本篇文章給大家分享的是有關深入淺析Android中的消息機制，小編覺得挺實用的，因此分享給大家學習，希望大家閱讀完這篇文章后可以有所收獲，話不多說，跟著小編一起來看看吧。
一、簡介
Android的消息機制主要是指Handler的運行機制，那么什么是Handler的運行機制那？通俗的來講就是，使用Handler將子線程的Message放入主線程的Messagequeue中，在主線程使用。
二、學習內容
學習Android的消息機制，我們需要先了解如下內容。
平常我們接觸的大多是Handler和Message，今天就讓我們來深入的了解一下他們。
三、代碼詳解
一般而言我們都是這樣使用Handler的
xxHandler.sendEmptyMessage(xxx);
當然還有其他表示方法，但我們深入到源代碼中，會發現，他們最終都調用了一個方法
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);
}sendMessageAtTime()方法，但這依然不是結束，我們可以看到最后一句enqueueMessage(queue, msg, uptimeMillis);按字面意思來說插入一條消息，那么疑問來了，消息插入了哪里。
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;
}進入源代碼，我們發現，我們需要了解一個新類Messagequeue。
雖然我們一般把他叫做消息隊列，但是通過研究，我們發下，它實際上是一種單鏈表的數據結構，而我們對它的操作主要是插入和讀取。
看代碼33-44，學過數據結構，我們可以輕松的看出，這是一個單鏈表的插入末尾的操作。
這樣就明白了，我們send方法實質就是向Messagequeue中插入這么一條消息，那么另一個問題隨之而來，我們該如何處理這條消息。
處理消息我們離不開一個重要的，Looper。那么它在消息機制中又有什么樣的作用那？
Looper扮演著消息循環的角色，具體而言它會不停的從MessageQueue中查看是否有新消息如果有新消息就會立刻處理，否則就已知阻塞在那里，現在讓我們來看一下他的代碼實現。
首先是構造方法
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}可以發現，它將當前線程對象保存了起來。我們繼續
Looper在新線程創建過程中有兩個重要的方法looper.prepare() looper.loop
new Thread(){
public void run(){
Looper.prepare();
Handler handler = new Handler()；
Looper.loop();
}
}.start();我們先來看prepare()方法
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));
}咦，我們可以看到這里面又有一個ThreadLocal類，我們在這簡單了解一下，他的特性，set()，get()方法。
首先ThreadLocal是一個線程內部的數據存儲類，通過它可以在指定的線程中存儲數據，數據存儲后，只有在制定線程中可以獲取存儲的數據，對于其他線程而言則無法獲取到數據。簡單的來說。套用一個列子：
private ThreadLocal<Boolean> mBooleanThreadLocal = new ThreadLocal<Boolean>();//
mBooleanThreadLocal.set(true);
Log.d(TAH,"Threadmain"+mBooleanThreadLocal.get());
new Thread("Thread#1"){
public void run(){
mBooleanThreadLocal.set(false);
Log.d(TAH,"Thread#1"+mBooleanThreadLocal.get());
}；
}.start();
new Thread("Thread#2"){
public void run(){
Log.d(TAH,"Thread#2"+mBooleanThreadLocal.get());
}；
}.start();上面的代碼運行后，我們會發現，每一個線程的值都是不同的，即使他們訪問的是同意個ThreadLocal對象。
那么我們接下來會在之后分析源碼，為什么他會不一樣?，F在我們跳回prepare()方法那一步，loop()方法源碼貼上
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;
// 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 block
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);
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();
}
}首先loop()方法，獲得這個線程的Looper，若沒有拋出異常。再獲得新建的Messagequeue，在這里我們有必要補充一下Messagequeue的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;
}
}從24-30我們可以看到，他遍歷了整個queue找到msg，若是msg為null，我們可以看到50，他把nextPollTimeoutMillis = -1;實際上是等待enqueueMessage的nativeWake來喚醒。較深的源碼涉及了native層代碼，有興趣可以研究一下。簡單來說next()方法，在有消息是會返回這條消息，若沒有，則阻塞在這里。
我們回到loop()方法27msg.target.dispatchMessage(msg);我們看代碼
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}msg.target實際上就是發送這條消息的Handler，我們可以看到它將msg交給dispatchMessage(),最后調用了我們熟悉的方法handleMessage(msg);
以上就是深入淺析Android中的消息機制，小編相信有部分知識點可能是我們日常工作會見到或用到的。希望你能通過這篇文章學到更多知識。更多詳情敬請關注億速云行業資訊頻道。
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