簡介
消息驅(qū)動(dòng)是一種進(jìn)程/線程的運(yùn)行模式,內(nèi)部或者外部的消息事件被放到進(jìn)程/線程的消息隊(duì)列中按序處理是現(xiàn)在的操作系統(tǒng)普遍采用的機(jī)制.Android也是采用了消息驅(qū)動(dòng)的機(jī)制來處理各種外部按鍵,觸屏,系統(tǒng)Intent,廣播事件等消息.
Android的消息隊(duì)列是線程相關(guān)的,每啟動(dòng)一個(gè)線程,都可以在內(nèi)部創(chuàng)建一個(gè)消息隊(duì)列,然后在消息隊(duì)列中不斷循環(huán)檢查是否有新的消息需要處理,如果有,則對(duì)該消息進(jìn)行處理,如果沒有,線程就進(jìn)入休眠狀態(tài)直到有新的消息需要處理為止.
數(shù)據(jù)模型
Android中與消息機(jī)制相關(guān)的類主要有Looper,MessageQueue,Handler,Message,相關(guān)的代碼主要在以下文件中:
- frameworks/base/core/java/android/os/Looper.java
- frameworks/base/core/java/android/os/Message.java
- frameworks/base/core/java/android/os/MessageQueue.java
- frameworks/base/core/java/android/os/Handler.java
- frameworks/base/core/jni/android_os_MessageQueue.cpp
- system/core/libutils/Looper.cpp
- Looper
Looper對(duì)象是用來創(chuàng)建消息隊(duì)列并進(jìn)入消息循環(huán)處理的.每個(gè)線程只能有一個(gè)Looper對(duì)象，同時(shí)對(duì)應(yīng)著一個(gè)MessageQueue，發(fā)送到該線程的消息都將存放在該隊(duì)列中，并由Looper循環(huán)處理。Android默認(rèn)只為主線程)(UI線程)創(chuàng)建了Looper，所以當(dāng)我們新建線程需要使用消息隊(duì)列時(shí)必須手動(dòng)創(chuàng)建Looper. - MessageQueue
MessageQueue即消息隊(duì)列，由Looper創(chuàng)建管理,一個(gè)Looper對(duì)象對(duì)應(yīng)一個(gè)MessageQueue對(duì)象. - Handler
Handler是消息的接收與處理者，Handler將Message添加到消息隊(duì)列，同時(shí)也通過Handler的回調(diào)方法handleMessage()處理對(duì)應(yīng)的消息.一個(gè)Handler對(duì)象只能關(guān)聯(lián)一個(gè)Looper對(duì)象，但多個(gè)Handler對(duì)象可以關(guān)聯(lián)到同一個(gè)Looper.默認(rèn)情況下Handler會(huì)關(guān)聯(lián)到實(shí)例化Handler線程的Lopper，也可以通過Handler的構(gòu)造函數(shù)的Looper參數(shù)指定Handler關(guān)聯(lián)到某個(gè)線程的Looper，即發(fā)送消息到某個(gè)指定線程并在該線程中回調(diào)Handler處理該消息. - Message
Message是消息的載體，Parcelable的派生類，通過其成員變量target關(guān)聯(lián)到Handler對(duì)象.
它們之間關(guān)系如下圖示：
在代碼中我們一般如下使用線程的消息機(jī)制:
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
線程消息隊(duì)列的創(chuàng)建
線程的消息隊(duì)列通過Looper創(chuàng)建并維護(hù)的，主線程中調(diào)用Looper.prepareMainLooper(),其他子線程中調(diào)用Looper.prepare()來創(chuàng)建消息隊(duì)列.一個(gè)線程多次調(diào)用prepareMainLooper()或prepare()將會(huì)拋出異常.
在介紹消息隊(duì)列創(chuàng)建之前,首先了解一下Looper與MessageQueue,再看消息隊(duì)列創(chuàng)建的流程.
- Looper類的主要成員變量與方法如下:
public final class Looper {
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;
final MessageQueue mQueue;
final Thread mThread;
public static void prepare() {...}
private static void prepare(boolean quitAllowed) {...}
public static void prepareMainLooper() {...}
public static Looper getMainLooper() {...}
public static void loop() {...}
}
- sThreadLocal是靜態(tài)成員變量,用于保存線程私有的Looper對(duì)象
- sMainLooper是主線程的Looper對(duì)象.在prepareMainLooper()中賦值,可通過調(diào)用getMainLooper獲取
- mQueue即消息隊(duì)列,在Looper構(gòu)造函數(shù)中初始化
- mThread即Looper所在的線程
- MessageQueue類的主要成員變量與方法如下:
public final class MessageQueue {
private final boolean mQuitAllowed;
private long mPtr;
Message mMessages;
MessageQueue(boolean quitAllowed) {...}
boolean enqueueMessage(Message msg, long when) {...}
Message next() {...}
}
- mQuitAllowed代表是否允許退出消息循環(huán),主線程中默認(rèn)為false,子線程默認(rèn)false
- mPtr保存的是NativeMessageQueue的地址,通過該地址就可以找到j(luò)ava層MessageQueue所對(duì)應(yīng)native的MessageQueue.
- mMessages即消息隊(duì)列,通過mMessages可以遍歷整個(gè)消息隊(duì)列
- 消息隊(duì)列的創(chuàng)建:
消息隊(duì)列的創(chuàng)建從Looper.prepare()/Looper.prepareMainLooper()開始
public static void prepare() {
prepare(true);
}
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
通過調(diào)用prepare()或prepareMainLooper()創(chuàng)建Looper對(duì)象,然后保存到sThreadLocal中,sThreadLocal是模板類ThreadLocal<T>,它通過線程ID與對(duì)象關(guān)聯(lián)的方式實(shí)現(xiàn)線程本地存儲(chǔ)功能.這樣放入sThreadLocal對(duì)象中的Looper對(duì)象就與創(chuàng)建它的線程關(guān)聯(lián)起來了.所以可以從sThreadLocal中獲取到保存的Looper對(duì)象:
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
主線程的Loopper對(duì)象保存在sMainLooper,可以通過getMainLooper獲取
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
創(chuàng)建Looper同時(shí)會(huì)創(chuàng)建Looper關(guān)聯(lián)的MessageQueue并賦值給成員變量mQueue,接下來再看new MessageQueue(quitAllowed)的過程:
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
可以看到,直接調(diào)用了nativeInit().這個(gè)JNI方法定義在android_os_MessageQueue.cpp
static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return 0;
}
nativeMessageQueue->incStrong(env);
return reinterpret_cast<jlong>(nativeMessageQueue);
}
nativeInit()中首先創(chuàng)建了nativeMessageQueue,然后又將nativeMessageQueue的地址賦值給java層的mPtr,所以java層的MessageQueue就可以通過mPtr找到nativeMessageQueue了.
再看new NativeMessageQueue()過程,NativeMessageQueue的構(gòu)造如下:
NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}
它首先通過Looper::getForThread()判斷當(dāng)前線程是否已創(chuàng)建過Looper對(duì)象,如果沒有則創(chuàng)建.注意,這個(gè)Looper對(duì)象是實(shí)現(xiàn)在JNI層的,與上面Java層的Looper是不一樣的,不過也是對(duì)應(yīng)的關(guān)系.JNI層的Looper對(duì)象的創(chuàng)建過程是在Looper.cpp中實(shí)現(xiàn)的.
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
mWakeEventFd = eventfd(0, EFD_NONBLOCK);
LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd. errno=%d", errno);
AutoMutex _l(mLock);
rebuildEpollLocked();
}
創(chuàng)建eventfd并賦值給mWakeEventFd,在以前的Android版本上,這里創(chuàng)建的是pipe管道.eventfd是較新的API,被用作一個(gè)事件等待/響應(yīng),實(shí)現(xiàn)了線程之間事件通知.
void Looper::rebuildEpollLocked() {
// Close old epoll instance if we have one.
if (mEpollFd >= 0) {
#if DEBUG_CALLBACKS
ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);
#endif
close(mEpollFd);
}
// Allocate the new epoll instance and register the wake pipe.
mEpollFd = epoll_create(EPOLL_SIZE_HINT);
LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno);
struct epoll_event eventItem;
memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeEventFd;
int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance. errno=%d",
errno);
for (size_t i = 0; i < mRequests.size(); i++) {
const Request& request = mRequests.valueAt(i);
struct epoll_event eventItem;
request.initEventItem(&eventItem);
int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);
if (epollResult < 0) {
ALOGE("Error adding epoll events for fd %d while rebuilding epoll set, errno=%d",
request.fd, errno);
}
}
}
rebuildEpollLocked中通過epoll_create創(chuàng)建了一個(gè)epoll專用的文件描述符,EPOLL_SIZE_HINT表示mEpollFd上能監(jiān)控的最大文件描述符數(shù).最后調(diào)用epoll_ctl監(jiān)控mWakeEventFd文件描述符的EPOLLIN事件，即當(dāng)eventfd中有內(nèi)容可讀時(shí)，就喚醒當(dāng)前正在等待的線程.
C++層的這個(gè)Looper對(duì)象創(chuàng)建好了之后，就返回到JNI層的NativeMessageQueue的構(gòu)造函數(shù)，再返回到Java層的消息隊(duì)列MessageQueue的創(chuàng)建過程,最后從Looper的構(gòu)造函數(shù)中返回.線程消息隊(duì)列的創(chuàng)建過程也就此完成.
總結(jié)一下:
- 首先在Java層創(chuàng)建了一個(gè)Looper對(duì)象，然后創(chuàng)建MessageQueue對(duì)象mQueue,進(jìn)入MessageQueue的創(chuàng)建過程
- MessageQueue在JNI層創(chuàng)建了一個(gè)NativeMessageQueue對(duì)象,并將這個(gè)對(duì)象保存在MessageQueue的成員變量mPtr中
- 在JNI層，創(chuàng)建了NativeMessageQueue對(duì)象時(shí),會(huì)創(chuàng)建了一個(gè)Looper對(duì)象，保存在JNI層的NativeMessageQueue對(duì)象的成員變量mLooper中，這個(gè)對(duì)象的作用是，當(dāng)Java層的消息隊(duì)列中沒有消息時(shí)，就使Android應(yīng)用程序線程進(jìn)入等待狀態(tài)，而當(dāng)Java層的消息隊(duì)列中來了新的消息后，就喚醒Android應(yīng)用程序的線程來處理這個(gè)消息
-
關(guān)于java層與JNI層的Looper,MessageQueue對(duì)象可以這樣理解,java層的Looper,MessageQueue主要實(shí)現(xiàn)了消息隊(duì)列發(fā)送處理邏輯,而JNI層的主要實(shí)現(xiàn)是線程的等待/喚醒.在邏輯上他們還是一一對(duì)應(yīng)的關(guān)系,只不過側(cè)重點(diǎn)不同.
java與jni層Looper,MessageQueue關(guān)系
線程消息隊(duì)列的循環(huán)
當(dāng)線程消息隊(duì)列創(chuàng)建完成后,即進(jìn)入消息隊(duì)列循環(huán)處理過程中,Android消息隊(duì)列的循環(huán)通過Loop.Loop()來實(shí)現(xiàn),整個(gè)流程如下圖示.
下面具體來看具體分析
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
msg.target.dispatchMessage(msg);
...
}
}
進(jìn)入loop前,首先通過myLooper()拿到前面創(chuàng)建的Looper對(duì)象,如果為null將會(huì)拋出異常,這也就是為什么必須在Looper.loop()之前調(diào)用Looper.prepare()或者Looper.prepareMainLooper()的原因.接下來通過me.mQueue拿到MessageQueue對(duì)象,而后進(jìn)入到無盡循環(huán)處理中.在循環(huán)中通過queue.next()從隊(duì)列中取消息,再調(diào)用msg.target.dispatchMessage(msg)處理.下面看一下queue.next()流程.
Message next() {
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1;
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
return msg;
}
} else {
nextPollTimeoutMillis = -1;
}
if (mQuitting) {
dispose();
return null;
}
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null;
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf("MessageQueue", "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
pendingIdleHandlerCount = 0;
nextPollTimeoutMillis = 0;
}
}
先看一下開始定義的2個(gè)變量的含義,pendingIdleHandlerCount表示消息隊(duì)列空閑消息處理器(IdleHandler)的個(gè)數(shù),nextPollTimeoutMillis表示沒有消息處理時(shí),線程需睡眠等待的時(shí)間.nativePollOnce將會(huì)睡眠等待nextPollTimeoutMillis時(shí)間.從nativePollOnce返回后,再從消息隊(duì)列中取消息,如果沒有任何消息,那么nextPollTimeoutMillis賦值為-1,表示下一次nativePollOnce無限制等待直到其他線程把它喚醒.如果取到消息,比較消息處理的時(shí)間與當(dāng)前時(shí)間,如果消息處理的時(shí)間未到(now < msg.when),那么計(jì)算nextPollTimeoutMillis,等下一次時(shí)間到時(shí)再處理.如果消息處理時(shí)間已到,那么取出消息返回到Looperde的loop中處理.另外如果當(dāng)前沒有消息處理時(shí),會(huì)回調(diào)注冊(cè)的IdleHandler.
下面繼續(xù)分析nativePollOnce.
static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}
void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
mPollEnv = env;
mPollObj = pollObj;
mLooper->pollOnce(timeoutMillis);
mPollObj = NULL;
mPollEnv = NULL;
if (mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}
最終nativePollOnce調(diào)用的JNI層Looper的pollOnce
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
int result = 0;
for (;;) {
...
if (result != 0) {
...
return result;
}
result = pollInner(timeoutMillis);
}
}
在pollOnce中不斷的循環(huán)調(diào)用pollInner來檢查線程是否有新消息需要處理.如果有新消息處理或者timeoutMillis時(shí)間到,則返回到j(luò)ava層MessageQueue的next()繼續(xù)執(zhí)行.
int Looper::pollInner(int timeoutMillis) {
...
int result = POLL_WAKE;
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
...
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeEventFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
} else {
...
}
}
...
return result;
}
epoll_wait會(huì)監(jiān)聽前面創(chuàng)建的epoll實(shí)例的文件描述符上的IO讀寫事件,如果文件描述上沒有IO事件出現(xiàn),那么則等待timeoutMillis延時(shí),檢測(cè)到EPOLLIN事件即文件描述符上發(fā)生了寫事件,隨后調(diào)用awoken讀出數(shù)據(jù),以便接收新的數(shù)據(jù).
void Looper::awoken() {
uint64_t counter;
TEMP_FAILURE_RETRY(read(mWakeEventFd, &counter, sizeof(uint64_t)));
}
在awoken中讀出數(shù)據(jù).然后一步步返回到j(luò)ava層的MessageQueue繼續(xù)消息處理.
線程消息的發(fā)送
消息的發(fā)送是通過Handler來執(zhí)行的,下面我們從new Handler()開始,一步步分析消息的發(fā)送過程
首先看一下Handler類的主要數(shù)據(jù)成員與方法:
public class Handler {
final MessageQueue mQueue;
final Looper mLooper;
public Handler() {...}
public Handler(Looper looper, Callback callback) {...}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {...}
public void handleMessage(Message msg) {...}
public final boolean sendMessage(Message msg){...}
public final boolean sendEmptyMessage(int what){...}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {...}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {...}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {...}
...
public final boolean post(Runnable r){...}
public final boolean postAtFrontOfQueue(Runnable r){...}
public final boolean postAtTime(Runnable r, long uptimeMillis){...}
public final boolean postDelayed(Runnable r, long delayMillis){...}
}
- mQueue handler對(duì)應(yīng)的MessageQueue對(duì)象,通過handler發(fā)送的消息都將插入到mQueue隊(duì)列中
- mLooper handler對(duì)應(yīng)的Looper對(duì)象,如果創(chuàng)建Handler前沒有實(shí)例化Looper對(duì)象將拋出異常.
Handler是與Looper對(duì)象相關(guān)聯(lián)的,我們創(chuàng)建的Handler對(duì)象都會(huì)關(guān)聯(lián)到某一Looper,默認(rèn)情況下,Handler會(huì)關(guān)聯(lián)到創(chuàng)建Handler對(duì)象所在線程的Looper對(duì)象,也可通過Handler的構(gòu)造函數(shù)來指定關(guān)聯(lián)到的Looper.Handler發(fā)送消息有二類接口,post類與send類,一般send類用來發(fā)送傳統(tǒng)帶消息ID的消息，post類用來發(fā)送帶消息處理方法的消息.
下面來看消息發(fā)送的具體流程
Handler或Post類方法最終都會(huì)調(diào)用enqueueMessage將消息發(fā)送到消息隊(duì)列
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
Message的成員變量target賦值為this，即關(guān)聯(lián)到handler.然后繼續(xù)調(diào)用MessageQueue的enqueueMessage方法
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
/// M: Add message protect mechanism @{
if (msg.hasRecycle) {
Log.wtf("MessageQueue", "Warning: message has been recycled. msg=" + msg);
return false;
}
/// Add message protect mechanism @}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
MessageQueue中的enqueueMessage主要工作是將message插入到隊(duì)列，然后根據(jù)情況判斷是否應(yīng)該調(diào)用nativeWake喚醒目標(biāo)線程.當(dāng)前隊(duì)列為空或者插入消息處理時(shí)間延時(shí)為0或者處理時(shí)間小于隊(duì)頭處理時(shí)間時(shí)，消息被插入到頭部，否則按時(shí)間遍歷插入到對(duì)應(yīng)位置，并設(shè)置needWake標(biāo)志，needWake是根據(jù)mBlocked來判斷的，mBlocked記錄了當(dāng)前線程是否處于睡眠狀態(tài)，如果消息插入隊(duì)頭且線程在睡眠中，neeWake為true，調(diào)用nativeWake喚醒目標(biāo)線程.
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
return nativeMessageQueue->wake();
}
void NativeMessageQueue::wake() {
mLooper->wake();
}
void Looper::wake() {
uint64_t inc = 1;
ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
if (nWrite != sizeof(uint64_t)) {
if (errno != EAGAIN) {
ALOGW("Could not write wake signal, errno=%d", errno);
}
}
}
nativeWake最終會(huì)調(diào)用到j(luò)ni層的Looper對(duì)象的wake方法中,Looper wake方法的實(shí)現(xiàn)非常簡單,即向mWakeEventFd寫入一個(gè)uint64_t,這樣目標(biāo)線程就會(huì)因?yàn)閙WakeEventFd發(fā)生的IO事件而喚醒.消息的發(fā)送流程就此結(jié)束.
線程消息的處理
從前面的分析可以知道,當(dāng)線程沒有消息需要處理時(shí),會(huì)在c++層Looper對(duì)象的pollInner中進(jìn)入睡眠等待,當(dāng)有新消息喚醒該目標(biāo)線程時(shí)或這延時(shí)時(shí)間到,執(zhí)行流程將沿著pollInner調(diào)用路徑一直返回,直到j(luò)ava層Looper類的loop.
loop中將調(diào)用msg.target.dispatchMessage(msg)處理消息,這里的msg.target就是上面enqueueMessage中所賦值的handler,即進(jìn)入handler的dispatchMessage處理消息
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
dispatchMessage進(jìn)行消息處理,先檢查是否有設(shè)置msg.callback,如果有則執(zhí)行msg.callback處理消息,如果沒有則繼續(xù)判斷mCallback的執(zhí)行,最后才是handleMessage處理.
