本文主要參考：http://www.cnblogs.com/codingmyworld/archive/2011/09/12/2174255.html
前言說明
先看一段非常經典的代碼，我們從會這裏入手，慢慢分析源碼去找這五個類的相互關係，以及消息封裝，消息入隊，消息出隊，消息處理整個過程。
public class LooperThread extends Thread {
private Handler handler1;
private Handler handler2;
@Override
public void run() {
// 將當前線程初始化爲Looper線程
Looper.prepare();
// 實例化兩個handler
handler1 = new Handler();
handler2 = new Handler();
// 開始循環處理消息隊列
Looper.loop();
}
} 先貼一張，我理解的它們五者的關係圖：
當這個Looper線程啓動會依次會prepare()和looper這兩個方法。
我們會以這兩個方法爲主線，瀏覽源碼逐步分析他們之間的關係。
先看prepare().
private static final String TAG = "Looper";
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
/** 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);
}
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));
}聽起來有一點繞是吧，爲什麼不直接保存一下Looper對象呢？而ThreadLocal<Looper>這個類又是做什麼的？Looper與Thread又是什麼關係？
看來要把這幾個問題說清楚，還是要從sThreadLocal.set(new Looper(quitAllowed)); 這個方法說起啊。
/**
* Sets the value of this variable for the current thread. If set to
* {@code null}, the value will be set to null and the underlying entry will
* still be present.
*
* @param value the new value of the variable for the caller thread.
*/
public void set(T value) {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}
/**
* Gets Values instance for this thread and variable type.
*/
Values values(Thread current) {
return current.localValues;
}
/**
* Creates Values instance for this thread and variable type.
*/
Values initializeValues(Thread current) {
return current.localValues = new Values();
}
/**
* Sets entry for given ThreadLocal to given value, creating an
* entry if necessary.
*/
void put(ThreadLocal<?> key, Object value) {
cleanUp();
// Keep track of first tombstone. That's where we want to go back
// and add an entry if necessary.
int firstTombstone = -1;
for (int index = key.hash & mask;; index = next(index)) {
Object k = table[index];
if (k == key.reference) {
// Replace existing entry.
table[index + 1] = value;
return;
}
if (k == null) {
if (firstTombstone == -1) {
// Fill in null slot.
table[index] = key.reference;
table[index + 1] = value;
size++;
return;
}
// Go back and replace first tombstone.
table[firstTombstone] = key.reference;
table[firstTombstone + 1] = value;
tombstones--;
size++;
return;
}
// Remember first tombstone.
if (firstTombstone == -1 && k == TOMBSTONE) {
firstTombstone = index;
}
}
}從上面的代碼分析可以知道sThreadLocal.set(new Looper(quitAllowed)); 實質是將Thread類的localValues的成員變量設置一個新的Looper，所以也就是說一個線程只能有一個Looper。
消息出隊和消息處理
再來看一下Looper的looper()方法：
/**
* 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.recycle();
}這裏有五個關鍵的步驟
(1)final Looper me = myLooper(); 根據當前的線程獲取Looper對象。
(2)final MessageQueue queue = me.mQueue;根據當前的Looper獲取它MessageQueue類型的成員變量
(3)Message msg = queue.next();出隊例。
(4)msg.target.dispatchMessage(msg); 將msg傳遞給目標Handler，再出所在的Handler進行處理。
(5)msg.recycle(); 消息回收
再來仔細看一下Handler是怎樣處理消息的?
/**
* 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);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}(1) 先看Message的Runnable類型的callback成員變量是否爲空，如果不爲空。 則直接調用callback的run方法。
請注意這裏調用的是run方法哦，而不是我們經常用的start方法。所有說這裏雖然是使用了Runnable，但是我們沒有把它作爲另外一個線程來處理。run方法裏的執行仍然和Handler處在同一個線程。
(2)先是嘗試通過Callback回調的方法處理消息。如果處理成功(即Callback的handleMessage方法返回true)，則直接返回。如果處理失敗可在由重寫Handler的handleMessage方法來處理(親，雖然Callback和Handler處理消息的方法都叫handleMessage,但是Handler裏那個沒有返回布爾值哦)。
以上講的是消息MessageQueue出取出，在由Handler處理的過程。
消息封裝和消息入隊
下面要說的是怎樣把消息放入MessageQueue裏。我們經常會這樣寫：
(1)先是obtain一下
/**
* 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;
/**
* Sends this Message to the Handler specified by {@link #getTarget}.
* Throws a null pointer exception if this field has not been set.
*/
public void sendToTarget() {
target.sendMessage(this);
}通過以上兩步，消息就成功的進入到對應的MessageQueue裏啦。
/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
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);
}可以看到通過以上三個方法，Message就進入到了MessageQueue裏了。
哦，所以我們現在重點就是MessageQueue從哪裏來的？
我們在Handler的構造方法找到了如下代碼：
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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;
}(1)根據Handler得到當前運行的Thread線程
(2)通過Thread裏面的localValues成員變量找到線程裏面的Looper
(3)再拿到Looper裏面MessqgeQueue的引用就可以了。