「handler機制--讓線程變?yōu)椤坝绖訖C”」這一篇最后提到“線程已經(jīng)做好了一切準備劈榨，就等待著“各種事件“的到來了”史辙。那我們這篇就分別從Message“誕生”供璧，發(fā)送Message诵姜，收到Message汽煮，處理Message，回收Message 這五個過程來介紹Message之旅。（以下分析都是基于android 12代碼）
1. Message"誕生"
Message的”誕生“可不是你想象的那樣暇赤，隨便調用下Message的構造方法就可以創(chuàng)建一個Message心例，Message可是handler機制中”使用量“最大的對象了，如果對Message的創(chuàng)建不加以管控鞋囊，那隨著Message對象的大量創(chuàng)建止后，必然會涉及到大量Message回收的工作，這回收工作量越來越大越來越頻繁溜腐，到最后影響的可是系統(tǒng)的運行译株。因此Message的創(chuàng)建需要
受管控，并且創(chuàng)建了Message緩沖池挺益。
下面通過代碼來介紹下創(chuàng)建Message
/** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
*/
public Message() {
}
public static Message obtain(Handler h, int what) {
//先調用obtain方法歉糜，獲取Message
Message m = obtain();
m.target = h;
m.what = what;
return m;
}
public static Message obtain(Handler h, int what, Object obj) {
//先調用obtain方法，獲取Message
Message m = obtain();
m.target = h;
m.what = what;
m.obj = obj;
return m;
}
上面只是列舉了obtain的幾個方法望众，省略了其他的重載方法（除了參數(shù)的區(qū)別外现恼，其他都一樣）
如上面Message構造方法的官方注釋，創(chuàng)建Message最好的方式是調用Message的各種obtain方法
再來看下最核心的obtain方法黍檩，如下代碼
public static Message obtain() {
synchronized (sPoolSync) {
//sPool是一個對象緩沖池叉袍，如果不等與空，從緩沖池中獲取
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
//否則new一個Message
return new Message();
}
obtain方法在創(chuàng)建Message的時候刽酱，它的過程如下：
- 若sPool不為null喳逛，則從sPool中獲取，sPool是Message對象的緩存池棵里，認真看上面代碼润文，sPool并不是一個容器，而是以鏈表的結構來把Message串起來殿怜，Message有一個很重要的屬性next典蝌，而這個屬性才支持了鏈表這個數(shù)據(jù)結構
- 若sPool為null，則調用構造方法創(chuàng)建Message
小結
創(chuàng)建Message的方法是調用Message的各種重載obtain方法头谜，它可以控制創(chuàng)建Message的過程骏掀，先從sPool對象緩沖池中獲取，沒有則采取新創(chuàng)建柱告。
那我們就創(chuàng)建一個Message對象截驮，如下代碼：
//下面代碼位于主線程
//handler處理Message,Looper.getMainLooper()獲取的是主線程的Looper對象
Handler handler = new Handler(Looper.getMainLooper()){
@Override
public void handleMessage(@NonNull Message msg) {
super.handleMessage(msg);
if (msg.what == 1 ){
}
}
};
//Message.obtain方法獲取一個Message，該方法參數(shù)是handler（上面創(chuàng)建的）际度，what值為1
Message msg = Message.obtain(handler, 1);
2. 發(fā)送Message
上面一節(jié)創(chuàng)建了一個msg對象葵袭，那就來看下怎么把它發(fā)送到MessageQueue。
發(fā)送Message需要調用它的sendToTarget方法乖菱，msg.sendToTarget()就可以發(fā)送消息坡锡，那就從sendToTarget作為入口蓬网，來看下發(fā)送Message的邏輯
2.1 Message#sendToTarget
public void sendToTarget() {
target.sendMessage(this);
}
2.2 Handler#sendMessage
//msg要發(fā)送的Message
public final boolean sendMessage(@NonNull Message msg) {
//delayMillis:0
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
//SystemClock.uptimeMillis()獲取從開機到這個時刻的毫秒數(shù)（不包含系統(tǒng)深度睡眠的時間），因為delayMillis當前值為0,因此uptimeMillis的值就代表當前時刻要發(fā)這條Message
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
//queue不存在則拋異常
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(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
//msg的target設置為當前的Handler對象
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
//若是異步鹉勒，則把Message設置為異步
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//調用queue的enqueueMessage方法
return queue.enqueueMessage(msg, uptimeMillis);
}
2.3 MessageQueue#enqueueMessage
//when：代表這條msg什么時候發(fā)送
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//進入同步塊帆锋，下面的代碼處于多線程中
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
//MessageQueue處于離開的狀態(tài)，則直接返回
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
//為了安全贸弥，把msg標記為使用狀態(tài)
msg.markInUse();
//把when值賦值給msg的when屬性
msg.when = when;
//MessageQueue中真正把存儲Message的是它的mMessages屬性窟坐，它是一個單向鏈表，鏈表上的Message是按when值排序的绵疲，越靠近鏈表頭when值越小哲鸳，p持有的是鏈表的頭
Message p = mMessages;
//如持有MessageQueue的線程處于wait狀態(tài)則喚醒，否則不做處理
boolean needWake;
//當mMessages沒有消息盔憨，或者when==0,或者當前發(fā)送的Message的時間小于 鏈表頭Message的時間徙菠，則把msg加入mMessages并作為它的表頭
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
//把msg加入mMessages，并作為表頭
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.
// mBlocked為true代表處于阻塞狀態(tài)郁岩，p.target == null代表頭部消息為屏障消息婿奔，當前發(fā)送的消息是異步Message，則有必要去喚醒
needWake = mBlocked && p.target == null && msg.isAsynchronous();
//下面的邏輯是根據(jù)when值把當前發(fā)送的msg插入鏈表中
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
//若當前要插入的消息不是最早的異步消息问慎，則沒必要去 喚醒
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.
//需要去喚醒萍摊，則調用nativeWake方法喚醒
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
mMessages
在介紹這個方法的邏輯之前，先來說下mMessages如叼，放入MessageQueue的Message是放在mMessages的它是一個鏈表結構冰木，mMessages指向了鏈表的頭部，Message類有一個next屬性指向了下個Message笼恰。
mMessages鏈表是以Message的when值排序的踊沸，when值越小越排在前面，when值越小代表這條Message越早發(fā)出去社证。
為啥使用鏈表來存儲Message逼龟，主要原因是鏈表在插入，刪除的操作上要快于列表追葡，在MessageQueue中插入腺律，刪除Message的操作是很頻繁的。
這個方法的邏輯比較長辽俗，但所做的事情是比較簡單的：
- 如果mMessages沒有消息疾渣，或者when==0,或者當前發(fā)送的Message的時間小于 鏈表頭Message的時間，則把發(fā)送的Message放入鏈表的頭部
- 否則崖飘，在鏈表中根據(jù)when值查找當前發(fā)送的Message放入鏈表的位置。并且如果 線程處于阻塞狀態(tài) 并且 頭部消息為屏障消息 并且 當前發(fā)送的消息是異步Message 并且這個Message是所有異步Message的第一個杈女，則有必要執(zhí)行喚醒操作
- 如果需要喚醒朱浴，則去調用nativeWake方法進行喚醒操作
咱們就來看下喚醒操作吊圾，nativeWake方法最終會調用android_os_MessageQueue.cpp的方法
2.4 android_os_MessageQueue.cpp#android_os_MessageQueue_nativeWake
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->wake();
}
void NativeMessageQueue::wake() {
[2.5]
mLooper->wake();
}
2.5 Looper.cpp#wake
void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ wake", this);
#endif
//調用write方法給mWakeEventFd寫入一個1
uint64_t inc = 1;
ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd.get(), &inc, sizeof(uint64_t)));
if (nWrite != sizeof(uint64_t)) {
if (errno != EAGAIN) {
LOG_ALWAYS_FATAL("Could not write wake signal to fd %d (returned %zd): %s",
mWakeEventFd.get(), nWrite, strerror(errno));
}
}
}
不敢相信這個方法這么簡單，調用write方法給mWakeEventFd寫入一個1翰蠢。這樣就可以喚醒了项乒？是不是感覺有點太簡單了，關于更多的原理可以看下「handler機制--讓線程變?yōu)椤坝绖訖C”」的epoll機制和eventfd機制梁沧。
小結
到此發(fā)送Message并且喚醒流程就分析完了檀何，下面用一張時序圖總結下：
handler-send-message
3. 收到Message
上面第2步把Message發(fā)送到了MessageQueue中，并且調用nativeWake方法去喚醒阻塞的線程廷支，那就來看下频鉴，阻塞的線程是怎么被喚醒，并且收到Message的恋拍。
「handler機制--讓線程變?yōu)椤坝绖訖C”」這篇介紹過 MessageQueue.next--> android_os_MessageQueue_nativePollOnce --> nativeMessageQueue#pollOnce --> Looper#pollOnce --> Looper#pollInner --> epoll_wait,最終由于epoll_wait進入等待狀態(tài)垛孔，導致線程進入等待阻塞狀態(tài)，既然第2步已經(jīng)給mWakeEventFd寫入了數(shù)據(jù)施敢，那來看下epoll_wait被喚醒后是怎么來處理喚醒邏輯的
3.1 Looper.cpp#pollInner
int Looper::pollInner(int timeoutMillis) {
省略代碼......
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
//因為第2步周荐，調用write方法給mWakeEventFd寫數(shù)據(jù)了，epoll監(jiān)聽到mWakeEventFd上的數(shù)據(jù)僵娃，epoll_wait方法被喚醒概作，檢測到的event會被放入struct epoll_event數(shù)據(jù)結構中
int eventCount = epoll_wait(mEpollFd.get(), eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
// No longer idling.
mPolling = false;
// Acquire lock.
mLock.lock();
省略代碼......
//eventCount大于0
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
//因為是往mWakeEventFd寫入了數(shù)據(jù)，因此進入下面的邏輯
if (fd == mWakeEventFd.get()) {
//只關心EPOLLIN類型的事件
if (epollEvents & EPOLLIN) {
//調用awoken方法
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
} else {
//通過調用addFd方法加入的fd都會放在mRequests中默怨，從mRequests中查找是否有相應的fd請求
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex >= 0) {
int events = 0;
//根據(jù)事件類型讯榕，對events增加不同的事件值，EVENT_xxx 這些類型的事件是Looper中定義的
if (epollEvents & EPOLLIN) events |= EVENT_INPUT;
if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT;
if (epollEvents & EPOLLERR) events |= EVENT_ERROR;
if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP;
//把events和Request放入mResponse中先壕，為下一步分發(fā)做準備
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;
//mMessageEnvelopes：是一個包含native層的Message的隊列瘩扼，Message是通過調用Looper的sendMessage/sendMessageDelayed方法發(fā)送的。
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
//調用native層的MessageHandler的handleMessage方法把message分發(fā)出去
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.
//若mResponses的size大于0,則開始分發(fā)它的消息
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
// Invoke the callback. Note that the file descriptor may be closed by
// the callback (and potentially even reused) before the function returns so
// we need to be a little careful when removing the file descriptor afterwards.
//調用callback的handleEvent方法把事件發(fā)出去
int callbackResult = response.request.callback->handleEvent(fd, events, data);
if (callbackResult == 0) {
removeFd(fd, response.request.seq);
}
// 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;
}
在介紹上面方法的邏輯之前垃僚，先來介紹下下面幾個屬性:
mRequest
mRequest是用來存放Request對象的集绰，那Request來自何處呢？看下面代碼
int Looper::addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data) {
#if DEBUG_CALLBACKS
ALOGD("%p ~ addFd - fd=%d, ident=%d, events=0x%x, callback=%p, data=%p", this, fd, ident,
events, callback.get(), data);
#endif
if (!callback.get()) {
if (! mAllowNonCallbacks) {
ALOGE("Invalid attempt to set NULL callback but not allowed for this looper.");
return -1;
}
if (ident < 0) {
ALOGE("Invalid attempt to set NULL callback with ident < 0.");
return -1;
}
} else {
ident = POLL_CALLBACK;
}
{ // acquire lock
AutoMutex _l(mLock);
//用傳遞進來的參數(shù)初始化request
Request request;
//request的fd存儲了fd
request.fd = fd;
request.ident = ident;
request.events = events;
request.seq = mNextRequestSeq++;
//fd上有數(shù)據(jù)的時候谆棺，會調用callback回調
request.callback = callback;
request.data = data;
if (mNextRequestSeq == -1) mNextRequestSeq = 0; // reserve sequence number -1
//根據(jù)request來初始化epoll_event
struct epoll_event eventItem;
request.initEventItem(&eventItem);
//若mRequests中不存在fd對應的epoll_event,則下面的邏輯 調用epoll_ctl方法把fd對應的eventItem添加到mEpollFd上栽燕，這樣就可以通過epoll_wait方法監(jiān)聽fd上面的數(shù)據(jù)了
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex < 0) {
int epollResult = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, fd, &eventItem);
if (epollResult < 0) {
ALOGE("Error adding epoll events for fd %d: %s", fd, strerror(errno));
return -1;
}
把fd，request添加到mRequests中
mRequests.add(fd, request);
} else {
//若mRequests中存在改淑，則替換fd上的eventItem
int epollResult = epoll_ctl(mEpollFd.get(), EPOLL_CTL_MOD, fd, &eventItem);
省略代碼......
mRequests.replaceValueAt(requestIndex, request);
}
} // release lock
return 1;
}
上面addFd方法主要做的事情是：
- 根據(jù)傳遞的參數(shù)初始化一個Request對象碍岔，并且根據(jù)Request對象初始化epoll_event對象，再調用epoll_ctl方法把fd及對應的epoll_event對象添加到mEpollFd上朵夏，這樣mEpollFd就和當前的fd“綁定”在一起礼搁，在調用epoll_wait方法的時候褪秀，就可以監(jiān)聽當前fd上的數(shù)據(jù)了。
- 初始化的Request對象宽气，以fd為可以它的值為value添加到mRequests中，為啥要保存在mRequests中？主要目的是當epoll_wait監(jiān)聽到fd上的數(shù)據(jù)時候，會從mRequest中解析出它對應的Request對象，它存儲了callback回調對象衣屏，調用callback的handleEvent方法就可以fd，event等作為參數(shù)發(fā)送出去辩棒，epoll機制并沒有從fd中讀取數(shù)據(jù)的功能狼忱，要讀取數(shù)據(jù)需要在handleEvent方法中自己讀取
addFd的主要作用就是： 來輔助以管道或者其他類型管道方式實現(xiàn)線程之間/進程之間通信，把管道創(chuàng)建的其中一個fd通過epoll機制加入到它的“監(jiān)聽隊列”一睁，這樣就可以在epoll機制中監(jiān)聽fd上的數(shù)據(jù)從而實現(xiàn)線程之間/進程之間通信
mMessageEnvelopes
mMessageEnvelopes包含了native的Message對象钻弄，java層有Message，native層也有Message卖局，它的作用和java層是一樣的斧蜕，實現(xiàn)線程之間的通信⊙馀迹看下相關代碼:
//uptime,handler, message作為參數(shù) 發(fā)送一個消息和java層的是不是很類似
void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
const Message& message) {
#if DEBUG_CALLBACKS
ALOGD("%p ~ sendMessageAtTime - uptime=%" PRId64 ", handler=%p, what=%d",
this, uptime, handler.get(), message.what);
#endif
size_t i = 0;
{ // acquire lock
AutoMutex _l(mLock);
//從mMessageEnvelopes中批销，根據(jù)uptime來查找一個當前Message可存放的位置
size_t messageCount = mMessageEnvelopes.size();
while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) {
i += 1;
}
//把messageEnvelope放入mMessageEnvelopes
MessageEnvelope messageEnvelope(uptime, handler, message);
mMessageEnvelopes.insertAt(messageEnvelope, i, 1);
// Optimization: If the Looper is currently sending a message, then we can skip
// the call to wake() because the next thing the Looper will do after processing
// messages is to decide when the next wakeup time should be. In fact, it does
// not even matter whether this code is running on the Looper thread.
if (mSendingMessage) {
return;
}
} // release lock
// Wake the poll loop only when we enqueue a new message at the head.
//如果存放的位置是第一位，則調用wake方法喚醒
if (i == 0) {
wake();
}
}
上面方法所做的事情：從mMessageEnvelopes中染坯，根據(jù)uptime來查找一個當前Message可存放的位置均芽，初始化一個MessageEnvelope對象，并把它放入mMessageEnvelopes中单鹿；如果存放的位置是第一位掀宋，則調用wake方法喚醒
mResponses
該屬性主要是與上面mRequests屬性相對應的，就是對Request的一個響應仲锄。
在介紹完這幾個屬性后劲妙，下面接著pollInner方法的分析，epoll_wait方法監(jiān)聽event事件喚醒后儒喊，會依次做下面幾件事情：
- 處理epoll監(jiān)聽到的eventItems镣奋，依次從eventItems篩選相應的event，如果event的fd是mWakeEventFd怀愧，則單獨處理侨颈；否則從mRequests中查詢是否存在event的fd對應的Request，有則封裝Response芯义，等待后面的調用哈垢。為啥不現(xiàn)在直接處理這些Request而要放在Response中，等候處理呢扛拨？應該是基于處理mMessageEnvelopes的優(yōu)先級高于這個
- 處理native層的Message耘分，若mMessageEnvelopes中有Message，則加鎖開始處理mMessageEnvelopes中可以處理的Message，并且調用 handler->handleMessage方法把Message分發(fā)出去
- 處理mResponses陶贼，若mResponses中存在啤贩，則依次調用它對應的request.callback->handleEvent方法待秃，把事件分發(fā)出去
到此epoll_wait被喚醒拜秧，pollInner方法執(zhí)行完畢后，依次返回方法調用鏈的棧頂方法MessageQueue.next方法章郁，那來看下它的執(zhí)行
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();
}
//被喚醒枉氮，開始執(zhí)行它后面的邏輯
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
//msg指向鏈表頭部
Message msg = mMessages;
//同步屏障Message，這節(jié)暫時不討論下面的邏輯
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());
}
//因為在第2步暖庄，發(fā)送了一個Message聊替，因此msg不為null
if (msg != null) {
//若當前的時間戳小于msg的when，則代表這個msg還不到發(fā)送的時間培廓，再次調整nextPollTimeoutMillis值惹悄，調用nativePollOnce方法的時候，告訴它等待nextPollTimeoutMillis這么久的時間
if (now < msg.when) {
// 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 {
//拿到Message了肩钠，返回泣港，并設置mBlocked為非阻塞
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
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)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
//下面是idle類型Message，不屬于咱們這節(jié)的介紹价匠，后面會介紹
// 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();
} catch (Throwable t) {
Log.wtf(TAG, "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;
// 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;
}
}
next方法拿到Message后返回給調用它的方法Looper.loopOnce,那就來看下處理Message的過程
4. 處理Message
4.1 Looper#loopOnce
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
//從next方法返回Message
Message msg = me.mQueue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return false;
}
省略代碼......
try {
//調用target的dispatchMessage方法把msg分發(fā)給Handler当纱，讓它開始處理消息
[4.2]
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
省略代碼......
//回收msg
msg.recycleUnchecked();
return true;
}
4.2 Handler#dispatchMessage
public void dispatchMessage(@NonNull Message msg) {
//如果msg的callback不為null，則直接調用handleCallback方法
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//調用handleMessage方法把msg發(fā)出去
handleMessage(msg);
}
}
loopOnce方法把msg交給Handler的dispatchMessage方法后踩窖，Handler根據(jù)條件判斷調用handleMessage方法把msg交給真正處理者開始處理坡氯，咱們在第1步創(chuàng)建的Handler重寫了handleMessage，如下
Handler handler = new Handler(Looper.getMainLooper()){
@Override
public void handleMessage(@NonNull Message msg) {
super.handleMessage(msg);
if (msg.what == 1 ){
}
}
};
5. 回收Message
上面處理了Mssage后洋腮，會調用msg.recycleUnchecked()方法來回收msg箫柳，來看下代碼
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
//清除各種屬性的值
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = UID_NONE;
workSourceUid = UID_NONE;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
//加入到緩沖池的頭部
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
總結
到此Message從“誕生”，發(fā)送啥供，收到悯恍，處理，回收 這五個過程就分析完畢滤灯。但是Message之旅卻沒有結束坪稽，它還會不斷的重復上面的過程，因為Looper.loop方法它是一個死循環(huán)鳞骤，它還會不斷的從MessageQueue中取Message處理Message窒百。
也正因為Looper不斷的執(zhí)行著取消息/等待消息，處理消息這樣的循環(huán)豫尽，ActivityThread的main方法才會不斷的能循環(huán)起來篙梢，最終一個app才能運行起來。
阻塞/喚醒本質
MessageQueue中阻塞/喚醒的本質美旧，就是epoll渤滞，eventfd機制贬墩。eventfd創(chuàng)建返回的fd會通過epoll_ctl方法進行添加，這樣epoll就可以監(jiān)聽fd上的數(shù)據(jù)妄呕。當在其他線程中給MessageQueue添加Message后陶舞，若需要喚醒，則會最終給eventfd返回的fd上write一個int值绪励，epoll_wait方法因為fd上有數(shù)據(jù)導致它被喚醒肿孵。
