android解析json数据novalueformessage_全面解析FlutterPlatformChannel原理

作者：拍友2502876287 | 来源：互联网 | 2023-08-19 10:07

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欢迎关注专栏&＃xff1a;里面定期分享Android和Flutter架构技术知识点及解析&＃xff0c;还会不断更新的BATJ面试专题&＃xff0c;欢迎大家前来探讨交流&＃xff0c;如有好的文章也欢迎投稿。

Android高级进阶zhuanlan.zhihu.com

前言

我们知道Flutter中通过Platform Channel实现Flutter和原生端的数据传递&＃xff0c;那么这些数据是怎么传递的&＃xff0c;传递的过程都做了哪些操作&＃xff0c;本文将以Android为例带大家一起了解Platform Channel的工作原理。

Flutter定义了三种不同类型的Channel&＃xff0c;分别是

BasicMessageChannel&＃xff1a;用于传递字符串和半结构化的数据;
MethodChannel&＃xff1a;用于传递方法调用&＃xff1b;
EventChannel&＃xff1a;用于数据流的通信&＃xff1b;

本文以MethodChannel为例带大家一起进行源码分析。

MethodChannel源码解析

官方架构图

根据架构图&＃xff0c;我们可以看出在Flutter端&＃xff0c;MethodChannel允许发送与方法调用相对应的消息。在安卓和iOS原生端&＃xff0c;Android上的MethodChannel和iOS上的FlutterMethodChannel启用接收方法调用并发回结果给Flutter端。而这种数据传递方式还可以反向调用。为了保证用户界面保持相应而不卡顿&＃xff0c;消息和响应以异步的形式进行传递。

下面根据官方提供的demo一步一步来进行分析其具体实现。官方提供的demo代码地址如下&＃xff0c;实现了一个从Flutter端发起的方法调用&＃xff0c;从原生端获取电量并返回给Flutter端用于展示。

Writing custom platform-specific code

先来看一下Flutter中dart相关的代码

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;.";}}

Dart层方法调用的消息传递分析

首先&＃xff0c;dart中会先创建一个MethodChannel对象&＃xff0c;其名称为“samples.flutter.io/battery”&＃xff0c;这个名字很关键&＃xff0c;必须与原生端的名字相对应&＃xff0c;具体原因后边会有解释。通过异步方式调用invokeMethod方法传入方法名来获取电量信息platform.invokeMethod(&＃39;getBatteryLevel&＃39;);&＃xff0c;invokeMethod方法具体实现如下

../engine/shell/platform/android/io/flutter/plugin/common/MethodChannel.java

&＃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;}

通过BinaryMessages.send()方法来发送方法调用消息&＃xff0c;我们可以看到send方法有两个参数&＃xff0c;第一个是channel的名称&＃xff0c;第二个是ByteData对象&＃xff08;使用codec对根据方法名和参数构建的MethodCall对象进行编码得到的对象&＃xff09;&＃xff1b;codec对象是在MethodChannel对象创建时默认创建的StandardMethodCodec对象&＃xff0c;其对MethodCall对象的编码过程如下

../engine/shell/platform/android/io/flutter/plugin/common/StandardMethodCodec.java

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

通过messageCodec将调用的方法名和传递的参数写入到buffer中&＃xff0c;messageCodec是一个StandardMessageCodec对象&＃xff0c;在StandardMethodCodec对象创建时默认创建&＃xff0c;其writeValue方法的实现如下

../engine/shell/platform/android/io/flutter/plugin/common/StandardMessageCodec.java

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);}}

从上述代码看出&＃xff0c;Flutter与平台端的消息传递支持12种类型&＃xff0c;这12种类型分别与安卓和iOS中的类型相对应&＃xff0c;看下面表格更直观

writeValue方法其实就是将方法名和参数转化为对应的二进制数据写入buffer中&＃xff0c;方法名都是String类型&＃xff0c;我们就以String类型方法写入过程来进行简单说明&＃xff0c;如果判断一个value为String后&＃xff0c;

1、调用buffer.putUint8(_valueString);先写入对应的类型值&＃xff0c;_valueString &＃61; 7;&＃xff0c;所以将00000111二进制数据写入buffer&＃xff1b;

2、紧接着将value通过utf8编码为int数组&＃xff0c;然后将数组的长度数据通过writeSize(buffer, bytes.length);写入buffer&＃xff1b;

3、最后再将数组数据写入buffer&＃xff0c;至此一个方法名编码完成&＃xff1b;

其他类型的数据依次类推进行编码&＃xff0c;编码完成后&＃xff0c;将StandardMessageCodec对象编码的ByteData数据通过BinaryMessages.send()方法发送出去&＃xff0c;看下send方法的具体实现

../flutter/packages/flutter/lib/src/services/platform_messages.dart

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

会从_mockHandlers中查找是否缓存的有_MessageHandler对象&＃xff0c;如果没有则通过_sendPlatformMessage方法发送消息&＃xff0c;

../flutter/packages/flutter/lib/src/services/platform_messages.dart

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;}

其最终调用的是ui.window.sendPlatformMessage方法&＃xff0c;该方法中会传递回调方法对象&＃xff0c;在数据返回后会被回调从而得到结果数据。

../engine/lib/ui/window.dart

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;;

在以上代码中ui.window.sendPlatformMessage()方法最终会调用Dart本地接口方法_sendPlatformMessage&＃xff0c;这里可以将这个方法简单理解为类似于java的JNI的方法&＃xff0c;在c&＃43;&＃43;层会调用"Window_sendPlatformMessage"对应的方法。至此&＃xff0c;dart中的方法消息传递已经结束&＃xff0c;我们下面开始从Flutter engine源码中分析c&＃43;&＃43;层是如何对方法调用消息进行传递的。

c&＃43;&＃43;层消息的传递流程分析

我们在engine源码文件的./lib/ui/window/http://window.cc文件中找到了关于dart本地方法的注册代码块

../engine/lib/ui/window/http://window.cc

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},}); }

通过代码可以看到通过tonic::DartLibraryNatives的对象指针调用Register()方法对window对应的多个dart本地方法进行了注册&＃xff08;说明&＃xff1a;该注册方法的调用是在flutter引擎初始化后Dart虚拟机初始化时调用&＃xff0c;这里不再对这一块进行分析&＃xff0c;知道即可&＃xff09;。其中就有上面提到的“Window_sendPlatformMessage”&＃xff0c;该符号对应到的c&＃43;&＃43;方法为_SendPlatformMessage&＃xff0c;我们看下该方法中做了些什么&＃xff0c;

../engine/lib/ui/window/http://window.cc

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(); }

dart_state是一个UIDartState对象指针&＃xff0c;指向当前线程&＃xff08;UI thread&＃xff09;对应的isolate对象Root isolate&＃xff0c;回调对象callback不为null&＃xff0c;则会根据dart_state和callback创建一个tonic::DartPersistentValue对象&＃xff0c;然后根据当前线程的ui task runner创建一个平台消息响应对象response(该response会在消息响应结果返回时使用到)&＃xff0c;接下来走到代码中注释下面的代码块&＃xff0c;其dart_state->window()->client()返回的是Engine对象创建时创建的RuntimeController对象(这个也要回溯到引擎和DartVM初始化的过程&＃xff0c;这里不再展开&＃xff0c;知道即可)&＃xff0c;下面会调用该对象的HandlePlatformMessage()方法&＃xff0c;方法中传递的是包含有channel名、方法调用的相关数据和response对象(fml::RefPtr对象&＃xff0c;消息响应返回后会使用到)的fml::RefPtr对象。

../engine/runtime/http://runtime_controller.cc

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

接着调用client_的HandlePlatformMessage()方法&＃xff0c;client_是一个继承了RuntimeDelegate类的Engine对象&＃xff0c;

../engine/shell/common/http://engine.cc

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

由最开始的demo可知channel是我们自定义的名称为“samples.flutter.io/battery”的channel&＃xff0c;所以会执行else中的代码块&＃xff0c;这里的delegate_是指继承了Engine::Delegate类的Shell对象&＃xff0c;

../engine/shell/common/http://shell.cc

// |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));}}); }

由于Engine的创建是在UI task Runner中即UI thread中创建&＃xff0c;所以以上所有消息传递都是在UI thread中进行&＃xff0c;由于平台相关的api都是运行在主线程&＃xff0c;马上要将消息发送给平台&＃xff0c;所以此处会将消息交由platform task Runner执行&＃xff0c;即在platform thread中执行方法调用。platform_view_是一个继承了PlatformView类的PlatformViewAndroid对象&＃xff0c;该对象在创建AndroidShellHolder对象时被创建。view->HandlePlatformMessage执行以下方法&＃xff0c;

../engine/shell/platform/android/http://platform_view_android.cc

// |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 {...} }

消息响应对象response不为空时&＃xff0c;创建一个response_id并将其对应response保存到pending_responses_中&＃xff08;消息响应结果返回后会根据response_id取出response对象来处理响应结果&＃xff09;&＃xff0c;消息数据不为空时调用if代码块中的代码&＃xff0c;然后会调用http://platform_view_android_jni.cc中的以下方法&＃xff0c;view.obj()为java中的flutterJNI对象&＃xff0c;这个对象是在AndroidShellHolder对象创建时从java层传递过来的。最后通过env->CallVoidMethod()方法调用java层的flutterJNI对象的handlePlatformMessage方法&＃xff0c;将channel名称、消息内容和响应ID传给java层。

../engine/shell/platform/android/http://platform_view_android_jni.cc

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)); }

接下来我们开始分析java层接受到消息后的处理逻辑。

java层接受消息后的处理流程分析

通过以上分析&＃xff0c;c&＃43;&＃43;层通过调用flutterJNI的handlePlatformMessage方法将消息传递给java层&＃xff0c;我们来看一下FlutterJNI中的方法实现

../engine/shell/platform/android/io/flutter/embedding/engine/FlutterJNI.java

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

此时会调用platformMessageHandler的handleMessageFromDart()方法&＃xff0c;platformMessageHandler对象是在FlutterNativeView构造方法中创建FlutterJNI对象后设置进来的&＃xff0c;是一个实现了PlatformMessageHandler接口的FlutterNativeView.PlatformMessageHandlerImpl对象&＃xff0c;我们看一下它的handleMessageFromDart()方法实现

../engine/shell/platform/android/io/flutter/view/FlutterNativeView.java

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);} }

首先根据channel名称从mMessageHandlers中查找对应的BinaryMessageHandler对象&＃xff0c;如果找到则执行该对象的onMessage()方法&＃xff0c;那么mMessageHandlers中是怎么保存我们的channel名称为“samples.flutter.io/battery”的对象的呢&＃xff0c;我们看下开始所说的demo中的java模块相关代码&＃xff0c;

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();}}});

这块代码在MainActivity中的onCreate方法中&＃xff0c;创建一个名为“samples.flutter.io/battery”的MethodChannel对象&＃xff0c;然后设置对应的MethodCallHandler对象&＃xff0c;setMethodCallHandler的方法实现如下

../engine/shell/platform/android/io/flutter/plugin/common/MethodChannel.java

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);} }

到此&＃xff0c;我们发现在MainActivity的onCreate方法中实现的MethodCallHandler通过一系列操作被包装到IncomingMethodCallHandler对象中并设置进了mMessageHandlers中。那么我们上面所说的onMessage方法的调用即是IncomingMethodCallHandler对象的方法&＃xff0c;

../engine/shell/platform/android/io/flutter/plugin/common/MethodChannel.java

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));}}

方法中首先将从c&＃43;&＃43;层传递过来的消息通过codec解码为MethodCall对象&＃xff0c;然后调用MainActivity中实现的MethodHandler的onMethodCall方法&＃xff0c;改方法实现中会获取当前手机电量信息int batteryLevel &＃61; getBatteryLevel();&＃xff0c;然后调用result.success()方法&＃xff0c;通过reply.reply(MethodChannel.this.codec.encodeSuccessEnvelope(result));将结果数据编码后进行返回。reply方法中会调用FlutterNativeView.this.mFlutterJNI.invokePlatformMessageResponseCallback(replyId, reply, reply.position());方法将响应结果返回&＃xff0c;方法具体实现如下

../engine/shell/platform/android/io/flutter/embedding/engine/FlutterJNI.java

&＃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);

最终会调用JNI方法将数据返回给c&＃43;&＃43;层&＃xff0c;下面我们再接着看c&＃43;&＃43;层中接受到响应数据后的处理逻辑。

c&＃43;&＃43;层接收消息响应后的处理流程分析

根据JNI方法动态注册模块可知&＃xff0c;nativeInvokePlatformMessageResponseCallback方法对应以下c&＃43;&＃43;方法&＃xff0c;

../engine/shell/platform/android/http://platform_view_android_jni.cc

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 //); }

接着会调用AndroidShellHolder对象持有的PlatformViewAndroid对象的InvokePlatformMessageResponseCallback方法&＃xff0c;

../engine/shell/platform/android/http://platform_view_android.cc

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))); }

根据response_id从pending_responses_中查找对应的message_response对象&＃xff0c;通过对象指针调用其Complete方法处理响应结果&＃xff0c;根据以上过程中代码的分析可知该方法对应的是继承了PlatformMessageResponse类的PlatformMessageResponseDart类对象的Complete方法&＃xff0c;

../engine/lib/ui/window/http://platform_message_response_dart.cc

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});})); }

这里会将返回的数据处理通过uitaskrunner执行&＃xff0c;即会在UI thread中执行。callback即为上面分析的dart中对应的回调方法PlatformMessageResponseCallback的对象&＃xff0c;通过tonic::DartInvoke()方法将消息返回结果传递到dart层进行处理。

Dart层接收消息响应后的处理流程分析

通过以上Dart层传递消息分析可知PlatformMessageResponseCallback方法回调后对byte_buffer数据进行处理&＃xff0c;通过completer.complete()方法完成返回数据&＃xff0c;然后一步步返回到调用方法层&＃xff0c;在异步方法中通过await等待数据返回后&＃xff0c;再通过setState改变State中的变量值从而刷新页面数据将电量信息显示到屏幕上。至此&＃xff0c;整个flutter发消息给platform并接收消息处理的流程就完成了。

总结

先上一张消息传递流程图

通过整个源码流程的跟踪&＃xff0c;整个消息发送和接收结果的流程分为以下几步&＃xff1a;

Dart层通过以上提到的12种类型包含的类型数据进行编码&＃xff0c;然后通过dart的类似jni的本地接口方法传递给c&＃43;&＃43;层&＃xff1b;
c&＃43;&＃43;层通过持有java对象flutterJNI的方法调用将消息传递到java层&＃xff1b;
java层解码接收到的消息&＃xff0c;根据消息内容做指定的逻辑处理&＃xff0c;得到结果后进行编码通过jni方法将响应结果返回给c&＃43;&＃43;层&＃xff1b;
c&＃43;&＃43;层处理返回的响应结果&＃xff0c;并将结果通过发送时保存的dart响应方法对象回调给Dart层&＃xff1b;
Dart层通过回调方法对结果数据进行处理&＃xff0c;然后通过codec解码数据做后续的操作&＃xff1b;

转自&＃xff1a;全面解析Flutter Platform Channel原理