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。

 

共享内存缓冲区大小,大概缓存20个触屏事件。

framework/base/include/ui/Input.h

/*
 * Maximum number of pointers supported per motion event.
 * Smallest number of pointers is 1.
 * (We want at least 10 but some touch controllers obstensibly configured for 10 pointers
 * will occasionally emit 11.  There is not much harm making this constant bigger.)
 */
#define MAX_POINTERS 16

framework/base/libs/ui/InputTransport.cpp


#define ROUND_UP(value, boundary) (((value) + (boundary) - 1) & ~((boundary) - 1))
#define MIN_HISTORY_DEPTH 20

// Must be at least sizeof(InputMessage) + sufficient space for pointer data
static const int DEFAULT_MESSAGE_BUFFER_SIZE = ROUND_UP(
        sizeof(InputMessage) + MIN_HISTORY_DEPTH
                * (sizeof(InputMessage::SampleData) + MAX_POINTERS * sizeof(PointerCoords)),
        4096);

 

 

 

JNI文件记录

InputQueue.java

android_view_InputQueue.cpp

android_view_InputChannel.cpp

 

InputManager.java

com_android_server_InputManager.cpp

 

framework/base/services/input/InputManager.h

framework/base/services/input/InputManager.cpp

 

framework/base/services/input/InputDispatcher.cpp

framework/base/services/input/InputDispatcher.cpp

 

framework/base/libs/ui/InputTransport.cpp

 

 

 

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 serverChannel;
    sp 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& outServerChannel, sp& 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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