android解析json数据novalueformessage_全面解析FlutterPlatformChannel原理

static const platform &＃61; const MethodChannel(&＃39;samples.flutter.io/battery&＃39;);Future _getBatteryLevel() async {String batteryLevel;try {final int result &＃61; await platform.invokeMethod(&＃39;getBatteryLevel&＃39;);batteryLevel &＃61; &＃39;Battery level at $result % .&＃39;;} on PlatformException catch (e) {batteryLevel &＃61; "Failed to get battery level: &＃39;${e.message}&＃39;.";}}

&＃64;optionalTypeArgsFuture invokeMethod(String method, [dynamic arguments]) async {assert(method !&＃61; null);final ByteData result &＃61; await BinaryMessages.send(name,codec.encodeMethodCall(MethodCall(method, arguments)),);if (result &＃61;&＃61; null) {throw MissingPluginException(&＃39;No implementation found for method $method on channel $name&＃39;);}final T typedResult &＃61; codec.decodeEnvelope(result);return typedResult;}

&＃64;override
ByteData encodeMethodCall(MethodCall call) {final WriteBuffer buffer &＃61; WriteBuffer();messageCodec.writeValue(buffer, call.method);messageCodec.writeValue(buffer, call.arguments);return buffer.done();
}

void writeValue(WriteBuffer buffer, dynamic value) {if (value &＃61;&＃61; null) {buffer.putUint8(_valueNull);} else if (value is bool) {buffer.putUint8(value ? _valueTrue : _valueFalse);} else if (value is int) {if (-0x7fffffff - 1 <&＃61; value && value <&＃61; 0x7fffffff) {buffer.putUint8(_valueInt32);buffer.putInt32(value);} else {buffer.putUint8(_valueInt64);buffer.putInt64(value);}} else if (value is double) {buffer.putUint8(_valueFloat64);buffer.putFloat64(value);} else if (value is String) {buffer.putUint8(_valueString);final List bytes &＃61; utf8.encoder.convert(value);writeSize(buffer, bytes.length);buffer.putUint8List(bytes);} else if (value is Uint8List) {buffer.putUint8(_valueUint8List);writeSize(buffer, value.length);buffer.putUint8List(value);} else if (value is Int32List) {buffer.putUint8(_valueInt32List);writeSize(buffer, value.length);buffer.putInt32List(value);} else if (value is Int64List) {buffer.putUint8(_valueInt64List);writeSize(buffer, value.length);buffer.putInt64List(value);} else if (value is Float64List) {buffer.putUint8(_valueFloat64List);writeSize(buffer, value.length);buffer.putFloat64List(value);} else if (value is List) {buffer.putUint8(_valueList);writeSize(buffer, value.length);for (final dynamic item in value) {writeValue(buffer, item);}} else if (value is Map) {buffer.putUint8(_valueMap);writeSize(buffer, value.length);value.forEach((dynamic key, dynamic value) {writeValue(buffer, key);writeValue(buffer, value);});} else {throw ArgumentError.value(value);}}

static Future send(String channel, ByteData message) {final _MessageHandler handler &＃61; _mockHandlers[channel];if (handler !&＃61; null)return handler(message);return _sendPlatformMessage(channel, message);
}

static Future _sendPlatformMessage(String channel, ByteData message) {final Completer completer &＃61; Completer();ui.window.sendPlatformMessage(channel, message, (ByteData reply) {try {completer.complete(reply);} catch (exception, stack) {FlutterError.reportError(FlutterErrorDetails(exception: exception,stack: stack,library: &＃39;services library&＃39;,context: &＃39;during a platform message response callback&＃39;,));}});return completer.future;}

void sendPlatformMessage(String name,ByteData data,PlatformMessageResponseCallback callback) {final String error &＃61;_sendPlatformMessage(name, _zonedPlatformMessageResponseCallback(callback), data);if (error !&＃61; null)throw new Exception(error);}String _sendPlatformMessage(String name,PlatformMessageResponseCallback callback,ByteData data) native &＃39;Window_sendPlatformMessage&＃39;;

void Window::RegisterNatives(tonic::DartLibraryNatives* natives) {natives->Register({{"Window_defaultRouteName", DefaultRouteName, 1, true},{"Window_scheduleFrame", ScheduleFrame, 1, true},{"Window_sendPlatformMessage", _SendPlatformMessage, 4, true},{"Window_respondToPlatformMessage", _RespondToPlatformMessage, 3, true},{"Window_render", Render, 2, true},{"Window_updateSemantics", UpdateSemantics, 2, true},{"Window_setIsolateDebugName", SetIsolateDebugName, 2, true},});
}

void _SendPlatformMessage(Dart_NativeArguments args) {tonic::DartCallStatic(&SendPlatformMessage, args);
}Dart_Handle SendPlatformMessage(Dart_Handle window,const std::string& name,Dart_Handle callback,const tonic::DartByteData& data) {UIDartState* dart_state &＃61; UIDartState::Current();...fml::RefPtr response;if (!Dart_IsNull(callback)) {response &＃61; fml::MakeRefCounted(tonic::DartPersistentValue(dart_state, callback),dart_state->GetTaskRunners().GetUITaskRunner());}if (Dart_IsNull(data.dart_handle())) {dart_state->window()->client()->HandlePlatformMessage(fml::MakeRefCounted(name, response));} else {const uint8_t* buffer &＃61; static_cast(data.data());// data数据部位null&＃xff0c;会走下面这块代码dart_state->window()->client()->HandlePlatformMessage(fml::MakeRefCounted(name, std::vector(buffer, buffer &＃43; data.length_in_bytes()),response));}return Dart_Null();
}

void RuntimeController::HandlePlatformMessage(fml::RefPtr message) {client_.HandlePlatformMessage(std::move(message));
}

void Engine::HandlePlatformMessage(fml::RefPtr message) {if (message->channel() &＃61;&＃61; kAssetChannel) {HandleAssetPlatformMessage(std::move(message));} else {delegate_.OnEngineHandlePlatformMessage(std::move(message));}
}

// |shell::Engine::Delegate|
void Shell::OnEngineHandlePlatformMessage(fml::RefPtr message) {...task_runners_.GetPlatformTaskRunner()->PostTask([view &＃61; platform_view_->GetWeakPtr(), message &＃61; std::move(message)]() {if (view) {view->HandlePlatformMessage(std::move(message));}});
}

// |shell::PlatformView|
void PlatformViewAndroid::HandlePlatformMessage(fml::RefPtr message) {JNIEnv* env &＃61; fml::jni::AttachCurrentThread();fml::jni::ScopedJavaLocalRef view &＃61; java_object_.get(env);if (view.is_null())return;int response_id &＃61; 0;if (auto response &＃61; message->response()) {response_id &＃61; next_response_id_&＃43;&＃43;;pending_responses_[response_id] &＃61; response;}auto java_channel &＃61; fml::jni::StringToJavaString(env, message->channel());if (message->hasData()) {fml::jni::ScopedJavaLocalRef message_array(env, env->NewByteArray(message->data().size()));env->SetByteArrayRegion(message_array.obj(), 0, message->data().size(),reinterpret_cast(message->data().data()));message &＃61; nullptr;// This call can re-enter in InvokePlatformMessageXxxResponseCallback.FlutterViewHandlePlatformMessage(env, view.obj(), java_channel.obj(),message_array.obj(), response_id);} else {...}
}

static jmethodID g_handle_platform_message_method &＃61; nullptr;
void FlutterViewHandlePlatformMessage(JNIEnv* env,jobject obj,jstring channel,jobject message,jint responseId) {env->CallVoidMethod(obj, g_handle_platform_message_method, channel, message,responseId);FML_CHECK(CheckException(env));
}

private void handlePlatformMessage(String channel, byte[] message, int replyId) {if (this.platformMessageHandler !&＃61; null) {this.platformMessageHandler.handleMessageFromDart(channel, message, replyId);}
}

public void handleMessageFromDart(final String channel, byte[] message, final int replyId) {FlutterNativeView.this.assertAttached();BinaryMessageHandler handler &＃61; (BinaryMessageHandler)FlutterNativeView.this.mMessageHandlers.get(channel);if (handler !&＃61; null) {try {ByteBuffer buffer &＃61; message &＃61;&＃61; null ? null : ByteBuffer.wrap(message);handler.onMessage(buffer, new BinaryReply() {private final AtomicBoolean done &＃61; new AtomicBoolean(false);public void reply(ByteBuffer reply) {if (!FlutterNativeView.this.isAttached()) {Log.d("FlutterNativeView", "handleMessageFromDart replying ot a detached view, channel&＃61;" &＃43; channel);} else if (this.done.getAndSet(true)) {throw new IllegalStateException("Reply already submitted");} else {if (reply &＃61;&＃61; null) {FlutterNativeView.this.mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);} else {FlutterNativeView.this.mFlutterJNI.invokePlatformMessageResponseCallback(replyId, reply, reply.position());}}}});} catch (Exception var6) {Log.e("FlutterNativeView", "Uncaught exception in binary message listener", var6);FlutterNativeView.this.mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);}} else {FlutterNativeView.this.mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);}
}

private static final String CHANNEL &＃61; "samples.flutter.io/battery";new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(new MethodChannel.MethodCallHandler() {&＃64;Overridepublic void onMethodCall(MethodCall call, MethodChannel.Result result) {if (call.method.equals("getBatteryLevel")) {int batteryLevel &＃61; getBatteryLevel();if (batteryLevel !&＃61; -1) {result.success(batteryLevel);} else {result.error("UNAVAILABLE", "Battery level not available.", null);}} else {result.notImplemented();}}});

public void setMethodCallHandler(&＃64;Nullable MethodChannel.MethodCallHandler handler) {this.messenger.setMessageHandler(this.name, handler &＃61;&＃61; null ? null : new MethodChannel.IncomingMethodCallHandler(handler));
}
其中的messenger就是通过getFlutterView()获取到的实现了BinaryMessenger接口的FlutterView对象&＃xff0c;方法的第二个参数是通过handler对象包装的MethodChannel.IncomingMethodCallHandler对象&＃xff0c;看下FlutterView中对接口方法setMessageHandler()的实现../engine/shell/platform/android/io/flutter/view/FlutterView.java
public void setMessageHandler(String channel, BinaryMessageHandler handler) {this.mNativeView.setMessageHandler(channel, handler);
}
会调用对应的FlutterNativeView的setMessageHandler()方法&＃xff0c;FlutterNativeView同样实现了BinaryMessenger接口&＃xff0c;看下其中的方法实现../engine/shell/platform/android/io/flutter/view/FlutterNativeView.java
public void setMessageHandler(String channel, BinaryMessageHandler handler) {if (handler &＃61;&＃61; null) {this.mMessageHandlers.remove(channel);} else {this.mMessageHandlers.put(channel, handler);}
}

public void onMessage(ByteBuffer message, final BinaryReply reply) {MethodCall call &＃61; MethodChannel.this.codec.decodeMethodCall(message);try {this.handler.onMethodCall(call, new MethodChannel.Result() {public void success(Object result) {reply.reply(MethodChannel.this.codec.encodeSuccessEnvelope(result));}public void error(String errorCode, String errorMessage, Object errorDetails) {reply.reply(MethodChannel.this.codec.encodeErrorEnvelope(errorCode, errorMessage, errorDetails));}public void notImplemented() {reply.reply((ByteBuffer)null);}});} catch (RuntimeException var5) {Log.e("MethodChannel#" &＃43; MethodChannel.this.name, "Failed to handle method call", var5);reply.reply(MethodChannel.this.codec.encodeErrorEnvelope("error", var5.getMessage(), (Object)null));}}

&＃64;UiThread
public void invokePlatformMessageResponseCallback(int responseId, ByteBuffer message, int position) {this.ensureAttachedToNative();this.nativeInvokePlatformMessageResponseCallback(this.nativePlatformViewId, responseId, message, position);
}private native void nativeInvokePlatformMessageResponseCallback(long var1, int var3, ByteBuffer var4, int var5);

static void InvokePlatformMessageResponseCallback(JNIEnv* env,jobject jcaller,jlong shell_holder,jint responseId,jobject message,jint position) {ANDROID_SHELL_HOLDER->GetPlatformView()->InvokePlatformMessageResponseCallback(env, //responseId, //message, //position //);
}

void PlatformViewAndroid::InvokePlatformMessageResponseCallback(JNIEnv* env,jint response_id,jobject java_response_data,jint java_response_position) {if (!response_id)return;auto it &＃61; pending_responses_.find(response_id);if (it &＃61;&＃61; pending_responses_.end())return;uint8_t* response_data &＃61;static_cast(env->GetDirectBufferAddress(java_response_data));std::vector response &＃61; std::vector(response_data, response_data &＃43; java_response_position);auto message_response &＃61; std::move(it->second);pending_responses_.erase(it);message_response->Complete(std::make_unique(std::move(response)));
}

void PlatformMessageResponseDart::Complete(std::unique_ptr data) {if (callback_.is_empty())return;FML_DCHECK(!is_complete_);is_complete_ &＃61; true;ui_task_runner_->PostTask(fml::MakeCopyable([callback &＃61; std::move(callback_), data &＃61; std::move(data)]() mutable {std::shared_ptr dart_state &＃61;callback.dart_state().lock();if (!dart_state)return;tonic::DartState::Scope scope(dart_state);Dart_Handle byte_buffer &＃61; WrapByteData(std::move(data));tonic::DartInvoke(callback.Release(), {byte_buffer});}));
}