Dart和Java通信源码分析和实践

前言

DartJava 通信这块的知识点涵盖了 Dart&C 以及 Java&C 的通信,我们先有简单的业务组件的定义再到底层实现原理进行分析,我们现在从Flutter定义的三种 Channel 中的 MethodChannel 使用进行剖析。

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Dart和Java通信的实践

Java端ChannelPlugin的创建

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public class FileProviderPlugin implements MethodChannel.MethodCallHandler {
public final static String NAME = "plugins.flutter.io/file_plugin";
private final Registrar mRegistrar;
private Context application;
public FileProviderPlugin(Registrar mRegistrar, Context context) {
this.mRegistrar = mRegistrar;
this.application = context.getApplicationContext();
}

public static void registerWith(Registrar registrar, Context context) {
MethodChannel channel =
new MethodChannel(registrar.messenger(), NAME);
FileProviderPlugin instance = new FileProviderPlugin(registrar, context);
channel.setMethodCallHandler(instance);
}

@TargetApi(Build.VERSION_CODES.FROYO)
@Override
public void onMethodCall(MethodCall methodCall, Result result) {
String path;
switch (methodCall.method) {
case "getExternalStoragePublicDirectory":
path = Environment.getExternalStoragePublicDirectory(methodCall.arguments.toString()).getAbsolutePath();
result.success(path);
break;
}
}
}

Java端ChannelPlugin的注册

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public class MainActivity extends FlutterActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
GeneratedPluginRegistrant.registerWith(this);
FileProviderPlugin.registerWith(this.registrarFor(FileProviderPlugin.NAME), this);
}
}

Dart端ChannelPlugin的创建

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import 'dart:async';
import 'dart:io';
import 'package:flutter/services.dart';
const MethodChannel _channel = const MethodChannel('plugins.flutter.io/file_plugin');
const String DIRECTORY_DCIM = "DCIM";
Future<Directory> getExternalStoragePublicDirectory(String type) async {
if (Platform.isIOS)
throw new UnsupportedError("Functionality not available on iOS");
final String path = await _channel.invokeMethod('getExternalStoragePublicDirectory', type);
if (path == null) {
return null;
}
return new Directory(path);
}

Dart端ChannelPlugin的使用

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class Page extends StatelessWidget{
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text('Reading and Writing Files')),
body: Center(
child: Text("this page name is test"),
),
floatingActionButton: FloatingActionButton(
onPressed: _click,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}

void _click() {
_getExternalStoragePublicDirectory(FileUtils.DIRECTORY_DCIM).then((path) {
print(path);
});
}
//获取存储图片和视频文件目录
Future<String> _getExternalStoragePublicDirectory(String s) async {
final directory = await FileUtils.getExternalStoragePublicDirectory(s);
return directory.path;
}
}

结果输出

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I/flutter (16780): /storage/emulated/0/DCIM

源代码分析

Dart端代码

MethodChannel信息的封装

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class MethodChannel {
const MethodChannel(this.name, [this.codec = const StandardMethodCodec()]);
final String name;
final MethodCodec codec;
@optionalTypeArgs
Future<T> invokeMethod<T>(String method, [dynamic arguments]) async {
assert(method != null);
final ByteData result = await BinaryMessages.send(
name,
//构造一个MethodCall对象,并调用StandardMethodCodec的encode将其写入一个ByteData中。
codec.encodeMethodCall(MethodCall(method, arguments)),
);
if (result == null) {
throw MissingPluginException('No implementation found for method $method on channel $name');
}
final T typedResult = codec.decodeEnvelope(result);
return typedResult;
}
/******部分代码省略******/
}
//MethodCall对象
class MethodCall {
const MethodCall(this.method, [this.arguments]): assert(method != null);
final String method;
final dynamic arguments;
@override
String toString() => '$runtimeType($method, $arguments)';
}
//默认的对函数进行编解码工具
class StandardMethodCodec implements MethodCodec {
const StandardMethodCodec([this.messageCodec = const StandardMessageCodec()]);
final StandardMessageCodec messageCodec;
@override
ByteData encodeMethodCall(MethodCall call) {
final WriteBuffer buffer = WriteBuffer();
messageCodec.writeValue(buffer, call.method);
messageCodec.writeValue(buffer, call.arguments);
return buffer.done();
}
/******部分代码省略******/
}
//默认的对数据信息编解码工具
class StandardMessageCodec implements MessageCodec<dynamic> {
const StandardMessageCodec();
static const int _valueNull = 0;
static const int _valueTrue = 1;
static const int _valueFalse = 2;
static const int _valueInt32 = 3;
static const int _valueInt64 = 4;
static const int _valueLargeInt = 5;
static const int _valueFloat64 = 6;
static const int _valueString = 7;
static const int _valueUint8List = 8;
static const int _valueInt32List = 9;
static const int _valueInt64List = 10;
static const int _valueFloat64List = 11;
static const int _valueList = 12;
static const int _valueMap = 13;
//数据转化
void writeValue(WriteBuffer buffer, dynamic value) {
if (value == null) {
buffer.putUint8(_valueNull);
} else if (value is bool) {
buffer.putUint8(value ? _valueTrue : _valueFalse);
}/******部分代码省略******/
}

信息传到C层

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class BinaryMessages {
BinaryMessages._();
static final Map<String, _MessageHandler> _handlers =
<String, _MessageHandler>{};
static final Map<String, _MessageHandler> _mockHandlers =
<String, _MessageHandler>{};
static Future<ByteData> send(String channel, ByteData message) {
final _MessageHandler handler = _mockHandlers[channel];
if (handler != null)
return handler(message);
return _sendPlatformMessage(channel, message);
}

static Future<ByteData> _sendPlatformMessage(String channel, ByteData message) {
final Completer<ByteData> completer = Completer<ByteData>();
ui.window.sendPlatformMessage(channel, message, (ByteData reply) {
try {
completer.complete(reply);
} catch (exception, stack) {
FlutterError.reportError(FlutterErrorDetails(
exception: exception,
stack: stack,
library: 'services library',
context: 'during a platform message response callback',
));
}
});
return completer.future;
}
/******部分代码省略******/
}

class Window {
Window._();
/******部分代码省略******/
void sendPlatformMessage(String name,
ByteData data,
PlatformMessageResponseCallback callback) {
final String error =
_sendPlatformMessage(name, _zonedPlatformMessageResponseCallback(callback), data);
if (error != null)
throw new Exception(error);
}
String _sendPlatformMessage(String name,
PlatformMessageResponseCallback callback,
ByteData data) native 'Window_sendPlatformMessage';
/******部分代码省略******/
}

C层代码分析

Window_sendPlatformMessage方法

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//engine/lib/ui/window/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},
{"Window_reportUnhandledException", ReportUnhandledException, 2, true},
{"Window_setNeedsReportTimings", SetNeedsReportTimings, 2, true},
});
}
void _SendPlatformMessage(Dart_NativeArguments args){
//由DART_NATIVE_CALLBACK_STATIC宏生成的方法调用;
//详见:/fuchsia/tonic/dart_binding_macros.h
tonic::DartCallStatic(&SendPlatformMessage, args);
}
Dart_Handle SendPlatformMessage(Dart_Handle window, const std::string &name, Dart_Handle callback, Dart_Handle data_handle){
UIDartState *dart_state = UIDartState::Current();
//判断当前的window对象
if (!dart_state->window()){
return tonic::ToDart("Platform messages can only be sent from the main isolate");
}
//如果需要返回值构造返回体(PlatformMessageResponseDart),再向dart端做数据返回的时候需要
fml::RefPtr<PlatformMessageResponse> response;
if (!Dart_IsNull(callback)){
response = fml::MakeRefCounted<PlatformMessageResponseDart>(tonic::DartPersistentValue(dart_state, callback), dart_state->GetTaskRunners().GetUITaskRunner());
}
//调用window的client对象的HandlePlatformMessage方法处理消息
//window=>WindowClient=>RuntimeController(RuntimeDelegate=>Engine).
//所以最终调用的是Engine.HandlePlatformMessage
if (Dart_IsNull(data_handle)){
dart_state->window()->client()->HandlePlatformMessage(fml::MakeRefCounted<PlatformMessage>(name, response));
}else{
tonic::DartByteData data(data_handle);
const uint8_t *buffer = static_cast<const uint8_t *>(data.data());
dart_state->window()->client()->HandlePlatformMessage(fml::MakeRefCounted<PlatformMessage>(name, std::vector<uint8_t>(buffer, buffer + data.length_in_bytes()), response));
}
return Dart_Null();
}

Engine.HandlePlatformMessage

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//engine/shell/common/engine.h
private:Engine::Delegate& delegate_;
//engine/shell/common/engine.cc
static constexpr char kAssetChannel[] = "flutter/assets";
void Engine::HandlePlatformMessage(fml::RefPtr<PlatformMessage> message){
//判断当前的channel是不是flutter/assets
if (message->channel() == kAssetChannel){
HandleAssetPlatformMessage(std::move(message));
}else{
delegate_.OnEngineHandlePlatformMessage(std::move(message));
}
}

Engine对象的delegate_

Shell 作为 Engine::Delegate 的子类:

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//engine/shell/common/shell.h
class Shell final : public PlatformView::Delegate,
public Animator::Delegate,
public Engine::Delegate,
public Rasterizer::Delegate,
public ServiceProtocol::Handler{

/*******部分代码省略********/
// |Engine::Delegate|
void OnEngineHandlePlatformMessage(fml::RefPtr<PlatformMessage> message) override;
}

OnEngineHandlePlatformMessage方法实现

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// sengine/hell/common/shell.cc
constexpr char kSkiaChannel[] = "flutter/skia";
// |Engine::Delegate|
void Shell::OnEngineHandlePlatformMessage(
fml::RefPtr<PlatformMessage> message){
//判断Shell是不是进行过setup
FML_DCHECK(is_setup_);
FML_DCHECK(task_runners_.GetUITaskRunner()->RunsTasksOnCurrentThread());
//判断当前的channel是不是flutter/skia
if (message->channel() == kSkiaChannel){
HandleEngineSkiaMessage(std::move(message));
return;
}
//在主线程中post一个task
task_runners_.GetPlatformTaskRunner()->PostTask(
//platform_view_是shell创建时候传过来的,也就是在Flutterview进行attach的时候出现的
//在这里的PlatformView是PlatformViewAndroid
[view = platform_view_->GetWeakPtr(), message = std::move(message)]() {
if (view){
view->HandlePlatformMessage(std::move(message));
}
});
}

PlatformView

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/// engine/shell/platform/android/platform_view_android.cc
// |PlatformView|
void PlatformViewAndroid::HandlePlatformMessage(
fml::RefPtr<flutter::PlatformMessage> message){
JNIEnv *env = fml::jni::AttachCurrentThread();
fml::jni::ScopedJavaLocalRef<jobject> view = java_object_.get(env);
if (view.is_null())
return;
int response_id = 0;
//如果需要相应
if (auto response = message->response()){
response_id = next_response_id_++;
//将当前的response_id存储在pending_responses_
pending_responses_[response_id] = response;
}
auto java_channel = fml::jni::StringToJavaString(env, message->channel());
//带参数
if (message->hasData()){
fml::jni::ScopedJavaLocalRef<jbyteArray> message_array(
env, env->NewByteArray(message->data().size()));
env->SetByteArrayRegion(
message_array.obj(), 0, message->data().size(),
reinterpret_cast<const jbyte *>(message->data().data()));
message = nullptr;
// This call can re-enter in InvokePlatformMessageXxxResponseCallback.
FlutterViewHandlePlatformMessage(env, view.obj(), java_channel.obj(),
message_array.obj(), response_id);
} else{ // 不带参数
message = nullptr;
// This call can re-enter in InvokePlatformMessageXxxResponseCallback.
FlutterViewHandlePlatformMessage(env, view.obj(), java_channel.obj(), nullptr, response_id);
}
}

JNI_OnLoad

Android 程序中 so 的加载都会调用 so 中的 JNI_OnLoad 方法, 详细的知识点可以从 从JNI_OnLoad看so的加载 这篇文章中学习。

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//engine/shell/platform/android/library_loader.cc
JNIEXPORT jint JNI_OnLoad(JavaVM *vm, void *reserved){
// Initialize the Java VM.
fml::jni::InitJavaVM(vm);
JNIEnv *env = fml::jni::AttachCurrentThread();
bool result = false;
// Register FlutterMain.
result = flutter::FlutterMain::Register(env);
FML_CHECK(result);
// Register PlatformView
result = flutter::PlatformViewAndroid::Register(env);
FML_CHECK(result);
// Register VSyncWaiter.
result = flutter::VsyncWaiterAndroid::Register(env);
FML_CHECK(result);
return JNI_VERSION_1_4;
}

FlutterMain::Register

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// engine/shell/platform/android/platform_view_android_jni.cc
void FlutterViewHandlePlatformMessage(JNIEnv *env,
jobject obj,
jstring channel,
jobject message,
jint responseId){
//即调用FlutterJNI.handlePlatformMessage方法
env->CallVoidMethod(obj, g_handle_platform_message_method, channel, message,
responseId);
FML_CHECK(CheckException(env));
}

bool RegisterApi(JNIEnv *env){
/*******部分代码省略********/
//这里的g_handle_platform_message_method为io/flutter/embedding/engine/FlutterJNI
g_handle_platform_message_method =
env->GetMethodID(g_flutter_jni_class->obj(), "handlePlatformMessage",
"(Ljava/lang/String;[BI)V");
return true;
}

PlatformViewAndroid::Register

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//engine/shell/platform/android/flutter_main.cc
static std::unique_ptr<FlutterMain> g_flutter_main;
void FlutterMain::Init(JNIEnv *env,
jclass clazz,
jobject context,
jobjectArray jargs,
jstring kernelPath,
jstring appStoragePath,
jstring engineCachesPath){
/*******部分代码省略********/
g_flutter_main.reset(new FlutterMain(std::move(settings)));
g_flutter_main->SetupObservatoryUriCallback(env);
}

void FlutterMain::SetupObservatoryUriCallback(JNIEnv *env){
/*******部分代码省略********/
g_flutter_jni_class = new fml::jni::ScopedJavaGlobalRef<jclass>(
env, env->FindClass("io/flutter/embedding/engine/FlutterJNI"));
}

Java端代码

从上面我们看到了最终调用到了 FlutterJNI.handlePlatformMessage 方法:

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//engine/shell/platform/android/io/flutter/embedding/engine/FlutterJNI.java
// Called by native.
// TODO(mattcarroll): determine if message is nonull or nullable
@SuppressWarnings("unused")
private void handlePlatformMessage(@NonNull final String channel, byte[] message, final int replyId){
if (platformMessageHandler != null){
platformMessageHandler.handleMessageFromDart(channel, message, replyId);
}
}

@UiThread
public void setPlatformMessageHandler(@Nullable PlatformMessageHandler platformMessageHandler){
ensureRunningOnMainThread();
this.platformMessageHandler = platformMessageHandler;
}

我们现在找到 FlutterJNI 中的 platformMessageHandler 实例就行。

FlutterActivity中FlutterView的创建

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public class FlutterActivity extends Activity implements Provider, PluginRegistry, ViewFactory {
private static final String TAG = "FlutterActivity";
private final FlutterActivityDelegate delegate = new FlutterActivityDelegate(this, this);
private final FlutterActivityEvents eventDelegate;
private final Provider viewProvider;
private final PluginRegistry pluginRegistry;

public FlutterActivity() {
this.eventDelegate = this.delegate;
this.viewProvider = this.delegate;
this.pluginRegistry = this.delegate;
}
/*******部分代码省略********/
}

FlutterActivityDelegate 中的委托实现:

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public final class FlutterActivityDelegate implements FlutterActivityEvents, Provider, PluginRegistry {
/*******部分代码省略********/
public void onCreate(Bundle savedInstanceState) {
/*******部分代码省略********/
String[] args = getArgsFromIntent(this.activity.getIntent());
FlutterMain.ensureInitializationComplete(this.activity.getApplicationContext(), args);
this.flutterView = this.viewFactory.createFlutterView(this.activity);
if (this.flutterView == null) {
FlutterNativeView nativeView = this.viewFactory.createFlutterNativeView();
this.flutterView = new FlutterView(this.activity, (AttributeSet)null, nativeView);
this.flutterView.setLayoutParams(matchParent);
this.activity.setContentView(this.flutterView);
this.launchView = this.createLaunchView();
if (this.launchView != null) {
this.addLaunchView();
}
}
/*******部分代码省略********/
}
/*******部分代码省略********/
}

FlutterNativeView中platformMessageHandler的注册

FlutterNativeView 构造函数中我们直接看到 了往 FlutterJNI 中注册的是 FlutterNativeView.PlatformMessageHandlerImpl

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public class FlutterNativeView implements BinaryMessenger {
/*******部分代码省略********/
public FlutterNativeView(Context context, boolean isBackgroundView) {
this.mNextReplyId = 1;
this.mPendingReplies = new HashMap();
this.mContext = context;
this.mPluginRegistry = new FlutterPluginRegistry(this, context);
this.mFlutterJNI = new FlutterJNI();
this.mFlutterJNI.setRenderSurface(new FlutterNativeView.RenderSurfaceImpl());
this.mFlutterJNI.setPlatformMessageHandler(new FlutterNativeView.PlatformMessageHandlerImpl());
this.mFlutterJNI.addEngineLifecycleListener(new FlutterNativeView.EngineLifecycleListenerImpl());
this.attach(this, isBackgroundView);
this.assertAttached();
this.mMessageHandlers = new HashMap();
}
/*******部分代码省略********/
}

FlutterNativeView.PlatformMessageHandlerImpl

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private final class PlatformMessageHandlerImpl implements PlatformMessageHandler {
/*******部分代码省略********/
public void handleMessageFromDart(final String channel, byte[] message, final int replyId) {
FlutterNativeView.this.assertAttached();
//通过channel获取注册的BinaryMessageHandler
BinaryMessageHandler handler = (BinaryMessageHandler)FlutterNativeView.this.mMessageHandlers.get(channel);
if (handler != null) {
try {
ByteBuffer buffer = message == null ? null : ByteBuffer.wrap(message);
//调用BinaryMessageHandler的onMessage方法
handler.onMessage(buffer, new BinaryReply() {
/*******部分代码省略********/
});
}
/*******部分代码省略********/
}
}
/*******部分代码省略********/
}

上面根据 channel 获取对应的 BinaryMessagehandler 实例,那么这个实现是通过什么方式在 FlutterNativeView 中的 mMessageHandlers 注册的呢?

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//FlutterNativeView
public void setMessageHandler(String channel, BinaryMessageHandler handler) {
if (handler == null) {
this.mMessageHandlers.remove(channel);
} else {
this.mMessageHandlers.put(channel, handler);
}
}

我们发现是在调用 FlutterNativeViewsetMessageHandler 方法往 mMessageHandlers 添加 BinaryMessageHandler

BinaryMessagehandler的注册

我们先回头看一下我们自定的 FileProviderPlugin 的实现:

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public class FileProviderPlugin implements MethodChannel.MethodCallHandler {
public final static String NAME = "plugins.flutter.io/file_plugin";
/*******部分代码省略********/
public static void registerWith(Registrar registrar, Context context) {
MethodChannel channel =
new MethodChannel(registrar.messenger(), NAME);
FileProviderPlugin instance = new FileProviderPlugin(registrar, context);
channel.setMethodCallHandler(instance);
}
}
//FlutterActivity
public final Registrar registrarFor(String pluginKey) {
//pluginRegistry的实例是FlutterActivityDelegate对象
return this.pluginRegistry.registrarFor(pluginKey);
}
//FlutterActivityDelegate
public Registrar registrarFor(String pluginKey) {
return this.flutterView.getPluginRegistry().registrarFor(pluginKey);
}
//FlutterNativeView
public FlutterPluginRegistry getPluginRegistry() {
return this.mPluginRegistry;
}
//FlutterPluginRegistry
public Registrar registrarFor(String pluginKey) {
if (this.mPluginMap.containsKey(pluginKey)) {
throw new IllegalStateException("Plugin key " + pluginKey + " is already in use");
} else {
this.mPluginMap.put(pluginKey, (Object)null);
return new FlutterPluginRegistry.FlutterRegistrar(pluginKey);
}
}
//FlutterPluginRegistry.FlutterRegistrar
private class FlutterRegistrar implements Registrar {
public BinaryMessenger messenger() {
return FlutterPluginRegistry.this.mNativeView;
}
}
//MethodChannel
public final class MethodChannel {
public void setMethodCallHandler(@Nullable MethodChannel.MethodCallHandler handler) {
//FlutterNativeView.setMessageHandler
this.messenger.setMessageHandler(this.name, handler == null ? null : new MethodChannel.IncomingMethodCallHandler(handler));
}
}

BinaryMessageHandler.onMessage

我们找到了 BinaryMessageHandler 的实例对象为 MethodChannel.IncomingMethodCallHandler :

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private final class IncomingMethodCallHandler implements BinaryMessageHandler{
//这里的Handler为我们定义的FileProviderPlugin
private final MethodChannel.MethodCallHandler handler;
IncomingMethodCallHandler(MethodChannel.MethodCallHandler handler) {
this.handler = handler;
}
public void onMessage(ByteBuffer message, final BinaryReply reply) {
MethodCall call = MethodChannel.this.codec.decodeMethodCall(message);
try {
//FileProviderPlugin的onMethodCall方法
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#" + MethodChannel.this.name, "Failed to handle method call", var5);
reply.reply(MethodChannel.this.codec.encodeErrorEnvelope("error", var5.getMessage(), (Object)null));
}
}
}

消息回传到Dart端

我们 Java 在进行事件响应后执行 MethodChannel.Resultsuccess 或者 error 方法将结果传递给 Dart

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//FileProviderPlugin的onMethodCall方法
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);
}
});

上面代码中我们看到最终调用的是 BinaryReplyreply 方法:

BinaryReply 是我们在消息传递过程中 FlutterNativeView.PlatformMessageHandlerImpl 调用 handleMessageFromDart 方法中产生的实例:

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public void handleMessageFromDart(final String channel, byte[] message, final int replyId) {
FlutterNativeView.this.assertAttached();
BinaryMessageHandler handler = (BinaryMessageHandler)FlutterNativeView.this.mMessageHandlers.get(channel);
if (handler != null) {
try {
ByteBuffer buffer = message == null ? null : ByteBuffer.wrap(message);
handler.onMessage(buffer, new BinaryReply() {
private final AtomicBoolean done = new AtomicBoolean(false);

public void reply(ByteBuffer reply) {
if (!FlutterNativeView.this.isAttached()) {
Log.d("FlutterNativeView", "handleMessageFromDart replying ot a detached view, channel=" + channel);
} else if (this.done.getAndSet(true)) {
throw new IllegalStateException("Reply already submitted");
} else {
if (reply == null) {
FlutterNativeView.this.mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);
} else {
FlutterNativeView.this.mFlutterJNI.invokePlatformMessageResponseCallback(replyId, reply, reply.position());
}
}
}
});
}
/*******部分代码省略********/
}

可以看到当 replay 不为空的时候我们调用的是 FlutterNativeView.this.mFlutterJNI.invokePlatformMessageResponseCallback

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 @UiThread
public void invokePlatformMessageResponseCallback(int responseId) {
this.ensureAttachedToNative();
this.nativeInvokePlatformMessageResponseCallback(this.nativePlatformViewId, responseId);
}

从而调用 nativenativeInvokePlatformMessageEmptyResponseCallback ,这个方法在 flutterso 加载的时候已经被注册了。

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//engine/shell/platform/android/platform_view_android_jni.cc
{
.name = "nativeInvokePlatformMessageResponseCallback",
.signature = "(JI)V",
.fnPtr = reinterpret_cast<void*>(
&InvokePlatformMessageResponseCallback),
}

对应的 C++ 的方法是 InvokePlatformMessageEmptyResponseCallback

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//engine/shell/platform/android/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 //
);
}

这个方法有调用 PlatfromViewInvokePlatformMessageResponseCallback 方法,也就是:

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//engine/shell/platform/android/platform_view_android.cc
void PlatformViewAndroid::InvokePlatformMessageResponseCallback(
JNIEnv* env,
jint response_id,
jobject java_response_data,
jint java_response_position) {
if (!response_id)
return;
//找到存储在pending_responses_当中的PlatformMessageResponse
auto it = pending_responses_.find(response_id);
if (it == pending_responses_.end())
return;
uint8_t* response_data =
static_cast<uint8_t*>(env->GetDirectBufferAddress(java_response_data));
std::vector<uint8_t> response = std::vector<uint8_t>(
response_data, response_data + java_response_position);
auto message_response = std::move(it->second);
//删除pending_responses_当中的PlatformMessageResponse
pending_responses_.erase(it);
//调用PlatformMessageResponse的Complete方法
message_response->Complete(
std::make_unique<fml::DataMapping>(std::move(response)));
}

上面降到过我们在调用 Engine.HandlePlatformMessage 方法时构造的是 PlatformMessageResponse 对象是的 PlatformMessageResponseDart

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//engine/lib/ui/window/platform_message_response_dart.h
class PlatformMessageResponseDart : public PlatformMessageResponse {
FML_FRIEND_MAKE_REF_COUNTED(PlatformMessageResponseDart);

public:
// Callable on any thread.
void Complete(std::unique_ptr<fml::Mapping> data) override;
void CompleteEmpty() override;

protected:
explicit PlatformMessageResponseDart(
tonic::DartPersistentValue callback,
fml::RefPtr<fml::TaskRunner> ui_task_runner);
~PlatformMessageResponseDart() override;

tonic::DartPersistentValue callback_;
fml::RefPtr<fml::TaskRunner> ui_task_runner_;
};
//engine/lib/ui/window/platform_message_response_dart.cc
void PlatformMessageResponseDart::Complete(std::unique_ptr<fml::Mapping> data) {
//判断在构造PlatformMessageResponseDart时的callback是否为空
if (callback_.is_empty())
return;
FML_DCHECK(!is_complete_);
is_complete_ = true;
ui_task_runner_->PostTask(fml::MakeCopyable(
[callback = std::move(callback_), data = std::move(data)]() mutable {
std::shared_ptr<tonic::DartState> dart_state =
callback.dart_state().lock();
if (!dart_state)
return;
tonic::DartState::Scope scope(dart_state);
Dart_Handle byte_buffer = WrapByteData(std::move(data));
//带着byte_buffer参数调用callback方法,回调到dart端
tonic::DartInvoke(callback.Release(), {byte_buffer});
}));
}

总结

事件由 dartC 再到 Java ,相应由 JavaC 再到 dart 的过程可以简单用一下步骤叙述:

1、Application启动的时候加载flutter的so文件;
2、在加载so的时候注册了一系列的相关平台的函数以及操作类;
3、dart调用C层的方法顺便将数据传递给C层;
4、C层调用相关平台的注册的类的对应方法,
5、对应平台进行数据处理并返回数据;
6、事件到达系统底层之后找到事件的相应的句柄进行回调;

在整个源码分析过程不免想了解到系统的更底层,结果引出我也解决不了的问题。哈哈尴尬^_^~!

真的很头痛。。。。

问题一:tonic

为什么要弄这样的宏定义?

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//tonic/dart_class_library.h
#define DART_NATIVE_CALLBACK_STATIC(CLASS, METHOD) \
static void CLASS##_##METHOD(Dart_NativeArguments args) { \
tonic::DartCallStatic(&CLASS::METHOD, args); \
}
//tonic/dart_args.h
template <typename Sig>
void DartCallStatic(Sig func, Dart_NativeArguments args) {
DartArgIterator it(args, 0);
using Indices = typename IndicesForSignature<Sig>::type;
DartDispatcher<Indices, Sig> decoder(&it);
if (it.had_exception())
return;
decoder.Dispatch(func);
}

问题二:Dart_handle&DartInvoke

DartInvoke 调用 Dart_Handle 方法就能执行 dart 的函数?Dart_handle 到底在 C 这一层是一个什么样的结构体,它的作用有什么?

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//tonic/logging/dart_invoke.cc
Dart_Handle DartInvoke(Dart_Handle closure,
std::initializer_list<Dart_Handle> args) {
int argc = args.size();
Dart_Handle* argv = const_cast<Dart_Handle*>(args.begin());
Dart_Handle handle = Dart_InvokeClosure(closure, argc, argv);
LogIfError(handle);
return handle;
}
//sdk/runtime/include/dart_api.h
DART_EXPORT DART_WARN_UNUSED_RESULT Dart_Handle
Dart_InvokeClosure(Dart_Handle closure,
int number_of_arguments,
Dart_Handle* arguments);

//dart/sdk/runtime/vm/dart_api_impl.cc
DART_EXPORT Dart_Handle Dart_InvokeClosure(Dart_Handle closure,
int number_of_arguments,
Dart_Handle* arguments) {
DARTSCOPE(Thread::Current());
API_TIMELINE_DURATION(T);
CHECK_CALLBACK_STATE(T);
const Instance& closure_obj = Api::UnwrapInstanceHandle(Z, closure);
if (closure_obj.IsNull() || !closure_obj.IsCallable(NULL)) {
RETURN_TYPE_ERROR(Z, closure, Instance);
}
if (number_of_arguments < 0) {
return Api::NewError(
"%s expects argument 'number_of_arguments' to be non-negative.",
CURRENT_FUNC);
}

// Set up arguments to include the closure as the first argument.
const Array& args = Array::Handle(Z, Array::New(number_of_arguments + 1));
Object& obj = Object::Handle(Z);
args.SetAt(0, closure_obj);
for (int i = 0; i < number_of_arguments; i++) {
obj = Api::UnwrapHandle(arguments[i]);
if (!obj.IsNull() && !obj.IsInstance()) {
RETURN_TYPE_ERROR(Z, arguments[i], Instance);
}
args.SetAt(i + 1, obj);
}
// Now try to invoke the closure.
return Api::NewHandle(T, DartEntry::InvokeClosure(args));
}

文章到这里就全部讲述完啦,若有其他需要交流的可以留言哦~!~!

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