Handler機制情景分析

一. 概述

在整個Android的原始碼世界裡，有兩大利劍，其一是Binder IPC機制，，另一個便是訊息機制(由Handler/Looper/MessageQueue等構成的).

Android有大量的訊息驅動方式來進行互動，比如Android的四劍客Activity, Service, Broadcast, ContentProvider的啟動過程的互動，都離不開訊息機制，Android某種意義上也可以說成是一個以訊息驅動的系統。訊息機制涉及MessageQueue/Message/Looper/Handler這4個類。

1.1 模型

訊息機制主要包含:

• Message：訊息分為硬體產生的訊息(如按鈕、觸控)和軟體生成的訊息；
• MessageQueue：訊息佇列的主要功能向訊息池投遞訊息( MessageQueue.enqueueMessage )和取走訊息池的訊息( MessageQueue.next )；
• Handler：訊息輔助類，主要功能向訊息池傳送各種訊息事件( Handler.sendMessage )和處理相應訊息事件( Handler.handleMessage )；
• Looper：不斷迴圈執行( Looper.loop )，按分發機制將訊息分發給目標處理者。

1.2 架構圖

1.3 Demo

public class MainActivity extends AppCompatActivity {

private Button mButton;
private final String TAG="MessageTest";
private int ButtonCount = 0;
private MyThread myThread;
private Handler mHandler;
private int mMessageCount = 0;

class MyThread extends Thread {
private Looper mLooper;
@Override
public void run() {
super.run();
/* Initialize the current thread as a looper */
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
/* Run the message queue in this thread */
Looper.loop();
}

public Looper getLooper(){
if (!isAlive()) {
return null;
}

// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
}

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

mButton = (Button)findViewById(R.id.button);
mButton.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// Perform action on click
Log.d(TAG, "Send Message "+ ButtonCount);
ButtonCount++;
/* 按下按鍵後通過mHandler傳送一個訊息 */
Message msg = new Message();
mHandler.sendMessage(msg);
}
});

myThread = new MyThread();
myThread.start();

/* 建立一個handle例項(詳見4.3.2),這個handle為執行緒myThread服務,當收到mesg時會呼叫設定的回撥函式*/
mHandler = new Handler(myThread.getLooper(), new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
Log.d(TAG, "get Message "+ mMessageCount);
mMessageCount++;
return false;
}
});
}
}

大概流程:先建立的一個執行緒,該執行緒中呼叫了 Looper.prepare() (詳見2.1)和 Looper.loop() (詳見2.2)方法,接著啟動了該執行緒,緊接著初始化了一個Handler例項(詳見4.3.2).用於服務message,在按下按鍵後通過 mHandler 傳送了一個訊息(詳見4.2),此時 handleMessage 被回撥(詳見4.1).接下來進行詳細分析.

該Demo中有個兩點 getLooper 方法,當外界呼叫該方法時,他會判斷當前 mLooper 是否為空,空的話就會一直等待.

為什麼要這麼做?

因為在建立執行緒後去獲取 mLooper ,此時執行緒的 run 方法可能還為執行,所以此時 mLooper 值應該為null;

當運行了 Looper.prepare() 方法建立了 looper 後,通過 Looper.myLooper() 獲取到 mLooper ,再 notifyAll ;

二. Looper

2.1 Looper.prepare()

public static void prepare() {
prepare(true);①
}

private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {②
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));③
}

①：無參情況下呼叫 prepare(true) ,形參置true表示允許退出。

②： sThreadLocal 會先去獲取本地的資料，如果能獲取到說明已經prepare過，則丟擲異常。

③：設定sThreadLocal資料

sThreadLocal是ThreadLocal型別（static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();）

ThreadLocal: 執行緒本地儲存區，每個執行緒都有自己的私有本地儲存區域，不同的執行緒之間彼此不能訪問對方的儲存區。

接下來看下剛儲存的TLS區域的Looper物件：

private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);①
mThread = Thread.currentThread();②
}

①：建立一個訊息佇列

這裡為該執行緒建立了一個訊息佇列 MessageQueue 的建構函式中呼叫的hal層的本地方法：

MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}

這個流程的分析先到這。

2.2 Looper.loop()

public static void loop() {
final Looper me = myLooper();①
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;

// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();

for (;;) {
Message msg = queue.next(); // might block②
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}

// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}

msg.target.dispatchMessage(msg);③

if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}

// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}

msg.recycleUnchecked();④
}
}

①: 獲取TLS中儲存的Looper物件

②: 獲取訊息,沒有訊息的時候會阻塞(詳見3.1)

③: 分發訊息(詳見4.1)

④: 回收訊息到訊息池(詳見5.1)

三. MesageQueue

3.1 next()

Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}

int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}

nativePollOnce(ptr, nextPollTimeoutMillis);①

synchronized (this) {
// Try to retrieve the next message.Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier.Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());②
}
if (msg != null) {
if (now < msg.when) {③
// Next message is not ready.Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
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 {
// No more messages.
nextPollTimeoutMillis = -1;④
}

// Process the quit message now that all pending messages have been handled.
if (mQuitting) {⑤
dispose();
return null;
}

// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {⑥
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run.Loop and wait some more.
mBlocked = true;
continue;
}

if (mPendingIdleHandlers == null) {⑦
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}

// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler

boolean keep = false;
try {
keep = idler.queueIdle();⑧
} catch (Throwable t) {
Log.wtf("MessageQueue", "IdleHandler threw exception", t);
}

if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}

// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;⑨

// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}

①: 呼叫本地epoll方法, 當沒有訊息時會阻塞在這,阻塞時間為nextPollTimeoutMillis(詳見6.1.1)

②: 查詢訊息佇列中的非同步訊息(詳見4.2)

③: 如果當前時間小於非同步訊息的觸發時間,則設定下一輪poll的超時時間(相當於休眠時間),否則返回將要執行的非同步訊息.

④: 沒有非同步訊息,下輪poll則無限等待,直到新的訊息來臨

⑤: 檢測下退出標誌

⑥: 如果訊息佇列未空或是第一個msg(訊息剛放進佇列且未達到觸發時間),則執行空閒的handler

⑦: IdleHandler一個臨時存放陣列物件(下面可以看到一個列表轉陣列的方法被呼叫)

⑧: 執行空閒的handler(只有第一次迴圈時會執行idle handle)

⑨: 重置idle handler計數,防止下次執行

往往在第一次進入next函式迴圈時,在 nativePollOnce 阻塞之後,都會執行idle handle函式.

獲取到非同步訊息,立馬把該訊息返回給上一層,否則繼續迴圈等待新的訊息產生.

3.2 enqueueMessage()

boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {①
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {②
throw new IllegalStateException(msg + " This message is already in use.");
}

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) {③
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) {④
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);⑤
}
}
return true;
}

根據判斷當前阻塞標誌,來覺得是否需要喚醒;

四. Handler

4.1 訊息分發

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);①
} else {
if (mCallback != null) {②
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);③
}
}

①: 如果該msg設定了回撥函式,則直接呼叫回撥方法 message.callback.run() ;

②: 當handler設定了回撥函式,則回撥方法 mCallback.handleMessage(msg) ;

③: 呼叫handler自身的方法 handleMessage ,該方法預設為空,一般通過子類覆蓋來完成具體的邏輯;

我們Demo程式中,是使用第二種方法,設定回撥來實現具體的邏輯,分發訊息的本意是響應訊息的對應的執行方法.

4.2 訊息傳送

可以看到呼叫 sendMessage 方法後,最終呼叫的是 enqueueMessage 方法.

public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

可以看到傳送訊息時都有一個時間引數選擇,該引數就是我們前面分析的延時觸發時間(相對時間).

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;①
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;②
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}

①: 判斷handler建立時,傳進來的訊息對列是否為空(詳見4.3)

②: 訊息的 target 為該物件本身,handler型別

這裡有對發生的訊息進行非同步標誌設定,通過判斷 mAsynchronous 標誌,該標誌是在建立handler時初始化的(詳見4.3);

Handler.enqueueMessage 方法呼叫的是 MessageQueue.enqueueMessage 方法(詳見3.2);

4.3 建立Handler

4.3.1 無參構造

public Handler() {
this(null, false);
}

public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}

mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}

無參構造方式比起我們Demo中的方式,它自己回撥用 Looper.myLooper() 靜態方法獲取looper;

4.3.2 有參構造

public Handler(Looper looper, Callback callback) {
this(looper, callback, false);①
}

public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}

①: 呼叫有參建構函式建立 handler 且非同步標誌置 false 說明該 handler 傳送的訊息都為同步訊息.

Demo中的handler就是使用該方式建立,自己傳入 looper 引數.

五. Message

5.1 recycle()

public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();
}

void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;①
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;

synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {②
next = sPool;
sPool = this;
sPoolSize++;
}
}
}

①: 將該訊息標誌置為使用中並清除其他引數為default

將訊息回收到訊息池都是將訊息加入到訊息池的連結串列表頭.

5.2 obtain()

public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;①
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();②
}

①: 從訊息池表頭拿出一個訊息

可以看出每次從訊息池取出訊息都是從連結串列的表頭取出,再對訊息的計數做減法.

六. HAL層

native層本身也有一套完整的訊息機制,用於處理native的訊息;

在整個訊息機制中, MessageQueue 是連線java層和native層的紐帶;

6.1 MessageQueue

檔案

android_os_MessageQueue.c

static JNINativeMethod gMessageQueueMethods[] = {
/* name, signature, funcPtr */
{ "nativeInit", "()J", (void*)android_os_MessageQueue_nativeInit },
{ "nativeDestroy", "(J)V", (void*)android_os_MessageQueue_nativeDestroy },
{ "nativePollOnce", "(JI)V", (void*)android_os_MessageQueue_nativePollOnce },
{ "nativeWake", "(J)V", (void*)android_os_MessageQueue_nativeWake },
{ "nativeIsIdling", "(J)Z", (void*)android_os_MessageQueue_nativeIsIdling }
};

以上可以看出上層呼叫 nativePollOnce 方法實質是呼叫HAL層的 android_os_MessageQueue_nativePollOnce 方法

6.1.1 nativePollOnce

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jclass clazz,
jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, timeoutMillis);
}

void NativeMessageQueue::pollOnce(JNIEnv* env, int timeoutMillis) {
mInCallback = true;
mLooper->pollOnce(timeoutMillis);
mInCallback = false;
if (mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}

通過原始碼可以看出訊息佇列中的 pollOnce 實質是呼叫的 looper 中的 pollOnce 方法(詳見6.2.1)

6.1.2 nativeWake

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();
}

通過原始碼可以看出訊息佇列中的 wake 實質是呼叫的 looper 中的 wake 方法(詳見6.2.4)

6.1.3 nativeInit

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);
}


NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}

可以看到hal層和java層中建立looper的時序幾乎是一樣的,先建立一個訊息對列,再建立一個looper(Looper的構造詳見6.2.3);

6.1.4 nativeIsIdling

static jboolean android_os_MessageQueue_nativeIsIdling(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
return nativeMessageQueue->getLooper()->isIdling();
}

bool Looper::isIdling() const {
return mIdling;
}

還是呼叫looper中的方法,來看看這個標誌具體表示什麼狀態:

// We are about to idle.
mIdling = true;

struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

// No longer idling.
mIdling = false;

以上程式碼片為 Looper::pollInner 中的一段,在wait時是空閒,當有資料來臨時是非空閒的;

以前也用過這樣的方法來判斷執行緒是否在使用,想不到在這裡也看到了這種方法;

6.1.5 nativeDestroy

static void android_os_MessageQueue_nativeDestroy(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->decStrong(env);
}

6.2 Looper

Looper.cpp: system/core/lib/libutils

6.2.1 Looper::pollOnce

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
int result = 0;
for (;;) {
while (mResponseIndex < mResponses.size()) {
const Response& response = mResponses.itemAt(mResponseIndex++);
int ident = response.request.ident;
if (ident >= 0) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - returning signalled identifier %d: "
"fd=%d, events=0x%x, data=%p",
this, ident, fd, events, data);
#endif
if (outFd != NULL) *outFd = fd;
if (outEvents != NULL) *outEvents = events;
if (outData != NULL) *outData = data;
return ident;
}
}

if (result != 0) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - returning result %d", this, result);
#endif
if (outFd != NULL) *outFd = 0;
if (outEvents != NULL) *outEvents = 0;
if (outData != NULL) *outData = NULL;
return result;
}

result = pollInner(timeoutMillis);
}
}

Looper::pollOnce 是通過呼叫 Looper::pollInner 方法實現;

6.2.2 Looper::pollInner

int Looper::pollInner(int timeoutMillis) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);
#endif

// Adjust the timeout based on when the next message is due.
if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime);
if (messageTimeoutMillis >= 0
&& (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) {
timeoutMillis = messageTimeoutMillis;
}
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - next message in %lldns, adjusted timeout: timeoutMillis=%d",
this, mNextMessageUptime - now, timeoutMillis);
#endif
}

// Poll.
int result = POLL_WAKE;
mResponses.clear();
mResponseIndex = 0;

// We are about to idle.
mIdling = true;

struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);①

// No longer idling.
mIdling = false;

// Acquire lock.
mLock.lock();

// Check for poll error.
if (eventCount < 0) {②
if (errno == EINTR) {
goto Done;
}
ALOGW("Poll failed with an unexpected error, errno=%d", errno);
result = POLL_ERROR;
goto Done;
}

// Check for poll timeout.
if (eventCount == 0) {③
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - timeout", this);
#endif
result = POLL_TIMEOUT;
goto Done;
}

// Handle all events.
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
#endif

/* 處理epoll後的所有事件 */
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeReadPipeFd) {④
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
}
} else {
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex >= 0) {
int events = 0;
if (epollEvents & EPOLLIN) events |= EVENT_INPUT;
if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT;
if (epollEvents & EPOLLERR) events |= EVENT_ERROR;
if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP;
pushResponse(events, mRequests.valueAt(requestIndex));
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
"no longer registered.", epollEvents, fd);
}
}
}
Done: ;

// Invoke pending message callbacks.
mNextMessageUptime = LLONG_MAX;
while (mMessageEnvelopes.size() != 0) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);
if (messageEnvelope.uptime <= now) {
// Remove the envelope from the list.
// We keep a strong reference to the handler until the call to handleMessage
// finishes.Then we drop it so that the handler can be deleted *before*
// we reacquire our lock.
{ // obtain handler
sp<MessageHandler> handler = messageEnvelope.handler;
Message message = messageEnvelope.message;
mMessageEnvelopes.removeAt(0);
mSendingMessage = true;
mLock.unlock();

#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d",
this, handler.get(), message.what);
#endif
handler->handleMessage(message);
} // release handler

mLock.lock();
mSendingMessage = false;
result = POLL_CALLBACK;
} else {
// The last message left at the head of the queue determines the next wakeup time.
mNextMessageUptime = messageEnvelope.uptime;
break;
}
}

// Release lock.
mLock.unlock();

// Invoke all response callbacks.
for (size_t i = 0; i < mResponses.size(); i++) {
Response& response = mResponses.editItemAt(i);
if (response.request.ident == POLL_CALLBACK) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",
this, response.request.callback.get(), fd, events, data);
#endif
int callbackResult = response.request.callback->handleEvent(fd, events, data);
if (callbackResult == 0) {
removeFd(fd);
}
// Clear the callback reference in the response structure promptly because we
// will not clear the response vector itself until the next poll.
response.request.callback.clear();
result = POLL_CALLBACK;
}
}
return result;
}

①: 等待mEpollFd有事件產生,等待時間為timeoutMilli;

當上層發訊息時且判斷需要喚醒,則會往管道的讀端寫入資料用於喚醒(詳見6.2.3);

②: 檢測poll是否出錯;

③: 檢測poll是否超時;

④: 如果是因為往管道讀端寫入資料被喚醒,則都去並清空管道中的資料;

6.2.3 Looper::Looper()

Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
int wakeFds[2];
int result = pipe(wakeFds);①
LOG_ALWAYS_FATAL_IF(result != 0, "Could not create wake pipe.errno=%d", errno);

mWakeReadPipeFd = wakeFds[0];
mWakeWritePipeFd = wakeFds[1];
result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK);②
LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake read pipe non-blocking.errno=%d",
errno);
result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake write pipe non-blocking.errno=%d",
errno);

mIdling = false;

// Allocate the 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;③
eventItem.data.fd = mWakeReadPipeFd;
result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeReadPipeFd, & eventItem);④
LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake read pipe to epoll instance.errno=%d",
errno);
}

①: 建立一個無名管道, wakeFds[0]:讀檔案描述符, wakeFds[1]: 寫檔案描述符;

②: 更改為無阻塞方式;

③: EPOLLIN:連線到達,有資料來臨;

④: 監測管道讀端是否有資料來臨;

6.2.4 Looper::wake()

void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ wake", this);
#endif

ssize_t nWrite;
do {
nWrite = write(mWakeWritePipeFd, "W", 1);
} while (nWrite == -1 && errno == EINTR);

if (nWrite != 1) {
if (errno != EAGAIN) {
ALOGW("Could not write wake signal, errno=%d", errno);
}
}
}

喚醒只是向管道的寫端寫入一個位元組資料,epoll_wait則會得到返回;

總結

在這裡做個總結針對java層(因為native層的訊息機制未進行詳細分析不過估計和java層的流程差不多);

當呼叫j靜態方法 Looper.prepare() 初始化後,再呼叫 Looper.loop() 方法進行訊息迴圈處理;

Looper.loop() 方法中呼叫 MesageQueue.next() 方法檢索新訊息,沒有則阻塞,有則將訊息插入訊息連結串列頭後立即返回;

阻塞方式是呼叫本地的 nativePollOnce() 方法實現,其原理是利用epoll管道檔案描述符實現;

Looper.loop() 呼叫 dispatchMessage 方法實現訊息的分發處理;

傳送一個訊息的實質是呼叫個 MessageQueue.enqueueMessage() 方法往訊息連結串列中插入一個訊息,插入位置的條件為延時時間;

然後再呼叫一個本地方法 nativeWake 對前面阻塞的進行喚醒,實質是往管道中寫入一個位元組資料;

參考

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