Android 消息機(jī)制就是handler的運(yùn)行機(jī)制以及MessageQueue和Looper的工作過(guò)程耸三。
那么Android的消息機(jī)制為什么要用Handler呢？
在Android中泄鹏，UI線程是非線程安全的臊旭，所以規(guī)定只能在主線程中訪問(wèn)UI線程。而且主線程中不能進(jìn)行耗時(shí)的操作量没，所以把耗時(shí)操作放在異步線程中執(zhí)行艘款。那如何讓子線程中的獲取的數(shù)據(jù)在主線程中進(jìn)行使用呢持际，這個(gè)時(shí)候就用到了Handler。
Handler的運(yùn)行機(jī)制 少不了Handler Message MessageQueue Looper幾個(gè)類(lèi)的支撐哗咆，下面分別對(duì)每個(gè)類(lèi)進(jìn)行介紹蜘欲。
Handler
A Handler allows you to send and process {@link Message} and Runnable objects associated with a thread's {@link MessageQueue}. Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler, it is bound to the thread /message queue of the thread that is creating it -- from that point on, it will deliver messages and runnables to that message queue and execute them as they come out of the message queue.
大致意思：handler 通過(guò)關(guān)聯(lián)一個(gè)消息隊(duì)列來(lái)發(fā)送和處理消息，發(fā)送或處理Runnable對(duì)象
Handler的作用
There are two main uses for a Handler: (1) to schedule messages and runnables to be executed as some point in the future; and (2) to enqueue an action to be performed on a different thread than your own.
大致意思：handler 通過(guò)關(guān)聯(lián)一個(gè)消息隊(duì)列來(lái)發(fā)送和處理消息晌柬，發(fā)送或處理Runnable對(duì)象
- 在未來(lái)某個(gè)時(shí)間點(diǎn)執(zhí)行分發(fā)的message/runnable
- 在其他線程執(zhí)行action
介紹完handler 來(lái)看看handler的 constructor的實(shí)現(xiàn)
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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
首先檢測(cè)用戶創(chuàng)建的handler類(lèi)是不是static的姥份，如果不是則會(huì)提示The following Handler class should be static or leaks might occur郭脂，意思就說(shuō)會(huì)有導(dǎo)致內(nèi)存泄露的可能。
在子線程中如果沒(méi)有創(chuàng)建looper澈歉，則會(huì)拋出異痴辜Γ“has not called Looper.prepare()”，因?yàn)樵谥骶€程中闷祥，系統(tǒng)默認(rèn)就會(huì)創(chuàng)建looper，所以我們平時(shí)使用handler時(shí)傲诵，沒(méi)有創(chuàng)建looper是不報(bào)錯(cuò)的凯砍，但是在子線程中會(huì)拋出異常的，接著就是將handler和MessageQueue進(jìn)行關(guān)聯(lián)拴竹。
Handler的主要工作包含消息的發(fā)送和接收悟衩，先分析發(fā)送：
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
//上面的方法最終都會(huì)走到這里，這個(gè)方法主要功能還是實(shí)現(xiàn)了enqueueMessage
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);
}
接著分析enqueueMessage：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以看到栓拜，Handler發(fā)送消息的過(guò)程就是向MessageQueue中插入一條消息座泳，下面會(huì)講到MessageQueue通過(guò)next方法獲取出消息，然后返回給looper幕与，在looper的loop方法中執(zhí)行了msg.target.dispatchMessage(msg); msg.target 就是指的handler挑势。那么接著分析dispatchMessage：
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
Message的callback就是一個(gè)Runnable對(duì)象，日常使用handler時(shí)會(huì)重寫(xiě)handlerMessage方法去處理消息啦鸣，而callback的意義就是當(dāng)我們不想實(shí)現(xiàn)handlerMessage方法時(shí)潮饱，就可以用callback代替。
Looper
used to run a message loop for a thread 簡(jiǎn)單的說(shuō)就是 為一個(gè)線程進(jìn)行消息輪詢诫给，典型的使用案例如下代碼所示：
* <pre>
* 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();
* }
* }</pre>
在創(chuàng)建handler之前執(zhí)行Looper.prepare()香拉， 創(chuàng)建handler之后執(zhí)行Looper.loop()即可。
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
作用時(shí)在確定執(zhí)行輪詢（loop）之前中狂，創(chuàng)建handler并關(guān)聯(lián)looper凫碌，執(zhí)行prepare后確定執(zhí)行l(wèi)oop，結(jié)束用quit胃榕。接著跟蹤下：
prepare(boolean)方法
這里面的入?yún)oolean表示Looper是否允許退出盛险，true就表示允許退出，對(duì)于false則表示Looper不允許退出勋又。
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));
}
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
上面是子線程創(chuàng)建handler必須要調(diào)用的枉层，后者是應(yīng)用創(chuàng)建主線程時(shí)調(diào)用，使用者可不理會(huì)赐写。關(guān)于sThreadLocal 后面文章會(huì)繼續(xù)講解鸟蜡。接著看下Looper的構(gòu)造方法：
Looper(boolean)構(gòu)造函數(shù)
private Looper(boolean quitAllowed) {
// 創(chuàng)建MessageQueue對(duì)象
mQueue = new MessageQueue(quitAllowed);
// 記錄當(dāng)前線程
mThread = Thread.currentThread();
}
通過(guò)上面代碼，我們發(fā)現(xiàn)就是創(chuàng)建了一個(gè)MessageQueue挺邀，并且把當(dāng)前線程賦值給本地變量的mThread揉忘。這里就實(shí)現(xiàn)了Looper和MessageQueue跳座，Thread的關(guān)聯(lián)，也知道Looper類(lèi)不是通過(guò)構(gòu)造函數(shù)直接來(lái)創(chuàng)建looper泣矛，而是通過(guò)靜態(tài)方法調(diào)用間接創(chuàng)建Looper類(lèi)疲眷。前面我們講了Looper的作用就是為線程輪詢消息，接著看真正的實(shí)現(xiàn)：
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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;
boolean slowDeliveryDetected = false;
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
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 {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
msg.recycleUnchecked();
}
}
通過(guò)queue.next() 獲取消息您朽，當(dāng)消息為null的時(shí)候狂丝，MessageQueue就會(huì)調(diào)用quit或者quitSafety方法并被標(biāo)記為退出狀態(tài)，當(dāng)消息隊(duì)列被標(biāo)記為退出狀態(tài)時(shí)哗总，next方法就會(huì)返回null几颜。換句話說(shuō)，loop方法會(huì)調(diào)用MessageQueue的next方法來(lái)獲取新消息讯屈，next方法是一個(gè)阻塞操作蛋哭，當(dāng)沒(méi)有消息時(shí)，next方法會(huì)一致阻塞涮母，loop方法也會(huì)被阻塞谆趾，當(dāng)有新消息到達(dá)時(shí)，Looper就會(huì)繼續(xù)處理消息了叛本。
msg.target.dispatchMessage(msg)分發(fā)消息沪蓬，msg.target 就是指當(dāng)前線程綁定的handler，msg.recycleUnchecked() 將消息放入消息池来候。
Message
Defines a message containing a description and arbitrary data object that can be sent to a {@link Handler}. This object contains two extra int fields and an extra object field that allow you to not do allocations in many cases.
一個(gè)實(shí)現(xiàn)了序列化的對(duì)象怜跑。接著看一下obtion方法：
/**
* Same as {@link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
* @param h Handler to assign to the returned Message object's <em>target</em> member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
可以看到message.target = handler ,就可以理解了上面Looper.loop()方法分發(fā)消息的target 就是handler，可以知道先是handler發(fā)送消息吠勘，然后handler分發(fā)消息性芬，最后自己實(shí)現(xiàn)handleMessage方法處理消息，其他obtain方法大同小異剧防。
MessageQueue
MessageQueue主要包含兩個(gè)功能植锉，enqueueMessage和next。
先看一下enqueueMessage：
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) {
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;
}
從實(shí)現(xiàn)來(lái)看主要就是單鏈表的插入操作峭拘，接著再看next方法：
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);
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.
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;
}
}
next方法中也有一個(gè)for循環(huán)俊庇，如果有新消息，next方法會(huì)retrun這條消息并且從鏈表中移除鸡挠，否則的話會(huì)一直阻塞辉饱，直到有新消息。
總結(jié)：
Looper類(lèi)用來(lái)為一個(gè)線程開(kāi)啟一個(gè)消息循環(huán)拣展。默認(rèn)情況下android中新創(chuàng)建的線程是未開(kāi)啟消息循環(huán)的彭沼。（主線程除外，主線程系統(tǒng)會(huì)自動(dòng)為其創(chuàng)建Looper對(duì)象备埃，開(kāi)啟消息循環(huán)姓惑。）
Looper對(duì)象通過(guò)MessageQueue來(lái)存放消息和事件褐奴。一個(gè)線程只能有一個(gè)Looper，對(duì)應(yīng)一個(gè)MessageQueue于毙。（如果對(duì)一個(gè)已經(jīng)quit的Looper重新start會(huì)出現(xiàn)異常）通常是通過(guò)Handler與Looper進(jìn)行交互的敦冬，Handler可看做是Looper的一個(gè)接口，用來(lái)向指定的Looper發(fā)送消息及定義處理方法唯沮。
默認(rèn)情況下Handler會(huì)與其被定義時(shí)所在線程的Looper綁定脖旱，比如，Handler在主線程中定義介蛉，那么它是與主線程的Looper綁定萌庆。
mainHandler = new Handler() 等價(jià)于new Handler（Looper.myLooper()）.
Looper.myLooper()：獲取當(dāng)前進(jìn)程的looper對(duì)象，類(lèi)似的 Looper.getMainLooper() 用于獲取主線程的Looper對(duì)象甘耿。在非主線程中直接new Handler() 會(huì)報(bào)如下的錯(cuò)誤:
E/AndroidRuntime( 6173): Uncaught handler: thread Thread-8 exiting due to uncaught exception
E/AndroidRuntime( 6173): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
原因是非主線程中默認(rèn)沒(méi)有創(chuàng)建Looper對(duì)象踊兜，需要先調(diào)用Looper.prepare()啟用Looper竿滨。Looper.loop()讓Looper開(kāi)始工作佳恬，從消息隊(duì)列里取消息，處理消息于游。注意：寫(xiě)在Looper.loop()之后的代碼不會(huì)被執(zhí)行毁葱，這個(gè)函數(shù)內(nèi)部應(yīng)該是一個(gè)循環(huán)，當(dāng)調(diào)用mHandler.getLooper().quit()后贰剥，loop才會(huì)中止倾剿，其后的代碼才能得以運(yùn)行。
