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。

首先ViewRoot的SetView方法中的关键地方：

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：

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：

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：

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：

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

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。