Android应用程序键盘（Keyboard）消息处理机制分析（4）

Step 18. EventHub.getEvent

这个函数定义在frameworks/base/libs/ui/EventHub.cpp文件中：

boolEventHub::getEvent(RawEvent*outEvent)
{
outEvent->deviceId=0;
outEvent->type=0;
outEvent->scanCode=0;
outEvent->keyCode=0;
outEvent->flags=0;
outEvent->value=0;
outEvent->when=0;
//NotethatweonlyallowonecallertogetEvent(),sodon'tneed
//todolockinghere...onlywhenadding/removingdevices.
if(!mOpened){
mError=openPlatformInput()?NO_ERROR:UNKNOWN_ERROR;
mOpened=true;
mNeedToSendFinishedDeviceScan=true;
}
for(;;){
//Reportanydevicesthathadlastbeenadded/removed.
if(mClosingDevices!=NULL){
device_t*device=mClosingDevices;
LOGV("Reportingdeviceclosed:id=0x%x,name=%s\n",
device->id,device->path.string());
mClosingDevices=device->next;
if(device->id==mFirstKeyboardId){
outEvent->deviceId=0;
}else{
outEvent->deviceId=device->id;
}
outEvent->type=DEVICE_REMOVED;
outEvent->when=systemTime(SYSTEM_TIME_MONOTONIC);
deletedevice;
mNeedToSendFinishedDeviceScan=true;
returntrue;
}
if(mOpeningDevices!=NULL){
device_t*device=mOpeningDevices;
LOGV("Reportingdeviceopened:id=0x%x,name=%s\n",
device->id,device->path.string());
mOpeningDevices=device->next;
if(device->id==mFirstKeyboardId){
outEvent->deviceId=0;
}else{
outEvent->deviceId=device->id;
}
outEvent->type=DEVICE_ADDED;
outEvent->when=systemTime(SYSTEM_TIME_MONOTONIC);
mNeedToSendFinishedDeviceScan=true;
returntrue;
}
if(mNeedToSendFinishedDeviceScan){
mNeedToSendFinishedDeviceScan=false;
outEvent->type=FINISHED_DEVICE_SCAN;
outEvent->when=systemTime(SYSTEM_TIME_MONOTONIC);
returntrue;
}
//Grabthenextinputevent.
for(;;){
//Consumebufferedinputevents,ifany.
if(mInputBufferIndex<mInputBufferCount){
conststructinput_event&iev=mInputBufferData[mInputBufferIndex++];
constdevice_t*device=mDevices[mInputDeviceIndex];
LOGV("%sgot:t0=%d,t1=%d,type=%d,code=%d,v=%d",device->path.string(),
(int)iev.time.tv_sec,(int)iev.time.tv_usec,iev.type,iev.code,iev.value);
if(device->id==mFirstKeyboardId){
outEvent->deviceId=0;
}else{
outEvent->deviceId=device->id;
}
outEvent->type=iev.type;
outEvent->scanCode=iev.code;
if(iev.type==EV_KEY){
status_terr=device->layoutMap->map(iev.code,
&outEvent->keyCode,&outEvent->flags);
LOGV("iev.code=%dkeyCode=%dflags=0x%08xerr=%d\n",
iev.code,outEvent->keyCode,outEvent->flags,err);
if(err!=0){
outEvent->keyCode=AKEYCODE_UNKNOWN;
outEvent->flags=0;
}
}else{
outEvent->keyCode=iev.code;
}
outEvent->value=iev.value;
//Useaneventtimestampinthesametimebaseas
//java.lang.System.nanoTime()andandroid.os.SystemClock.uptimeMillis()
//asexpectedbytherestofthesystem.
outEvent->when=systemTime(SYSTEM_TIME_MONOTONIC);
returntrue;
}
//Finishreadingalleventsfromdevicesidentifiedinpreviouspoll().
//ThiscodeassumesthatmInputDeviceIndexisinitially0andthatthe
//reventsmemberofpollfdisinitializedto0whenthedeviceisfirstadded.
//SincemFDs[0]isusedforinotify,weprocessregulareventsstartingatindex1.
mInputDeviceIndex+=1;
if(mInputDeviceIndex>=mFDCount){
break;
}
conststructpollfd&pfd=mFDs[mInputDeviceIndex];
if(pfd.revents&POLLIN){
int32_treadSize=read(pfd.fd,mInputBufferData,
sizeof(structinput_event)*INPUT_BUFFER_SIZE);
if(readSize<0){
if(errno!=EAGAIN&&errno!=EINTR){
LOGW("couldnotgetevent(errno=%d)",errno);
}
}elseif((readSize%sizeof(structinput_event))!=0){
LOGE("couldnotgetevent(wrongsize:%d)",readSize);
}else{
mInputBufferCount=readSize/sizeof(structinput_event);
mInputBufferIndex=0;
}
}
}
......
mInputDeviceIndex=0;
//Pollforevents.Mindthewakelockdance!
//Weholdawakelockatalltimesexceptduringpoll().Thisworksduetosome
//subtlechoreography.Whenadevicedriverhaspending(unread)events,itacquires
//akernelwakelock.However,oncethelastpendingeventhasbeenread,thedevice
//driverwillreleasethekernelwakelock.Topreventthesystemfromgoingtosleep
//whenthishappens,theEventHubholdsontoitsownuserwakelockwhiletheclient
//isprocessingevents.Thusthesystemcanonlysleepiftherearenoevents
//pendingorcurrentlybeingprocessed.
release_wake_lock(WAKE_LOCK_ID);
intpollResult=poll(mFDs,mFDCount,-1);
acquire_wake_lock(PARTIAL_WAKE_LOCK,WAKE_LOCK_ID);
if(pollResult<=0){
if(errno!=EINTR){
LOGW("pollfailed(errno=%d)\n",errno);
usleep(100000);
}
}
}
}

这个函数比较长，我们一步一步来分析。

首先，如果是第一次进入到这个函数中时，成员变量mOpened的值为false，于是就会调用openPlatformInput函数来打开系统输入设备，在本文中，我们主要讨论的输入设备就是键盘了。打开了这些输入设备文件后，就可以对这些输入设备进行是监控了。如果不是第一次进入到这个函数，那么就会分析当前有没有输入事件发生，如果有，就返回这个事件，否则就会进入等待状态，等待下一次输入事件的发生。在我们这个场景中，就是等待下一次键盘事件的发生了。

我们先分析openPlatformInput函数的实现，然后回过头来分析这个getEvent函数的具体的实现。

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