描述
結合幾個問題去看源碼。 1.Handler, MessageQueue, Message, Looper, LocalThread這5者在android的消息傳遞過程中扮演了什麼樣的角色？ 2.一個線程中可以有多個Handler嗎？多個Looper呢？ 3.整個消息處理過程，消息是怎麼流動的？ 4.為什麼隻有UI線程可以更改UI？（就憑他叫UI線程？其實也可以叫主線程或者ActivityThread）
開始看源碼
1.View的繪制都是從ViewRootImpl這個類開始，我們在這個類中找找，為什麼異步線程不能更新UI，發現了這個方法
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
其中mThread就是大名鼎鼎的UI線程，這段代碼說明不是UI線程就直接抛異常，相當粗暴。接着我們發現
@Override
public void requestFitSystemWindows() {
checkThread();
mApplyInsetsRequested = true;
scheduleTraversals();
}
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
還有更多就不一一列舉了，對UI的操作都調用了這個檢查的方法，是以非UI線程不能更新UI。這樣做有什麼好處？大概有2點 a.防止耗時操作導緻的ANR，嚴重影響體驗 b.UI控件不是線程安全的，多線程通路會出問題
2.接着看Handler,當我們new出一個Handler時，系統在幹什麼。
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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
這個構造器最直接。首先需要一個Looper，哪裡來的？在子線程中，我們通常要執行Looper.prepare()
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));
}
發現系統建立了一個Looper對象，并且存在了threadLocal中。當Looper建立的時候，同時建立了消息隊列
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
當handler構造時，調用Looper.myLooper()方法，從threadLocal中取出建立的Looper對象。如果你沒用調用Looper.myLooper()，執行下邊代碼就遇到那個熟悉的異常了。
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
在主線程中沒用調用，為什麼沒有報錯？因為ActivityThread幫你調了
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
看Looper.prepareMainLooper()和Looper.loop()都調了。
3.再看下Handler的post和send方法，Message怎麼被Handler送進MessageQueue的。發現最後他們都進了這個方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
通過enqueueMessage方法将消息體也就是Message對象壓入MessageQueue中。
4，然後調用Looper.loop()，開始處理消息
/**
* 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;
// 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();
}
}
一個死循環，然後不斷通過queue.next()方法把Message從MessageQueue中取出來處理
5.将消息取出來後，通過調用msg.target.dispatchMessage(msg)來處理消息。msg.target指的是Handler對象，來具體看下：
/**
* 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);
}
}
這個方法說明當有Runnable對象(handler.post(new Runnable())設定)時,消息交給handleCallback()處理
private static void handleCallback(Message message) {
message.callback.run();
}
直接調用了Runnable的run方法。這也解釋了handler.post(new Runnable())時沒有建立線程的問題。當有mCallBack（new Handler(new CallBack())時設定），調用callBack的方法handlerMessage()。都沒有，則調用Handler的handlerMessage方法。
6.自此消息基本從 産生->加入隊列->處理 過程基本走通了。
總結
1.很多細節沒講到，主要是從源碼角度分析整個過程 2.handler可以在任意線程發送消息，這些消息會被添加到關聯的MessageQueue上 3.消息最終會交給産生handler的線程處理，是以子線程可以持handler的引用更新UI 4.MessageQueue和Looper和目前線程相關 5.handler可以建多個，Looper隻能有一個 5.ThreadLocal是個存儲類，從他儲存建立的Looper對象可以看出，以後細說
遺留問題
1.handler的回調中明明列印的目前是子線程，為什麼還是可以更新UI？ 2.但是調用延時的方法後又不行了？
實驗代碼：
Log.e("---1---", String.valueOf(Thread.currentThread())
+ "\nThreadID:" + Thread.currentThread().getId());
new Thread(new Runnable() {
@Override
public void run() {
try {
Looper.prepare();
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
Log.e("---3---", String.valueOf(Thread.currentThread())
+ "\nThreadID:" + Thread.currentThread().getId());
btn.setText("handlerMessage");
}
};
Log.e("---2---", String.valueOf(Thread.currentThread())
+ "\nThreadID:" + Thread.currentThread().getId());
handler.sendEmptyMessage(0);
handler.sendEmptyMessageDelayed(0, 2000);
handler.post(new Runnable() {
@Override
public void run() {
Log.e("---4---", String.valueOf(Thread.currentThread())
+ "\nThreadID:" + Thread.currentThread().getId());
btn.setBackgroundColor(Color.YELLOW);
}
});
Looper.loop();
} catch (Exception e) {
e.printStackTrace();
}
}
}).start();
結果： 01-07 14:55:40.688 21362-21362/com.empty.animationtest E/---1---: Thread[main,5,main] ThreadID:1 01-07 14:55:40.698 21362-21394/com.empty.animationtest E/---2---: Thread[Thread-37592,5,main] ThreadID:37592 01-07 14:55:40.698 21362-21394/com.empty.animationtest E/---3---: Thread[Thread-37592,5,main] ThreadID:37592 01-07 14:55:40.698 21362-21394/com.empty.animationtest E/---4---: Thread[Thread-37592,5,main] ThreadID:37592 01-07 14:55:42.698 21362-21394/com.empty.animationtest E/---3---: Thread[Thread-37592,5,main] ThreadID:37592 01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: 01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views. 01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6294) 01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: at android.view.ViewRootImpl.invalidateChildInParent(ViewRootImpl.java:878) 01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: at android.view.ViewGroup.invalidateChild(ViewGroup.java:4344) 01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: at android.view.View.invalidate(View.java:10957) 01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: at android.view.View.invalidate(View.java:10912)
結果分析： 執行到3，4确實是子線程，也完成了UI的更新。而延時的第二個3，不能成功更新UI，為什麼？