Fix bug #114.

android/src/main/kotlin/com/bruno/native_admob_flutter/banner/Controller.kt

@@ -26,6 +26,8 @@ class BannerAdController(
             "loadAd" -> {
                 channel.invokeMethod("loading", null)
                 loadRequested?.let { it(result) }
+                // Actually, no ads will be loaded here, so return false immediately
+                result.success(false)
             }
             else -> result.notImplemented()
         }

lib/src/banner/banner_ad_widget.dart

@@ -193,6 +193,8 @@ class _BannerAdState extends State<BannerAd>
   BannerAdOptions get options => widget.options;
   StreamSubscription? _onEventSub;
 
+  late Future _initFuture;
+
   @override
   void initState() {
     super.initState();
@@ -215,6 +217,8 @@ class _BannerAdState extends State<BannerAd>
       }
     });
     if (!controller.isLoaded) controller.load(timeout: widget.loadTimeout);
+
+    _initFuture = controller.waitForInitFinish();
   }
 
   @override
@@ -257,6 +261,18 @@ class _BannerAdState extends State<BannerAd>
     assertPlatformIsSupported();
     assertVersionIsSupported();
 
+    return FutureBuilder<void>(
+        future: _initFuture,
+        builder: (BuildContext context, AsyncSnapshot snapshot) {
+          if (snapshot.connectionState == ConnectionState.done) {
+            return _bannerAdWidget();
+          } else {
+            return SizedBox();
+          }
+        });
+  }
+
+  Widget _bannerAdWidget() {
     return LayoutBuilder(
       builder: (context, consts) {
         double height = widget.size.size.height;

lib/src/banner/controller.dart

@@ -155,6 +155,8 @@ class BannerAdController extends LoadShowAd<BannerAdEvent>
   /// For more info, [read the documentation](https://github.com/bdlukaa/native_admob_flutter/wiki/Using-the-controller-and-listening-to-banner-events#listening-to-events)
   Stream<Map<BannerAdEvent, dynamic>> get onEvent => super.onEvent;
 
+  FutureSyncExecutor _executor = FutureSyncExecutor();
+
   /// Creates a new native ad controller
   BannerAdController({
     Duration loadTimeout = kDefaultLoadTimeout,
@@ -167,7 +169,15 @@ class BannerAdController extends LoadShowAd<BannerAdEvent>
   /// Initialize the controller. This can be called only by the controller
   void init() {
     channel.setMethodCallHandler(_handleMessages);
-    MobileAds.pluginChannel.invokeMethod('initBannerAdController', {'id': id});
+    _executor.exec(() async {
+      await MobileAds.pluginChannel.invokeMethod('initBannerAdController', {'id': id});
+    }, null);
+  }
+
+  Future<void> waitForInitFinish() async {
+    return await _executor.exec(() {
+      return;
+    }, null);
   }
 
   /// Dispose the controller to free up resources.
@@ -203,6 +213,9 @@ class BannerAdController extends LoadShowAd<BannerAdEvent>
         });
         break;
       case 'onAdLoaded':
+        if (isLoaded) {
+          break;
+        }
         isLoaded = true;
         onEventController.add({BannerAdEvent.loaded: call.arguments});
         break;
@@ -227,16 +240,19 @@ class BannerAdController extends LoadShowAd<BannerAdEvent>
     ensureAdNotDisposed();
     assertMobileAdsIsInitialized();
     if (!debugCheckAdWillReload(isLoaded, force)) return false;
-    isLoaded = (await channel.invokeMethod<bool>('loadAd').timeout(
-      timeout ?? this.loadTimeout,
-      onTimeout: () {
-        if (!onEventController.isClosed && !isLoaded)
-          onEventController.add({
-            BannerAdEvent.loadFailed: AdError.timeoutError,
-          });
-        return false;
-      },
-    ))!;
+    isLoaded = await _executor.exec(() async {
+      return channel.invokeMethod<bool>('loadAd').timeout(
+        timeout ?? this.loadTimeout,
+        onTimeout: () {
+          if (!onEventController.isClosed && !isLoaded) {
+            onEventController.add({
+              BannerAdEvent.loadFailed: AdError.timeoutError,
+            });
+          }
+          return false;
+        },
+      );
+    }, null);
     if (isLoaded) lastLoadedTime = DateTime.now();
     return isLoaded;
   }

lib/src/native/controller/controller.dart

@@ -134,6 +134,9 @@ class NativeAdController extends LoadShowAd<NativeAdEvent>
   /// For more info, read the [documentation](https://github.com/bdlukaa/native_admob_flutter/wiki/Custom-mute-this-ad#check-if-custom-mute-this-ad-is-available)
   bool get isCustomMuteThisAdEnabled => _customMuteThisAdEnabled;
 
+  /// Asynchronous task synchronous executor
+  FutureSyncExecutor _syncExecutor= FutureSyncExecutor();
+
   /// Listen to the events the controller throws
   ///
   /// Usage:
@@ -261,7 +264,9 @@ class NativeAdController extends LoadShowAd<NativeAdEvent>
   /// Initialize the controller. This can be called only by the controller
   void init() {
     channel.setMethodCallHandler(_handleMessages);
-    MobileAds.pluginChannel.invokeMethod('initNativeAdController', {'id': id});
+    _syncExecutor.exec(() async {
+      await MobileAds.pluginChannel.invokeMethod('initNativeAdController', {'id': id});
+    }, null);
   }
 
   /// Dispose the controller to free up resources.
@@ -393,21 +398,24 @@ class NativeAdController extends LoadShowAd<NativeAdEvent>
     assertMobileAdsIsInitialized();
     if (!debugCheckAdWillReload(isLoaded, force)) return false;
     unitId ??= MobileAds.nativeAdUnitId ?? MobileAds.nativeAdTestUnitId;
-    isLoaded = (await channel.invokeMethod<bool>('loadAd', {
-      'unitId': unitId,
-      'options': (options ?? NativeAdOptions()).toJson(),
-      'nonPersonalizedAds': nonPersonalizedAds ?? this.nonPersonalizedAds,
-      'keywords': keywords,
-    }).timeout(
-      timeout ?? this.loadTimeout,
-      onTimeout: () {
-        if (!onEventController.isClosed && !isLoaded)
-          onEventController.add({
-            NativeAdEvent.loadFailed: AdError.timeoutError,
-          });
-        return false;
-      },
-    ))!;
+    isLoaded = await _syncExecutor.exec(() async {
+      // Invoke the loadAd method after initializing MethodChannel
+      return await channel.invokeMethod<bool>('loadAd', {
+        'unitId': unitId,
+        'options': (options ?? NativeAdOptions()).toJson(),
+        'nonPersonalizedAds': nonPersonalizedAds ?? this.nonPersonalizedAds,
+        'keywords': keywords,
+      }).timeout(
+        timeout ?? this.loadTimeout,
+        onTimeout: () {
+          if (!onEventController.isClosed && !isLoaded)
+            onEventController.add({
+              NativeAdEvent.loadFailed: AdError.timeoutError,
+            });
+          return false;
+        },
+      );
+    }, null);
     if (isLoaded) lastLoadedTime = DateTime.now();
     return isLoaded;
   }

lib/src/utils.dart

@@ -290,3 +290,23 @@ class ServerSideVerificationOptions {
         'customData': this.customData,
       };
 }
+
+/// Future synchronous executor
+class FutureSyncExecutor {
+  Future? future;
+
+  Future exec(Function function, List<dynamic>? positionalArguments,
+      [Map<Symbol, dynamic>? namedArguments]) {
+    final applyFunction = () {
+      return Function.apply(function, positionalArguments, namedArguments);
+    };
+
+    if (future == null) {
+      future = Future(applyFunction);
+      return future!;
+    } else {
+      future = future!.then((value) => applyFunction());
+      return future!;
+    }
+  }
+}