Android输入输出机制之来龙去脉

openInputChannelPair(

阅读本文的前提条件是知道匿名管道和匿名共享内存是怎么一回事，否则阅读相应的文章。

Anonymous pipes 和Anonymous Shared Memory。

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首先ViewRoot的SetView方法中的关键地方：

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第一处是创建：

 mInputChannel = new InputChannel(); try {                    res = sWindowSession.add(mWindow, mWindowAttributes,                            getHostVisibility(), mAttachInfo.mContentInsets,                            mInputChannel);}

第二处是注册：

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 InputQueue.registerInputChannel(mInputChannel, mInputHandler,                            Looper.myQueue());

创建部分的第一个方法InputChanel()构造函数是个空函数。重要的是第二个函数，

                    res = sWindowSession.add(mWindow, mWindowAttributes,                            getHostVisibility(), mAttachInfo.mContentInsets,                            mInputChannel);

?这个函数调用的是系统服务，所谓的系统服务，就是运行在SYstem进程的服务程序。代码进入到了android系统服务进程的WindowManagerService类的Session类的add方法，下面是add方法：

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        public int add(IWindow window, WindowManager.LayoutParams attrs,                int viewVisibility, Rect outContentInsets, InputChannel outInputChannel) {            return addWindow(this, window, attrs, viewVisibility, outContentInsets,                    outInputChannel);        }

?add调用addWindow，下面进入addWindow，addWindow比较长，仅仅列出重要的几行代码：

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            if (outInputChannel != null) {                String name = win.makeInputChannelName();                InputChannel[] inputChannels = InputChannel.openInputChannelPair(name);                win.mInputChannel = inputChannels[0];                inputChannels[1].transferToBinderOutParameter(outInputChannel);                                mInputManager.registerInputChannel(win.mInputChannel);            }

?这里就牵涉到了匿名管道了，进入OpenInputChannelPair来看，调用了nativeOpenInputChannelPair，下面看nativeOpenInputChannelPair做了什么事情：

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static jobjectArray android_view_InputChannel_nativeOpenInputChannelPair(JNIEnv* env,        jclass clazz, jstring nameObj) {    const char* nameChars = env->GetStringUTFChars(nameObj, NULL);    String8 name(nameChars);    env->ReleaseStringUTFChars(nameObj, nameChars);    sp<InputChannel> serverChannel;    sp<InputChannel> clientChannel;    status_t result = InputChannel::openInputChannelPair(name, serverChannel, clientChannel);}

?最重要的是

? status_t result = InputChannel::openInputChannelPair(name, serverChannel, clientChannel);这一行

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status_t InputChannel::openInputChannelPair(const String8& name,        sp<InputChannel>& outServerChannel, sp<InputChannel>& outClientChannel) {    status_t result;    int serverAshmemFd = ashmem_create_region(name.string(), DEFAULT_MESSAGE_BUFFER_SIZE);    if (serverAshmemFd < 0) {        result = -errno;        LOGE("channel '%s' ~ Could not create shared memory region. errno=%d",                name.string(), errno);    } else {        result = ashmem_set_prot_region(serverAshmemFd, PROT_READ | PROT_WRITE);        if (result < 0) {            LOGE("channel '%s' ~ Error %d trying to set protection of ashmem fd %d.",                    name.string(), result, serverAshmemFd);        } else {            // Dup the file descriptor because the server and client input channel objects that            // are returned may have different lifetimes but they share the same shared memory region.            int clientAshmemFd;            clientAshmemFd = dup(serverAshmemFd);            if (clientAshmemFd < 0) {                result = -errno;                LOGE("channel '%s' ~ Could not dup() shared memory region fd. errno=%d",                        name.string(), errno);            } else {                int forward[2];                if (pipe(forward)) {                    result = -errno;                    LOGE("channel '%s' ~ Could not create forward pipe.  errno=%d",                            name.string(), errno);                } else {                    int reverse[2];                    if (pipe(reverse)) {                        result = -errno;                        LOGE("channel '%s' ~ Could not create reverse pipe.  errno=%d",                                name.string(), errno);                    } else {                        String8 serverChannelName = name;                        serverChannelName.append(" (server)");                        outServerChannel = new InputChannel(serverChannelName,                                serverAshmemFd, reverse[0], forward[1]);                        String8 clientChannelName = name;                        clientChannelName.append(" (client)");                        outClientChannel = new InputChannel(clientChannelName,                                clientAshmemFd, forward[0], reverse[1]);                        return OK;                    }                    ::close(forward[0]);                    ::close(forward[1]);                }                ::close(clientAshmemFd);            }        }        ::close(serverAshmemFd);    }    outServerChannel.clear();    outClientChannel.clear();    return result;}

?? 这段代码又长又臭，总而言之就是创建用来【发送和接受信号】的接受和发送描述符，和生成用来【传递事件】的匿名共享内存，生成InputChannel对象。创建好之后，AddWindow方法通过BInder机制返回给【用户进程】。?? 客户端对应的是【应用程序】（读），服务端对应的是【InputDispatcher】（写）。

?? 理解本段代码的关键是：代码中的 reverse和forward是相对于server来说的。对于server来说，后向管道用来接收，前向管道用来发送。函数pipe出来的值，数组的0索引对应的描述符是发送端。1对应的是接收端。

?? 上面的介绍基本上就结束了。后面也许，我们更想知道的是这两个InputChannel如何通信的。一个在ViewRoot中，一个在InputDiapacher中。通信方式几本上就是，

?? InputReader（InputReader.cpp中）启动无限循环，读取一个事件，发送给InputDispacher，InputDispatcher把事件写入到共享内存，并通过管道发送信号给ViewRoot中的InputChannel，InputChannel收到信号后，通过InputConsumer的consume方法来把事件发送给VIewRoot中的InputChannel。

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