Java代码
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- 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:
Java代码
- mInputChannel = new InputChannel();
- try {
- res = sWindowSession.add(mWindow, mWindowAttributes,
- getHostVisibility(), mAttachInfo.mContentInsets,
- mInputChannel);
第二处是注册:
Java代码
- 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,
Java代码
- 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:
Java代码
- 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:
Java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:
Cpp代码
- 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
Cpp代码
- 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 {
-
-
- 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。