1. Handler通過sent或者post方法將目標(biāo)Message推送至MessageQueue。
這里涉及到隊(duì)列優(yōu)先級問題。Handler推送目標(biāo)Message時(shí)，會(huì)先通過循環(huán)將目標(biāo)Message的執(zhí)行時(shí)間與MessageQueue里原有Message的執(zhí)行時(shí)間進(jìn)行一一對比，并按照執(zhí)行時(shí)間先后順序?qū)⒛繕?biāo)Message插入MessageQueue。
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.");
}
synchronized (this) {
//如果當(dāng)前消息循環(huán)已經(jīng)結(jié)束，直接退出
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;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;//頭部消息
boolean needWake;
//如果隊(duì)列中沒有消息，或者當(dāng)前進(jìn)入的消息比消息隊(duì)列中的消息等待時(shí)間短，那么就放在消息隊(duì)列的頭部
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 {
//判斷喚醒條件，當(dāng)前當(dāng)前消息隊(duì)列頭部消息是屏障消息，且當(dāng)前插入的消息為異步消息
//且當(dāng)前消息隊(duì)列處于無消息可處理的狀態(tài)
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
//循環(huán)遍歷消息隊(duì)列，把當(dāng)前進(jìn)入的消息放入合適的位置（比較等待時(shí)間）
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
//將消息插入合適的位置
msg.next = p;
prev.next = msg;
}
//調(diào)用nativeWake，以觸發(fā)nativePollOnce函數(shù)結(jié)束等待
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
2.Looper通過loop（）方法獲取MessageQueue里的Message。
loop（）方法會(huì)通過死循環(huán)不斷調(diào)用MessageQueue的next方法來獲取Message，并判斷等待時(shí)間，如果還需要等待則等到相應(yīng)時(shí)間后提取Message。
Message next() {
//如果退出消息消息循環(huán)，那么就直接退出
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();
}
//執(zhí)行native層消息機(jī)制層,
//timeOutMillis參數(shù)為超時(shí)等待時(shí)間。如果為-1，則表示無限等待，直到有事件發(fā)生為止。
//如果值為0，則無需等待立即返回。該方法可能會(huì)阻塞
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
//獲取系統(tǒng)開機(jī)到現(xiàn)在的時(shí)間，如果使用System.currentMillis()會(huì)有誤差，
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;//頭部消息
//判斷是否是柵欄，同時(shí)獲取消息隊(duì)列最近的異步消息
if (msg != null && msg.target == null) {
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
//獲取消息，判斷等待時(shí)間，如果還需要等待則等待相應(yīng)時(shí)間后喚醒
if (msg != null) {
if (now < msg.when) {//判斷當(dāng)前消息時(shí)間，是不是比當(dāng)前時(shí)間大，計(jì)算時(shí)間差
// 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 {
// 不需要等待時(shí)間或者等待時(shí)間已經(jīng)到了，那么直接返回該消息
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 {
//沒有更多的消息了
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
//判斷是否已經(jīng)退出了
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.
//獲取空閑時(shí)處理任務(wù)的handler 用于發(fā)現(xiàn)線程何時(shí)阻塞等待更多消息的回調(diào)接口。
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
//如果空閑時(shí)處理任務(wù)的handler個(gè)數(shù)為0，繼續(xù)讓線程阻塞
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
//判斷當(dāng)前空閑時(shí)處理任務(wù)的handler是否是為空
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
//只有第一次迭代的時(shí)候，才會(huì)執(zhí)行下面代碼
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);
}
//如果不保存空閑任務(wù)，執(zhí)行完成后直接刪除
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// 重置空閑的handler個(gè)數(shù)，因?yàn)椴恍枰貜?fù)執(zhí)行
pendingIdleHandlerCount = 0;
//當(dāng)執(zhí)行完空閑的handler的時(shí)候，新的native消息可能會(huì)進(jìn)入，所以喚醒Native消息機(jī)制層
nextPollTimeoutMillis = 0;
}
}
Looper取出Message后會(huì)執(zhí)行message.target.dispatchMessage()方法，即handler的dispatchMessage（）方法，并在該方法里執(zhí)行handleMessage。至此，Handler的一個(gè)消息循環(huán)結(jié)束。
參考文檔：
http://www.itdecent.cn/p/219701879fe4
http://www.itdecent.cn/p/3d8f7ec1017a
