文章将分析Android 的Input Event 子系统的来龙去脉。
Android 系统里面有很多小工具，运行这些工具，我们对它们有一个感性的认识，进而阅读和分析这些小工具源代码，再顺藤摸瓜，就可以把整个子系统的来龙去脉弄清楚。
1.运行toolbox的getevent 工具。
# getevent -help
getevent -help
Usage: getevent [-t] [-n] [-s switchmask] [-S] [-v [mask]] [-p] [-q] [-c count] [-r] [device]
-t: show time stamps
-n: don‘t print newlines
-s: print switch states for given bits
-S: print all switch states
-v: verbosity mask (errs=1, dev=2, name=4, info=8, vers=16, pos. events=32)
-p: show possible events (errs, dev, name, pos. events)
-q: quiet (clear verbosity mask)
-c: print given number of events then exit
-r: print rate events are received
# getevent -c 20
getevent -c 20
add device 1: /dev/input/event4
name: "sensor-input"
add device 2: /dev/input/event3
name: "88pm860x_hook"
add device 3: /dev/input/event2
name: "88pm860x_on"
add device 4: /dev/input/event1
name: "88pm860x-touch"
add device 5: /dev/input/event0
name: "pxa27x-keypad"
/dev/input/event0: 0001 0066 00000001
/dev/input/event0: 0000 0000 00000000
/dev/input/event0: 0001 0066 00000000
/dev/input/event0: 0000 0000 00000000
/dev/input/event1: 0003 0000 00000c48
/dev/input/event1: 0003 0001 00000751
/dev/input/event1: 0001 014a 00000001
/dev/input/event1: 0000 0000 00000000
/dev/input/event1: 0003 0000 00000c67
/dev/input/event1: 0003 0001 000006f9
/dev/input/event1: 0000 0000 00000000
/dev/input/event1: 0003 0000 00000c9e
/dev/input/event1: 0003 0001 0000069e
/dev/input/event1: 0000 0000 00000000
/dev/input/event1: 0003 0000 00000cc4
/dev/input/event1: 0003 0001 00000620
/dev/input/event1: 0000 0000 00000000
/dev/input/event1: 0003 0000 00000ce8
/dev/input/event1: 0003 0001 000005ba
/dev/input/event1: 0000 0000 00000000
运行这个工具，然后按键或者滑动触摸屏，会看到程序会实时打印event。从上面的输出来看，系统有5个input 子系统。它们分别是
add device 1: /dev/input/event4
name: "sensor-input"
#Sensor input 子系统
add device 2: /dev/input/event3
name: "88pm860x_hook"
#耳机Hook键子系统。可支持接电话挂电话的耳机上面有一个按键，对应的就是这个input 子系统。
add device 3: /dev/input/event2
name: "88pm860x_on"
#开机键 input 子系统
add device 4: /dev/input/event1
name: "88pm860x-touch"
#Touch Screen input 子系统
add device 5: /dev/input/event0
name: "pxa27x-keypad"
#按键子系统，包括Home/Menu/Back等按键。
可以尝试多种event，实际感觉一下出来的log。
2.阅读getevent的代码。代码为./core/toolbox/getevent.c
从代码中，我们知道，程序在while(1)的一个死循环里，不断地在读取 (select 操作)/dev/input 下面的文件，检查是否Kernel往里面更新内容，如果有内容更新，就把它打印出来。并且从代码中，我们还知道，任何一个event都有三种属性，type,code,value.
while(1) {
pollres = poll(ufds, nfds, -1);
//printf("poll %d, returned %d/n", nfds, pollres);
if(ufds[0].revents & POLLIN) {
read_notify(device_path, ufds[0].fd, print_flags);
}
for(i = 1; i < nfds; i++) {
if(ufds[i].revents) {
if(ufds[i].revents & POLLIN) {
res = read(ufds[i].fd, &event, sizeof(event));
if(res < (int)sizeof(event)) {
fprintf(stderr, "could not get event/n");
return 1;
}
if(get_time) {
printf("%ld-%ld: ", event.time.tv_sec, event.time.tv_usec);
}
if(print_device)
printf("%s: ", device_names[i]);
printf("%04x %04x %08x", event.type, event.code, event.value);
if(sync_rate && event.type == 0 && event.code == 0) {
int64_t now = event.time.tv_sec * 1000000LL + event.time.tv_usec;
if(last_sync_time)
printf(" rate %lld", 1000000LL / (now - last_sync_time));
last_sync_time = now;
}
printf("%s", newline);
if(event_count && --event_count == 0)
return 0;
}
}
}
3.问题来了，Android Framework是否也是一样的原理呢？？猜测应该是一样的才对，不然这个工具就没有调试的价值了。
我们来阅读和分析framework中input event的相关代码。
我们从Kernel层往上看，先看看Framework中，直接操纵/dev/input设备的代码。
在.frameworks/base/libs/ui/EventHub.cpp 中，我们看到跟getevent工具类似的代码。
bool EventHub::getEvent(int32_t* outDeviceId, int32_t* outType,
int32_t* outScancode, int32_t* outKeycode, uint32_t *outFlags,
int32_t* outValue, nsecs_t* outWhen)
{
....
while(1) {
....
release_wake_lock(WAKE_LOCK_ID);
pollres = poll(mFDs, mFDCount, -1);
acquire_wake_lock(PARTIAL_WAKE_LOCK, WAKE_LOCK_ID);
if (pollres <= 0) {
if (errno != EINTR) {
LOGW("select failed (errno=%d)/n", errno);
usleep(100000);
}
continue;
}
....
// mFDs[0] is used for inotify, so process regular events starting at mFDs[1]
for(i = 1; i < mFDCount; i++) {
if(mFDs[i].revents) {
LOGV("revents for %d = 0x%08x", i, mFDs[i].revents);
if(mFDs[i].revents & POLLIN) {
res = read(mFDs[i].fd, &iev, sizeof(iev));
if (res == sizeof(iev)) {
LOGV("%s got: t0=%d, t1=%d, type=%d, code=%d, v=%d",
mDevices[i]->path.string(),
....
}
4.那么framework中那个模块再调用EventHub呢，接着往下查。
在framework目录中，输入下面的命令查找
# find . -name "*.cpp" |grep -v EventHub | xargs grep EventHub
./base/services/jni/com_android_server_KeyInputQueue.cpp:#include <ui/EventHub.h>
./base/services/jni/com_android_server_KeyInputQueue.cpp:static sp<EventHub> gHub;
./base/services/jni/com_android_server_KeyInputQueue.cpp: sp<EventHub> hub = gHub;
./base/services/jni/com_android_server_KeyInputQueue.cpp: hub = new EventHub;
./base/services/jni/com_android_server_KeyInputQueue.cpp: sp<EventHub> hub = gHub;
./base/services/jni/com_android_server_KeyInputQueue.cpp: hub = new EventHub;
5.从查找结果中得知，在jni文件com_android_server_KeyInputQueue.cpp文件中有对EventHub进行调用。
打开并阅读com_android_server_KeyInputQueue.cpp文件得知，在下面的函数中调用了EventHub的getEvent函数
static jboolean
android_server_KeyInputQueue_readEvent(JNIEnv* env, jobject clazz,
jobject event)
{
gLock.lock();
sp<EventHub> hub = gHub;
if (hub == NULL) {
hub = new EventHub;
gHub = hub;
}
gLock.unlock();
int32_t deviceId;
int32_t type;
int32_t scancode, keycode;
uint32_t flags;
int32_t value;
nsecs_t when;
bool res = hub->getEvent(&deviceId, &type, &scancode, &keycode,
&flags, &value, &when);
env->SetIntField(event, gInputOffsets.mDeviceId, (jint)deviceId);
env->SetIntField(event, gInputOffsets.mType, (jint)type);
env->SetIntField(event, gInputOffsets.mScancode, (jint)scancode);
env->SetIntField(event, gInputOffsets.mKeycode, (jint)keycode);
env->SetIntField(event, gInputOffsets.mFlags, (jint)flags);
env->SetIntField(event, gInputOffsets.mValue, value);
env->SetLongField(event, gInputOffsets.mWhen,
(jlong)(nanoseconds_to_milliseconds(when)));
return res;
}
6.根据jni的调用规则，在本文件中查找对于的java函数。
static JNINativeMethod gInputMethods[] = {
/* name, signature, funcPtr */
{ "readEvent", "(Landroid/view/RawInputEvent;)Z",
(void*) android_server_KeyInputQueue_readEvent },
....
7. 接着顺藤摸瓜，找到对应的java文件,base/services/java/com/android/server/KeyInputQueue.java
private static native boolean readEvent(RawInputEvent outEvent);
在一个线程中会调用readEvent函数。
Thread mThread = new Thread("InputDeviceReader") {
public void run() {
if (DEBUG) Slog.v(TAG, "InputDeviceReader.run()");
android.os.Process.setThreadPriority(
android.os.Process.THREAD_PRIORITY_URGENT_DISPLAY);
RawInputEvent ev = new RawInputEvent();
while (true) {
try {
InputDevice di;
// block, doesn‘t release the monitor
readEvent(ev);
boolean send = false;
boolean configChanged = false;
if (false) {
Slog.i(TAG, "Input event: dev=0x"
+ Integer.toHexString(ev.deviceId)
+ " type=0x" + Integer.toHexString(ev.type)
+ " scancode=" + ev.scancode
+ " keycode=" + ev.keycode
+ " value=" + ev.value);
}
8.那是谁启动这个线程呢？？？查找mThread变量，得知在KeyInputQueue的构造函数中会启动这个线程。
KeyInputQueue(Context context, HapticFeedbackCallback hapticFeedbackCallback) {
if (MEASURE_LATENCY) {
lt = new LatencyTimer(100, 1000);
}
Resources r = context.getResources();
BAD_TOUCH_HACK = r.getBoolean(com.android.internal.R.bool.config_filterTouchEvents);
JUMPY_TOUCH_HACK = r.getBoolean(com.android.internal.R.bool.config_filterJumpyTouchEvents);
mHapticFeedbackCallback = hapticFeedbackCallback;
readExcludedDevices();
PowerManager pm = (PowerManager)context.getSystemService(
Context.POWER_SERVICE);
mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,
"KeyInputQueue");
mWakeLock.setReferenceCounted(false);
mFirst = new QueuedEvent();
mLast = new QueuedEvent();
mFirst.next = mLast;
mThread.start();
}
9.那这个KeyInputQueue是在哪里被实例化呢？
而且查看KeyInputQueue类的声明，得知它是一个abstract class.
public abstract class KeyInputQueue
{
.....
}
说明它肯定会被某个类继承.接着查找。
/frameworks$ find . -name "*.java" |grep -v KeyInputQueue | xargs grep KeyInputQueue
./policies/base/phone/com/android/internal/policy/impl/KeyguardViewMediator.java: * {@link com.android.server.KeyInputQueue}‘s and {@link android.view.WindowManager}‘s.
./base/services/java/com/android/server/PowerManagerService.java: && !"KeyInputQueue".equals(tag))) {
./base/services/java/com/android/server/WindowManagerService.java:import com.android.server.KeyInputQueue.QueuedEvent;
./base/services/java/com/android/server/WindowManagerService.java: implements Watchdog.Monitor, KeyInputQueue.HapticFeedbackCallback {
./base/services/java/com/android/server/WindowManagerService.java: return KeyInputQueue.getSwitchState(sw);
./base/services/java/com/android/server/WindowManagerService.java: return KeyInputQueue.getSwitchState(devid, sw);
./base/services/java/com/android/server/WindowManagerService.java: return KeyInputQueue.hasKeys(keycodes, keyExists);
./base/services/java/com/android/server/WindowManagerService.java: private class KeyQ extends KeyInputQueue
./base/services/java/com/android/server/WindowManagerService.java: implements KeyInputQueue.FilterCallback {
./base/services/java/com/android/server/InputDevice.java: // For use by KeyInputQueue for keeping track of the current touch
./base/services/java/com/android/server/InputDevice.java: if (KeyInputQueue.BAD_TOUCH_HACK) {
./base/services/java/com/android/server/InputDevice.java: Slog.i("KeyInputQueue", "Updating: " + currentMove);
./base/services/java/com/android/server/InputDevice.java: Slog.i("KeyInputQueue", "Updating: " + currentMove);
10.从上面的查找结果得知，会在WindowManagerService.java中有一个KeyQ类继承KeyInputQueue类，再在这个文件中查找KeyQ类在哪里定义并实例化的，找到在其构造函数里实例化的。
private WindowManagerService(Context context, PowerManagerService pm,
boolean haveInputMethods) {
if (MEASURE_LATENCY) {
lt = new LatencyTimer(100, 1000);
}
....
mQueue = new KeyQ();
mInputThread = new InputDispatcherThread();
PolicyThread thr = new PolicyThread(mPolicy, this, context, pm);
...
}
至此，基本上把Input event的Framework的流程全部走完了。WindowManagerService是属于System server进程里面起的一个Service.一开机就会运行，当然其构造函数一开机就能会运行。
至此，整个流程如下：
WindowManagerService
|
|
//
KeyQ
|
|
//
KeyInputQueue
|
|
//
EventHub
|
|
//
Kernel device (/dev/input)
后续的文章将介绍/dev/input在Kernel中的实现。