在Android 系統(tǒng)中通常都說有四大組件：Activity，Service，Broadcast和Content Provider，但是還有兩個最重要的東西是支撐整個Android 系統(tǒng)的進程間通信和線程間通信：Binder和消息隊列。
Binder，是Android 系統(tǒng)使用的進程間通信，不論是應(yīng)用層Java代碼編寫的進程還是C/C++編寫的native進程都是通過Binder進行進程間通信的，Binder通信的實現(xiàn)也是很復(fù)雜，牽扯到驅(qū)動，runtime，native和Java Service，最基本的實現(xiàn)是通過進程間的共享內(nèi)存來實現(xiàn)的，我們這次不討論Binder。
消息隊列，主要負責整個Android系統(tǒng)中消息的處理和分發(fā)，比如整個Android 系統(tǒng)InputManagerService就是依賴于Handler機制實現(xiàn)的，以及不同線程間通信的橋梁。
重要的幾個類
Looper: 不斷循環(huán)執(zhí)行(
Looper.loop), 按分發(fā)機制將消息分發(fā)給目標處理者Handler: 消息輔助類，主要功能是向消息池發(fā)送各種消息事件(
Handler.sendMessage)和處理相應(yīng)的消息事件(Handle.handleMessage)MessageQueue: 消息隊列的主要功能是向消息池投遞消息(
MessageQueue.enqueueMessage)和取走消息池的消息(MessageQueue.next)Message: 消息分為硬件產(chǎn)生的消息(如按鈕，觸摸)和軟件生產(chǎn)的消息
這幾個類在源代碼的位置：
framework/base/core/java/andorid/os/
- Handler.java
- Looper.java
- Message.java
- MessageQueue.java
類的框架圖：
Looper有一個MessageQueue消息隊列
MessageQueue有一組待處理的Message
Message中有一個用于消息處理的Handler
Handler中有Looper和MessageQueue
Handler線程間通信實例
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();
}
}
可以看到LooperThread是一個子線程，在這個線程的run方法中有實現(xiàn)了Handler的handleMessage方法，這個方法就是用來接收消息然后做相應(yīng)的處理，主線程就可以通過LooperThread的mHandler來發(fā)送消息到子線程中，從而實現(xiàn)線程間通信。
下面就圍繞著這段代碼來解析一下源代碼
Looper
Looper.prepare()
代碼里面直接調(diào)用的是Looper.prepare()，默認調(diào)用的是prepare(true)，標識的是這個Looper允許退出，false則表示不允許退出。
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
public static void prepare() {
prepare(true);
}
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));
}
這里的sThreadLocal是ThreadLocal類型。
ThreadLocal: 線程本地存儲區(qū)(Thread Local Storage, 簡稱為TLS)，每個線程都有自己的私有本地存儲區(qū)域，不同線程之間彼此不能訪問對方的TLS區(qū)域。
ThreadLocal的get()和set()方法操作的類型都是泛型，
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
可見sThreadLocal通過get和set的是Looper類型。
Looper.prepare()在每個線程中只允許執(zhí)行一次，該方法會創(chuàng)建Looper對象，Looper的構(gòu)造方法中會創(chuàng)建一個MessageQueue對象，再通過Looper對象保存到當前線程的TLS。
Looper構(gòu)造方法
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed); //創(chuàng)建MessageQueue對象
mThread = Thread.currentThread(); //記錄當前線程
}
Looper.loop()
public static void loop() {
final Looper me = myLooper(); //從TLS中獲取當前Looper對象
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue; //獲取Looper對象中的消息隊列
// 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();
//確保在權(quán)限檢查時基于本地進程，而不是調(diào)用進程
final long ident = Binder.clearCallingIdentity();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
for (;;) { //進入loop的主循環(huán)
Message msg = queue.next(); //可能會阻塞，取決于消息池中有無可用消息
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
//用于給target分發(fā)消息使用
msg.target.dispatchMessage(msg);
//記錄分發(fā)消息用了多長時間
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
}
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.
// 恢復(fù)調(diào)用者信息
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);
}
//講Message放入到消息池來循環(huán)利用
msg.recycleUnchecked();
}
}
loop()進入循環(huán)，不斷的重復(fù)下面的操作，知道MessageQueue.next()方法返回的是null
讀取MessageQueue的下一條Message
把Message分發(fā)給相應(yīng)的target
再把分發(fā)后的Message回收到消息池，以便重復(fù)利用
消息處理的核心部分就干這么幾件事情，
Looper.quit()
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
最終是調(diào)用到MessageQueue.quit()方法
void quit(boolean safe) {
//當mQuitAllowed為false，標識不允許退出，調(diào)用quit會拋出異常
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
//移除尚未觸發(fā)的所有消息
removeAllFutureMessagesLocked();
} else {
//移除所有的消息
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
消息退出的方式有兩種：
safe=true時，只移除上位觸發(fā)的所有消息，對于正在觸發(fā)的消息不移除safe=false時，移除所有的消息
Looper.myLooper
用于偶去TLS存儲的Looper對象
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
Handler
構(gòu)造Handler
無參構(gòu)造
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
//匿名類、內(nèi)部類或本地類都必須申明為static，否則會警告可能出現(xiàn)的內(nèi)存泄漏
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());
}
}
//必須先執(zhí)行Looper.prepare()，才能獲取Looper對象，否則為null
mLooper = Looper.myLooper(); //從當前線程的TLS中獲取Looper對象
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; //消息隊列，來自Looper對象
mCallback = callback; //回調(diào)方法
mAsynchronous = async; //設(shè)置消息是否為異步處理方式
}
對于Handler的無參構(gòu)造方法，默認采用當前線程TLS中的Looper對象，并且callback回調(diào)方法為null，且消息為同步處理方式。只要執(zhí)行的Looper.prepare()方法，那么便可以獲取有效的Looper對象。
有參構(gòu)造
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Handler類在構(gòu)造方法中，可指定Looper，Callback回調(diào)方法以及消息的處理方式(同步或異步)，對于無參的Handler，默認是當前線程的Looper。
消息分發(fā)機制
在Looper.loop()中，當發(fā)現(xiàn)有消息時，調(diào)用消息的目標handler，執(zhí)行dispatchMessage()方法來分發(fā)消息。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//當Message存在回調(diào)方法，回調(diào)msg.callback.run()方法
handleCallback(msg);
} else {
if (mCallback != null) {
//當Handler存在Callback成員變量時，回調(diào)方法handleMessage()
if (mCallback.handleMessage(msg)) {
return;
}
}
//Handler自身的回調(diào)方法handleMessage()
handleMessage(msg);
}
}
分發(fā)消息流程：
當
Message的回調(diào)方法不為空時，則回調(diào)方法msg.callback.run，其中callback數(shù)據(jù)類型為Runnable，否則進入步驟2當
Handler的mCallback成員變量不為空時，則回調(diào)方法mCallback.handleMessage(msg)，否則進入步驟3調(diào)用
Handler自身的回調(diào)方法handleMessage()，該方法默認為空，Handler子類通過覆寫該方法來完成具體的邏輯
對于很多情況下，消息分發(fā)后的處理方式是第3種情況，即Handler.hahndleMessage()，一般往往通過覆寫改方法從而實現(xiàn)自己的業(yè)務(wù)邏輯。
消息發(fā)送
發(fā)送消息調(diào)用流程
其實可以從圖中看出，利用Handler發(fā)送消息，最終調(diào)用到的是MessageQueue.enqueueMessage()
sendEmptyMessage
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
sendEmptyMessageDelayed
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
sendMessageDelayed
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
sendMessageAtTime
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
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);
}
sendMessageAtFrontOfQueue
public final boolean sendMessageAtFrontOfQueue(Message msg) {
MessageQueue queue = mQueue;
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, 0);
}
post
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
postAtFrontOfQueue
public final boolean postAtFrontOfQueue(Runnable r) {
return sendMessageAtFrontOfQueue(getPostMessage(r));
}
enqueueMessage
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
Handler.sendEmptyMessage()等系列方法最終調(diào)用MessageQueue.enqueueMessage(msg, uptimeMills)，將消息添加到消息隊列中，其中uptimeMillis為系統(tǒng)當前的運行時間。
Handler類還有一些其他的方法
obtainMessage最終調(diào)用Message.obtainMessage(this)，其中this為當前的Handler對象removeMessage
Handler是消息機制中非常重要的輔助類，更多的是實現(xiàn)MessageQueue, Message中的方法。
MessageQueue
MessageQueue是消息機制的Java層和C++層的連接紐帶，大部分核心工作都是通過JNI中的native方法去實現(xiàn)的，涉及到下面列的方法：
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
private native void nativePollOnce(long ptr, int timeoutMillis);
private native static void nativeWake(long ptr);
private native static boolean nativeIsPolling(long ptr);
private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);
創(chuàng)建MessageQueue
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
// 調(diào)用nativeInit獲取底層的指針對象的地址mPtr，可以通過mPtr來調(diào)用底下native的接口
mPtr = nativeInit();
}
next()
獲取下一條message
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) { //native層的指針對象被銷毀之后就推出循環(huán)
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//調(diào)用底層阻塞函數(shù)nativePollOnce來等待消息的到來
//等待超時時間為nextPollTimeoutMillis，等待消息來喚醒
nativePollOnce(ptr, nextPollTimeoutMillis);
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());
}
if (msg != null) {
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 {
// Got a message.
// 獲取一條消息，然后返回
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);
}
// 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;
}
}
nativePollOnce是阻塞操作，其中nextPollTimeoutMillis代表下一個消息到來前，還需要等待的時長，當nextPollTimeoutMills=-1時，代表消息隊列中沒有消息，會無限等待下去。
當處于空閑時，會執(zhí)行IdleHandler中的方法，當nativePollOnce()返回后，next()從mMessages中提取一個消息。
核心是nativePollOnce()做的事情，底下通過epoll的機制實現(xiàn)。
enqueueMessage
往消息池中添加一條消息，并且喚醒等待隊列。
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) {
if (mQuitting) { //正在退出
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;
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是按照Message觸發(fā)時間的先后順序排列的，對頭的消息是最早要觸發(fā)的。當有消息要加入消息隊列的時候，會遍歷隊列，尋找時間點插入要加入的消息，保證按照時間先后排序。
removeMessages
void removeMessages(Handler h, int what, Object object) {
if (h == null) {
return;
}
synchronized (this) {
Message p = mMessages;
// Remove all messages at front.
// 從頭尋找符合條件的消息移除
while (p != null && p.target == h && p.what == what
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycleUnchecked();
p = n;
}
// Remove all messages after front.
// 移除剩余符合要求的消息
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && n.what == what
&& (object == null || n.obj == object)) {
Message nn = n.next;
n.recycleUnchecked();
p.next = nn;
continue;
}
}
p = n;
}
}
}
這個一出消息的方法，采用了兩個while循環(huán)，第一個循環(huán)是從隊頭開始，移除符合條件的消息，第二個循環(huán)是從頭部移除完連續(xù)的滿足條件的消息后，再從隊列后面繼續(xù)查詢會否有滿足條件的消息需要移除。
postSyncBarrier
removeSyncBarrier
前面說每一個Message必須有一個target，對于特殊的message是沒有target，即同步barrier token，這個消息的價值就是用于攔截同步消息，所以并不會喚醒Looper
Message
消息對象
每一條Message的內(nèi)容;
| 數(shù)據(jù)類型 | 成員變量 | 解釋 |
|---|---|---|
| int | what | 消息類別 |
| long | when | 消息觸發(fā)時間 |
| int | arg1 | 參數(shù)1 |
| int | arg2 | 參數(shù)2 |
| Object | obj | 消息內(nèi)容 |
| Handler | target | 消息響應(yīng)方 |
| Runnable | callback | 回調(diào)方法 |
創(chuàng)建一個新消息，就是去填充這些內(nèi)容。
消息池
為了Message可以高效的重復(fù)利用，系統(tǒng)提供了消息池，當消息池不為空時，可以直接從消息池中獲取Message對象，而不是直接創(chuàng)建，提高效率。
靜態(tài)變量sPoll的數(shù)據(jù)類型為Message，通過next成員變量，維護一個消息池；靜態(tài)變量MAX_POLL_SIZE代表消息池的可用大小，消息池的默認大小為50
private static final int MAX_POOL_SIZE = 50;
消息池常用的操作方法是obtain()和recycle()
obtain
從消息池中獲取消息
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next; //從sPoll中取出一個Message對象
m.next = null; //斷開消息鏈
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
recyle
把不用的消息放入消息池，循環(huán)利用
public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();
}
/**
* Recycles a Message that may be in-use.
* Used internally by the MessageQueue and Looper when disposing of queued Messages.
*/
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;
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) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
recycle()，將Message加入到消息池的過程，都是把Message加到鏈表的表頭。
總結(jié)
Handler通過sendMessage()發(fā)送Message到MessageQueue隊列
Looper通過loop()，不斷提取出達到觸發(fā)條件的Message，并將Message交給target來處理
經(jīng)過dispatchMessage()后，交回給Handler的handleMessage()進行相應(yīng)的處理
將Message加入MessageQueue時，往管道寫入字符，來喚醒loop線程；如果MessageQueue中沒有Message，并處于Idle狀態(tài)，則會執(zhí)行IdleHanler接口中的方法，往往用于一些清理性的工作
消息分發(fā)的優(yōu)先級
Message的回調(diào)方法：
message.callback.run()Handler的回調(diào)方法：
Handler.mCallback.handleMessage(msg)Handler的默認方法：
Handler.handleMessage(msg)
