通过上文分析,我们知道FlutterViewHandlePlatformMessage()实际上是通过JNI的方式最终调用了FlutterJNI.java中的handlePlatformMessage()方法,该方法接受三个来自Native层的参数:
- channel: String类型,表示Channel名称.
- message: 字节数组,表示方法调用中的数据,如方法名和参数.
- replyId: int类型,在将此次调用的响应数据从Java层写回到Native层时用到
public class FlutterJNI {
private PlatformMessageHandler platformMessageHandler;
@UiThread
public void setPlatformMessageHandler(@Nullable PlatformMessageHandler platformMessageHandler) {
this.platformMessageHandler = platformMessageHandler;
}
// Called by native.
@SuppressWarnings("unused")
private void handlePlatformMessage(final String channel, byte[] message, final int replyId) {
if (platformMessageHandler != null) {
platformMessageHandler.handleMessageFromDart(channel, message, replyId);
}
}
}
FlutterJNI类定义了Java层和Flutter C/C++引擎之间的相关接口.此类目前处于实验性质,随着后续的发展可能会被不断的重构和优化,不保证一直存在,不建议开发者调用该类.
为了建立Android应用和Flutter C/C++引擎的连接,需要创建FlutterJNI实例,然后将其attach到Native,常见的使用方法如下:
// 1.创建FlutterJNI实例
FlutterJNI flutterJNI = new FlutterJNI();
// 2.建立和Native层的连接
flutterJNI.attachToNative();
......
// 3.断开和Native层的连接,并释放资源
flutterJNI.detachFromNativeAndReleaseResources();
FlutterJNI中handlePlatformMessage(),在该方法中首先判断platformMessageHandler是否为null,不为null,则调用其handleMessageFromDart()方法.其中platformMessageHandler需要通过FlutterJNI中的setPlatformMessageHandler()方法来设置.在FlutterNativeView中调用的。
public class FlutterNativeView implements BinaryMessenger {
private final Map mMessageHandlers;
private int mNextReplyId = 1;
private final Map mPendingReplies = new HashMap<>();
private final FlutterPluginRegistry mPluginRegistry;
private FlutterView mFlutterView;
private FlutterJNI mFlutterJNI;
private final Context mContext;
private boolean applicationIsRunning;
public FlutterNativeView(Context context, boolean isBackgroundView) {
mContext = context;
mPluginRegistry = new FlutterPluginRegistry(this, context);
// 创建FlutterJNI实例
mFlutterJNI = new FlutterJNI();
mFlutterJNI.setRenderSurface(new RenderSurfaceImpl());
// 将PlatformMessageHandlerImpl实例赋值给FlutterJNI中的platformMessageHandler属性
mFlutterJNI.setPlatformMessageHandler(new PlatformMessageHandlerImpl());
mFlutterJNI.addEngineLifecycleListener(new EngineLifecycleListenerImpl());
attach(this, isBackgroundView);
assertAttached();
mMessageHandlers = new HashMap<>();
}
.......
}
在FlutterNativeView的构造函数中,首先创建FlutterJNI实例mFlutterJNI,然后调用setPlatformMessageHandler()并把PlatformMessageHandlerImpl实例作为参数传入.因此在FlutterJNI的handlePlatformMessage()方法中,最终调用PlatformMessageHandlerImpl实例的handleMessageFromDart()来处理来自Flutter中的消息.
public class FlutterNativeView implements BinaryMessenger {
private final Map mMessageHandlers;
......
private final class PlatformMessageHandlerImpl implements PlatformMessageHandler {
// Called by native to send us a platform message.
public void handleMessageFromDart(final String channel, byte[] message, final int replyId) {
// 1.根据channel名称获取对应的BinaryMessageHandler对象.每个Channel对应一个Handler对象
BinaryMessageHandler handler = mMessageHandlers.get(channel);
if (handler != null) {
try {
// 2.将字节数组对象封装为ByteBuffer对象
final ByteBuffer buffer = (message == null ? null : ByteBuffer.wrap(message));
// 3.调用handler对象的onMessage()方法来分发消息
handler.onMessage(buffer, new BinaryReply() {
private final AtomicBoolean done = new AtomicBoolean(false);
@Override
public void reply(ByteBuffer reply) {
// 4.根据reply的情况,调用FlutterJNI中invokePlatformMessageXXX()方法将响应数据发送给Flutter层
if (reply == null) {
mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);
} else {
mFlutterJNI.invokePlatformMessageResponseCallback(replyId, reply, reply.position());
}
}
});
} catch (Exception exception) {
mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);
}
return;
}
mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);
}
}
以Channel名称作为key,以BinaryMessageHandler类型为value.在handleMessageFromDart()方法中,首先根据Channel名称从mMessageHandlers取出对应的二进制消息处理器BinaryMessageHandler,然后将字节数组message封装为ByteBuffer对象,然后调用BinaryMessageHandler实例的onMessage()方法处理ByteBuffer,并进行响应.
BinaryReply是一个接口,主要用来将ByteBuffer类型的响应数据reply从Java层写回到Flutter层.根据reply是否为null,调用FlutterJNI实例不同的方法.
BinaryMessageHandler是如何添加到mMessageHandler中:
public class FlutterNativeView implements BinaryMessenger {
private final Map mMessageHandlers;
......
@Override
public void setMessageHandler(String channel, BinaryMessageHandler handler) {
if (handler == null) {
mMessageHandlers.remove(channel);
} else {
mMessageHandlers.put(channel, handler);
}
}
.......
}
public class MainActivity extends FlutterActivity {
// 1.定义Channel的名称,该名称作为Channel的唯一标识符
private static final String CHANNEL = "samples.flutter.io/battery";
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// 2.创建MethodChannel对象channel
MethodChannel channel = new MethodChannel(getFlutterView(), CHANNEL);
// 3.调用MethodChannel实例的setMethodCallHandler()方法为当前channel设置Handler
channel.setMethodCallHandler(
new MethodCallHandler() {
@Override
public void onMethodCall(MethodCall call, Result result) {
// TODO
}
});
}
}
接下来是 MethodChannel 定义
public final class MethodChannel {
// 二进制信使
private final BinaryMessenger messenger;
// Channel名称
private final String name;
// 方法编码
private final MethodCodec codec;
public MethodChannel(BinaryMessenger messenger, String name) {
this(messenger, name, StandardMethodCodec.INSTANCE);
}
public MethodChannel(BinaryMessenger messenger, String name, MethodCodec codec) {
assert messenger != null;
assert name != null;
assert codec != null;
this.messenger = messenger;
this.name = name;
this.codec = codec;
}
......
public void setMethodCallHandler(final @Nullable MethodCallHandler handler) {
messenger.setMessageHandler(name,
handler == null ? null : new IncomingMethodCallHandler(handler));
}
......
private final class IncomingMethodCallHandler implements BinaryMessageHandler {
private final MethodCallHandler handler;
IncomingMethodCallHandler(MethodCallHandler handler) {
this.handler = handler;
}
@Override
public void onMessage(ByteBuffer message, final BinaryReply reply) {
// 1.使用codec对来自Flutter方法调用数据进行解码,并将其封装为MethodCall对象.
// MethodCall中包含两部分数据:method表示要调用的方法;arguments表示方法所需参数
final MethodCall call = codec.decodeMethodCall(message);
try {
// 2.调用自定义MethodCallHandler中的onMethodCall方法继续处理方法调用
handler.onMethodCall(call, new Result() {
@Override
public void success(Object result) {
// 调用成功时,需要回传数据给Flutter层时,使用codec对回传数据result
// 进行编码
reply.reply(codec.encodeSuccessEnvelope(result));
}
@Override
public void error(String errorCode, String errorMessage, Object errorDetails) {
// 调用失败时,需要回传错误数据给Flutter层时,使用codec对errorCode,
// errorMessage,errorDetails进行编码
reply.reply(codec.encodeErrorEnvelope(errorCode, errorMessage, errorDetails));
}
@Override
public void notImplemented() {
// 方法没有实现时,调用该方法后,flutter将会受到相应的错误消息
reply.reply(null);
}
});
} catch (RuntimeException e) {
Log.e(TAG + name, "Failed to handle method call", e);
reply.reply(codec.encodeErrorEnvelope("error", e.getMessage(), null));
}
}
}
}
在上述代码中,首先使用codec对来自Flutter层的二进制数据进行解码,并将其封装为MethodCall对象,然后调用MethodCallHandler.onMethodCall()方法.
调用过程
Java->Native
public class FlutterJNI {
private Long nativePlatformViewId;
......
@UiThread
public void invokePlatformMessageResponseCallback(int responseId, ByteBuffer message, int position) {
// 1.检查FlutterJNI是否已经attach到Native层,如若没有则抛出异常
ensureAttachedToNative();
// 2.继续调用nativeInvokePlatformMessageResponseCallback()
nativeInvokePlatformMessageResponseCallback(
nativePlatformViewId,
responseId,
message,
position
);
}
private native void nativeInvokePlatformMessageResponseCallback(
long nativePlatformViewId,
int responseId,
ByteBuffer message,
int position
);
......
private void ensureAttachedToNative() {
// FlutterJNI attach到Native层后,会返回一个long类型的值用来初始化nativePlatformViewId
if (nativePlatformViewId == null) {
throw new RuntimeException("Cannot execute operation because FlutterJNI is not attached to native.");
}
}
}
当数据需要写回时,数据首先通过codec被编码成ByteBuffer类型,然后调用reply的reply()方法.在reply()方法中,对于非null类型的ByteBuffer,会调用FlutterJNI中的invokePlatformMessageResponseCallback().
在上述invokePlatformMessageResponseCallback()方法中,首先检查当前FlutterJNI实例是否已经attach到Native层,然后调用Native方法nativeInvokePlatformMessageResponseCallback()向JNI层写入数据
void PlatformViewAndroid::InvokePlatformMessageResponseCallback(
JNIEnv* env,
jint response_id,
jobject java_response_data,
jint java_response_position) {
if (!response_id)
return;
// 1.通过response_id从pending_responses_中取出response
auto it = pending_responses_.find(response_id);
if (it == pending_responses_.end())
return;
// 2.GetDirectBufferAddress函数返回一个指向被传入的ByteBuffer对象的地址指针
uint8_t* response_data =
static_cast(env->GetDirectBufferAddress(java_response_data));
std::vector response = std::vector(
response_data, response_data + java_response_position);
auto message_response = std::move(it->second);
// 3.从pending_responses_中移除该response
pending_responses_.erase(it);
// 4.调用response的Complete()方法将二进制结果返回
message_response->Complete(
std::make_unique(std::move(response)));
}
Native -> Dart
void PlatformMessageResponseDart::Complete(std::unique_ptr data) {
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 dart_state =
callback.dart_state().lock();
if (!dart_state)
return;
tonic::DartState::Scope scope(dart_state);
// 将Native层的二进制数据data转为Dart中的二进制数据byte_buffer
Dart_Handle byte_buffer = WrapByteData(std::move(data));
tonic::DartInvoke(callback.Release(), {byte_buffer});
}));
}