List<DownloadTask> _downloadTaskListFromJson(String jsonString) {
+    return List<DownloadTask>.from(jsonDecode(jsonString,
+        reviver: (key, value) => switch (key) {
+              int _ => Task.createFromJson(value as Map<String, dynamic>),
+              _ => value
+            }));
+  }
+
+  /// Helper to create a list of Tasks from a list of JSON strings, for use with [compute]
+  ///
+  /// Used for the 'allTasks' message
+  static List<Task> _taskListFromListStrings(List<dynamic> jsonStrings) {
+    return jsonStrings
+        .map((e) => Task.createFromJson(jsonDecode(e as String)))
+        .toList();
+  }
+
+  /// Helper to create a TaskStatusUpdate from a JSON string, for use with [compute]
+  static TaskStatusUpdate _taskStatusUpdateFromJson(String jsonString) {
+    return TaskStatusUpdate.fromJsonString(jsonString);
+  }
+
+  /// Helper to encode a Task to a JSON string, for use with [compute]
+  static String _taskToJsonString(Task task) {
+    return jsonEncode(task.toJson());
+  }
+
+  /// Helper to encode a TaskStatusUpdate to a JSON string, for use with [compute]
+  static String _taskStatusUpdateToJsonString(TaskStatusUpdate update) {
+    return jsonEncode(update.toJson());
+  }
 }
 
 /// Android native downloader
@@ -733,31 +766,34 @@ void initCallbackDispatcher() {
     switch (call.method) {
       case 'beforeTaskStartCallback':
         final taskJsonString = call.arguments as String;
-        final task = Task.createFromJson(jsonDecode(taskJsonString));
+        final task =
+            await compute(NativeDownloader._taskFromJson, taskJsonString);
         final callBack = task.options?.beforeTaskStartCallBack;
         final taskStatusUpdate = await callBack?.call(task);
         if (taskStatusUpdate == null) {
           return null;
         }
-        return jsonEncode(taskStatusUpdate.toJson());
+        return await compute(
+            NativeDownloader._taskStatusUpdateToJsonString, taskStatusUpdate);
 
       case 'onTaskStartCallback':
       case 'onAuthCallback':
         final taskJsonString = call.arguments as String;
-        final task = Task.createFromJson(jsonDecode(taskJsonString));
+        final task =
+            await compute(NativeDownloader._taskFromJson, taskJsonString);
         final callBack = call.method == 'onTaskStartCallback'
             ? task.options?.onTaskStartCallBack
             : task.options?.auth?.onAuthCallback;
         final newTask = await callBack?.call(task);
         if (newTask == null) {
           return null;
         }
-        return jsonEncode(newTask.toJson());
+        return await compute(NativeDownloader._taskToJsonString, newTask);
 
       case 'onTaskFinishedCallback':
         final taskUpdateJsonString = call.arguments as String;
-        final taskStatusUpdate =
-            TaskStatusUpdate.fromJsonString(taskUpdateJsonString);
+        final taskStatusUpdate = await compute(
+            NativeDownloader._taskStatusUpdateFromJson, taskUpdateJsonString);
         final callBack = taskStatusUpdate.task.options?.onTaskFinishedCallBack;
         await callBack?.call(taskStatusUpdate);
         return null;