Fix enqueue, resume and retry callback for Android.

README.md

@@ -273,6 +273,7 @@ void initState() {
 
 @override
 void dispose() {
+  FlutterDownloader.registerCallback(null);
 	IsolateNameServer.removePortNameMapping('downloader_send_port');
 	super.dispose();
 }
@@ -284,6 +285,7 @@ static void downloadCallback(String id, DownloadTaskStatus status, int progress)
 
 ````
 
+- Note: set `callback` as `null` to remove listener. You should clean up callback to prevent from leaking references.
 
 #### Load all tasks:
 

example/lib/main.dart

@@ -129,6 +129,7 @@ class _MyHomePageState extends State<MyHomePage> {
 
   @override
   void dispose() {
+    FlutterDownloader.registerCallback(null);
     _unbindBackgroundIsolate();
     super.dispose();
   }

lib/src/downloader.dart

@@ -383,13 +383,28 @@ class FlutterDownloader {
   ///
   /// {@end-tool}
   ///
-  static registerCallback(DownloadCallback callback) {
+  static registerCallback(DownloadCallback? callback) {
     assert(_initialized, 'FlutterDownloader.initialize() must be called first');
 
-    final callbackHandle = PluginUtilities.getCallbackHandle(callback)!;
-    assert(callbackHandle != null,
-        'callback must be a top-level or a static function');
-    _channel.invokeMethod(
-        'registerCallback', <dynamic>[callbackHandle.toRawHandle()]);
+    if (callback != null) {
+      // remove previous setting
+      _channel.setMethodCallHandler(null);
+      _channel.setMethodCallHandler((MethodCall call) async {
+        if (call.method == 'updateProgress') {
+          String id = call.arguments['task_id'];
+          int status = call.arguments['status'];
+          int process = call.arguments['progress'];
+          callback(id, DownloadTaskStatus(status), process);
+        }
+      });
+
+      final callbackHandle = PluginUtilities.getCallbackHandle(callback)!;
+      assert(callbackHandle != null,
+          'callback must be a top-level or a static function');
+      _channel.invokeMethod(
+          'registerCallback', <dynamic>[callbackHandle.toRawHandle()]);
+    } else {
+      _channel.setMethodCallHandler(null);
+    }
   }
 }