1、如果 Flutter 要获取设备的电量信息怎么办？
2、如果 Flutter 要实时监控网络状态怎么办？
由于 Flutter 特点如下：
Flutter is Google’s UI toolkit for building beautiful, natively compiled applications for mobile, web, and desktop from a single codebase.
1、Flutter 是一个跨平台的 UI 库，专注于构建高效的 UI。
2、多平台的支持，下图是 Flutter 目前支持的平台，每个平台的都有自己的平台特性。
基于以上两点，目前 Flutter 如果要和平台相关的部分通信需要有一个通道即 PlatformChannel。
三种类型的 channel 都定义在 platform_channel.dart 中，从源码中可以看到三种 channel 都用到了以下三个属性。
name：String 类型，表示 channel 的名字，全局唯一（The logical channel on which communication happens）codec：MessageCodec 类型，消息的编码解码器（The message codec used by this channel）binaryMessenger：BinaryMessenger类型，用于发送数据（The messenger used by this channel to send platform messages）channel 的名字，每个 Flutter 应用可能有多个 channel，但是每个 channel 必须有一个唯一的名字。
codec 用来对数据编码解码，以便两端可以正确读取数据。
用于发送数据
method_channel_page.dart 主要代码
第一步
static const methodChannel = MethodChannel("method_channel_sample");
第二步
Future<dynamic> getUserInfo(String method, {String userName}) async {
return await methodChannel.invokeMethod(method, userName);
}
第三步
MaterialButton(
color: Colors.blue,
textColor: Colors.white,
child: new Text('获取 snow 用户信息'),
onPressed: () {
getUserInfo("getInfo", userName: "snow")
..then((result) {
setState(() {
messageFromNative = result;
});
});
},
),
MainActivity.java 主要代码
private void addMethodChannel() {
mMethodChannel = new MethodChannel(getFlutterView(), "method_channel_sample");
mMethodChannel.setMethodCallHandler((methodCall, result) -> {
String method = methodCall.method;
if ("getInfo".equals(method)) {
String userName = (String) methodCall.arguments;
if (userName.equals("rocx")) {
String user = "name:rocx, age:18";
result.success(user);
} else {
result.success("user not found");
invokeSayHelloMethod();
}
}
});
}
从以上代码可以看出
Dart 调用 Android 代码分三步。首先在 Dart 端定义 MethodChannel 名字为 method_channel_sample。然后定义getUserInfo方法，传入要调用的方法名和参数。最后点击按钮执行方法，获取用户信息。
在 Android 端定一个 MethodChannel 名字和 Dart 端保持一致。设置 MethodCallHandler。当调用的是getInfo方法时，根据参数返回信息。
MainActivity.java 主要代码
private void invokeSayHelloMethod() {
mMethodChannel.invokeMethod("sayHello", "", new MethodChannel.Result() {
@Override
public void success(Object o) {
Toast.makeText(MainActivity.this, o.toString(), Toast.LENGTH_LONG).show();
}
@Override
public void error(String s, String s1, Object o) {
}
@Override
public void notImplemented() {
}
});
}
method_channel_page.dart 主要代码
Future<dynamic> addHandler(MethodCall call) async {
switch (call.method) {
case "sayHello":
return "Hello from Flutter";
break;
}
}
@override
void initState() {
super.initState();
methodChannel.setMethodCallHandler(addHandler);
}
从代码可以看出，在 Dart 端设置 MethodCallHandler 然后在 Android 端调用即可。
basic_message_channel_page.dart 主要代码
第一步
static const basicMessageChannel = BasicMessageChannel(
"basic_message_channel_sample", StandardMessageCodec());
第二步
Future<dynamic> sayHelloToNative(String message) async {
String reply = await basicMessageChannel.send(message);
setState(() {
msgReplyFromNative = reply;
});
return reply;
}
第三步
MaterialButton(
color: Colors.blue,
textColor: Colors.white,
child: new Text('say hello to native'),
onPressed: () {
sayHelloToNative("hello");
},
),
MainActivity.java 主要代码
private void addBasicMessageChannel() {
mBasicMessageChannel = new BasicMessageChannel<>(getFlutterView(), "basic_message_channel_sample", StandardMessageCodec.INSTANCE);
mBasicMessageChannel.setMessageHandler((object, reply) -> {
reply.reply("receive " + object.toString() + " from flutter");
mBasicMessageChannel.send("native say hello to flutter");
});
}
从以上代码可以看出
Dart 向 Android 发送消息依然分为三步。首先在 Dart 端定义 BasicMessageChannel 名字为 basic_message_channel_sample。然后定义发送消息的方法sayHelloToNative。最后点击按钮向 Android 端发送消息。
在 Android 端定一个 BasicMessageChannel 名字和 Dart 端保持一致。设置 MethodCallHandler。当收到消息时发一个回复。
MainActivity.java 主要代码
mBasicMessageChannel.send("native say hello to flutter");
basic_message_channel_page.dart 主要代码
Future<dynamic> addHandler(Object result) async {
setState(() {
msgReceiveFromNative = result.toString();
});
}
void addMessageListener() {
basicMessageChannel.setMessageHandler(addHandler);
}
@override
void initState() {
super.initState();
addMessageListener();
}
从代码可以看出，在 Dart 端设置 MessageHandler 然后在 Android 端直接发送消息即可。
event_channel_page.dart 主要代码
第一步
static const eventChannel = EventChannel("event_channel_sample");
void _onEvent(Object event) {
setState(() {
if (_streamSubscription != null) {
eventMessage = event.toString();
}
});
}
void _onError(Object error) {
setState(() {
if (_streamSubscription != null) {
eventMessage = "error";
}
});
}
@override
void initState() {
super.initState();
eventMessage = "";
第二步
_streamSubscription = eventChannel
.receiveBroadcastStream()
.listen(_onEvent, onError: _onError);
}
MainActivity.java 主要代码
private void addEventChannel() {
mEventChannel = new EventChannel(getFlutterView(), "event_channel_sample");
mEventChannel.setStreamHandler(new EventChannel.StreamHandler() {
@Override
public void onListen(Object o, EventChannel.EventSink eventSink) {
task = new TimerTask() {
@Override
public void run() {
runOnUiThread(() -> eventSink.success("i miss you " + System.currentTimeMillis()));
}
};
timer = new Timer();
timer.schedule(task, 2000, 3000);
}
@Override
public void onCancel(Object o) {
task.cancel();
timer.cancel();
task = null;
timer = null;
}
});
}
Dart 接受 Android stream event。首先在 Dart 端定义 EventChannel 名字为 event_channel_sample。然后设置receiveBroadcastStream监听，当 Android 端有消息发过来会回调_onEvent方法。
在 Android 端启动一个定时器，每隔3s向 Dart 端发送一次消息。
如下图，在 Dart 与 Platform 通信过程中，通过 channel name 找到对方，然后把消息通过 codec 进行编解码，最后通过 binaryMessenger 进行发送。
@optionalTypeArgs
Future<T> invokeMethod<T>(String method, [ dynamic arguments ]) async {
assert(method != null);
final ByteData result = await binaryMessenger.send(
name,
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;
}
@override
Future<ByteData> send(String channel, ByteData message) {
final MessageHandler handler = _mockHandlers[channel];
if (handler != null)
return handler(message);
return _sendPlatformMessage(channel, message);
}
Future<ByteData> _sendPlatformMessage(String channel, ByteData message) {
final Completer<ByteData> completer = Completer<ByteData>();
// ui.window is accessed directly instead of using ServicesBinding.instance.window
// because this method might be invoked before any binding is initialized.
// This issue was reported in #27541. It is not ideal to statically access
// ui.window because the Window may be dependency injected elsewhere with
// a different instance. However, static access at this location seems to be
// the least bad option.
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: ErrorDescription('during a platform message response callback'),
));
}
});
return completer.future;
}
void sendPlatformMessage(String name,
ByteData data,
PlatformMessageResponseCallback callback) {
final String error =
_sendPlatformMessage(name, _zonedPlatformMessageResponseCallback(callback), data);
if (error != null)
throw Exception(error);
}
String _sendPlatformMessage(String name,
PlatformMessageResponseCallback callback,
ByteData data) native 'Window_sendPlatformMessage';
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) {
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();
if (!dart_state->window()) {
return tonic::ToDart(
"Platform messages can only be sent from the main isolate");
}
fml::RefPtr<PlatformMessageResponse> response;
if (!Dart_IsNull(callback)) {
response = fml::MakeRefCounted<PlatformMessageResponseDart>(
tonic::DartPersistentValue(dart_state, callback),
dart_state->GetTaskRunners().GetUITaskRunner());
}
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();
}
WindowClient* client() const { return client_; }
class RuntimeController final : public WindowClient {
...
// |WindowClient|
void HandlePlatformMessage(fml::RefPtr<PlatformMessage> message) override;
...
}
void RuntimeController::HandlePlatformMessage(
fml::RefPtr<PlatformMessage> message) {
client_.HandlePlatformMessage(std::move(message));
}
RuntimeDelegate& p_client
class Engine final : public RuntimeDelegate {
...
// |RuntimeDelegate|
void HandlePlatformMessage(fml::RefPtr<PlatformMessage> message) override;
...
}
void Engine::HandlePlatformMessage(fml::RefPtr<PlatformMessage> message) {
if (message->channel() == kAssetChannel) {
HandleAssetPlatformMessage(std::move(message));
} else {
delegate_.OnEngineHandlePlatformMessage(std::move(message));
}
}
// |Engine::Delegate|
void OnEngineHandlePlatformMessage(
fml::RefPtr<PlatformMessage> message) override;
// |Engine::Delegate|
void Shell::OnEngineHandlePlatformMessage(
fml::RefPtr<PlatformMessage> message) {
FML_DCHECK(is_setup_);
FML_DCHECK(task_runners_.GetUITaskRunner()->RunsTasksOnCurrentThread());
if (message->channel() == kSkiaChannel) {
HandleEngineSkiaMessage(std::move(message));
return;
}
task_runners_.GetPlatformTaskRunner()->PostTask(
[view = platform_view_->GetWeakPtr(), message = std::move(message)]() {
if (view) {
view->HandlePlatformMessage(std::move(message));
}
});
}
class PlatformViewAndroid final : public PlatformView {
...
// |PlatformView|
void HandlePlatformMessage(
fml::RefPtr<flutter::PlatformMessage> message) override;
...
}
// |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;
if (auto response = message->response()) {
response_id = next_response_id_++;
pending_responses_[response_id] = response;
}
auto java_channel = fml::jni::StringToJavaString(env, message->channel());
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 {
message = nullptr;
// This call can re-enter in InvokePlatformMessageXxxResponseCallback.
FlutterViewHandlePlatformMessage(env, view.obj(), java_channel.obj(),
nullptr, response_id);
}
}
static jmethodID g_handle_platform_message_method = 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));
}
g_handle_platform_message_method =
env->GetMethodID(g_flutter_jni_class->obj(), "handlePlatformMessage",
"(Ljava/lang/String;[BI)V");
g_flutter_jni_class = new fml::jni::ScopedJavaGlobalRef<jclass>(
env, env->FindClass("io/flutter/embedding/engine/FlutterJNI"));
if (g_flutter_jni_class->is_null()) {
FML_LOG(ERROR) << "Failed to find FlutterJNI Class.";
return false;
}
private void handlePlatformMessage(@NonNull String channel, byte[] message, int replyId) {
if (this.platformMessageHandler != null) {
this.platformMessageHandler.handleMessageFromDart(channel, message, replyId);
}
}
public void handleMessageFromDart(@NonNull String channel, @Nullable byte[] message, int replyId) {
Log.v("DartMessenger", "Received message from Dart over channel '" + channel + "'");
BinaryMessageHandler handler = (BinaryMessageHandler)this.messageHandlers.get(channel);
if (handler != null) {
try {
Log.v("DartMessenger", "Deferring to registered handler to process message.");
ByteBuffer buffer = message == null ? null : ByteBuffer.wrap(message);
handler.onMessage(buffer, new DartMessenger.Reply(this.flutterJNI, replyId));
} catch (Exception var6) {
Log.e("DartMessenger", "Uncaught exception in binary message listener", var6);
this.flutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);
}
} else {
Log.v("DartMessenger", "No registered handler for message. Responding to Dart with empty reply message.");
this.flutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);
}
}
Writing custom platform-specific code
platform channel 官方示例
深入理解Flutter Platform Channel
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