爲什麼像寫Handler，基本網上已經一大堆了，一個是自己歸納一下，加深點記憶，一個是都工作這麼久了，好像沒有自己好好看看源碼總結下，感覺怪怪的。
首先，看看Handler
默認構造函數，在當前線程中，通過looper關聯當前handler。
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
通過備註，我們能抓到一個關鍵點Looper，這個Looper是什麼？等下再看，通過this繼續追下去看看Handler
/**
* 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 respect 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;
}
中間關鍵代碼：mLooper = Looper.myLooper(); mQueue = mLooper.mQueue;
這個時候構造方法中，創建looper，又通過成員變量mLooper獲取的MessagerQueue，這時候handler基本出現了形狀了。
mLooper = Looper.myLooper();
mQueue = mLooper.mQueue;
我們繼續看myLooper（）：return sThreadLocal.get();
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
自此，進入了Looper類，Looper是什麼呢？看一下Looper類的介紹
備註可以看出來，該類用於爲線程運行消息循環。線程默認情況沒有關聯消息循環，需要創建一個。
prepare（）使線程中運行一個循環，loop()處理消息直到循環停止。
/**
* Class used to run a message loop for a thread. Threads by default do
* not have a message loop associated with them; to create one, call
* {@link #prepare} in the thread that is to run the loop, and then
* {@link #loop} to have it process messages until the loop is stopped.
*
* <p>Most interaction with a message loop is through the
* {@link Handler} class.
*
* <p>This is a typical example of the implementation of a Looper thread,
* using the separation of {@link #prepare} and {@link #loop} to create an
* initial Handler to communicate with the Looper.
*
* <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>
*/
public final class Looper {。。。。}
通過備註，出現關鍵的方法，Prepare（），Loop（），先看prepare（）
該方法初始化當前looper，注意，這個方法一定執行在loop（）方法之後，
/** 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));
}
prepare（）創建一個looper對象並將looper對象設置給了sThreadLocal中，保證每一個線程looper的唯一性。
通過不同的線程，訪問同一個ThreadLocal，不管是set方法還是get方法，他們對ThreadLocal做的讀寫操作僅限與各自線程內部。
注意：sThreadLocal.set(new Looper(quitAllowed));和前面的return sThreadLocal.get(); 對應
此時：Looper和線程關聯了！！！
看看Loop（）：其實他就是創建了一個消息隊列循環，逐個獲取消息，處理消息的過程。
運行消息隊列，for循環中next（）循環讀取消息。
/**
* 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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}
msg.target.dispatchMessage(msg);
消息進行分發處理了，那麼這個target又什麼呢？我們進入到Message類中，發現它就是Handler
/*package*/ Handler target;
繞了一圈，又回到Handler。其實handler把消息傳遞給了消息隊列，消息隊列循環又把消息分發給Handler處理。
Handler作用兩個就是發送消息，處理消息。
追到Handler裏面的 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);
}
}
msg.callback是否爲空，callback其實也就是一個Runable對象
/*package*/ Runnable callback;
爲空，線程run方法
private static void handleCallback(Message message) {
message.callback.run();
}
不爲空，走mCallback.handleMessage(msg)，由handlerMessage處理
/**
* 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);
}
Handler.sentMessage(Message msg)方法發送一個消息
所發送的消息將加入消息隊列的末端
返回值Boolean，true表示消息成功加入消息管道Message queue，返回false失敗。Message queue的出口是looper 處理
/**
* 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);
}
looper中quit（）
/**
* Quits the looper.
* <p>
* Causes the {@link #loop} method to terminate without processing any
* more messages in the message queue.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p class="note">
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
* </p>
*
* @see #quitSafely
*/
public void quit() {
mQueue.quit(false);
}
Handler引起的內存泄漏：
靜態內部類持有外部類的匿名引用，導致外部activity無法釋放，然後就內存泄漏了~
解決辦法：handler改爲靜態內部類，或者調用handler.removeCallbake()
private Handler mHandler=new Handler();
改成：
private static Handler mHandler=new Handler();