public class Handler {
final MessageQueue mQueue;
final Looper mLooper;
final Callback mCallback;
final boolean mAsynchronous;
IMessenger mMessenger;
/*参数中不带looper的构造方法会回调Handler(Callback callback, *boolean *async) 这个构造方法*/
public Handler() { this(null, false); }
public Handler(Callback callback){ this(callback, false);}
public Handler(boolean async) { this(null, async); }
/*参数中带looper的构造方法会回调Handler(Looper looper,Callback *callback, boolean *async) 这个构造方法*/
public Handler(Looper looper) {this(looper, null, false);}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
}在看Handler(Callback callback, boolean async){}方法
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) {
//这句话的意思是 handler要设置为静态，否则可能会发生内存泄露
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
/*没有Looper.prepare()的线程是不能创建handler的即需要调用
*Looper.prepare()对mLooper 进行初始化*/
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
/* 重点：直接把关联looper的MQ作为自己的MQ，因此它的消息将发送到关联*looper的MQ上*/
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}1)Looper.prepare()
public final class Looper {
static final ThreadLocal<Looper> sThreadLocal = new
ThreadLocal<Looper>();
private static Looper sMainLooper;
final MessageQueue mQueue;
final Thread mThread;
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
/*ThreadLocal用于线程间的数据隔离，每个线程内部维护一个独立内存区域，数据为线程独有，不共享>*/
if (sThreadLocal.get() != null) {
//一个线程只能有一个looper
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
}由上述代码可知，Looper.prepare()是一个将线程转换为looperThread的过程，为了迎合一个thread只能有一个looper对象这一原则，首先会校验当前线程是否已经有looper对象，有则抛出异常，没有则创建looper对象，并把它存储在sThreadLocal中，以供looper.myLooper（）获取当前线程的looper对象。
2）Looper.loop()
public static void loop() {
final Looper me = myLooper();
if (me == null) {
//looper没有初始化，即没有调用了looper.prepare().
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) {
//MessageQueue 中没有Message 时，退出消息队列读取消息的循环
// 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);
}
/*由message的target（即handler）来处理从MessageQueue中取出的 *message*/
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);
}
// 回收message资源
msg.recycle();
}
}调用loop方法后，Looper线程就开始真正工作了，它不断从自己的MQ中取出队头的消息(也叫任务)执行。
3)looper的其他方法
//这个方法不需要我们自己调用，系统会自动调用
public static void prepareMainLooper() {
/* prepare(false)结合Messagequeue的quit(boolean)方法可知主线程是不能 *被quit的*/
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}4）LooperThread 样板
public class LooperThread extends Thread {
private Handler handler1;
private Handler handler2;
@Override
public void run() {
// 将当前线程初始化为Looper线程
Looper.prepare();
// ...其他处理，如实例化handler
handler1 = new Handler();
handler2 = new Handler();
// 开始循环处理消息队列
Looper.loop();
}
}由此可知：一个线程可以有多个Handler，但是只能有一个Looper！
5)handler发送消息
post(Runnable)
postAtTime(Runnable, long)
postDelayed(Runnable, long)
sendEmptyMessage(int)
sendMessage(Message)
sendMessageAtTime(Message, long)
sendMessageDelayed(Message, long)这些方法向MQ上发送消息了
这里介绍一下post(Runnable)
public final boolean post(Runnable r){
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
}
public final boolean sendMessageDelayed(Message msg,
long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}由上述代码可知post发出的Runnable对象最后都被封装成message对象，在这个过程中由getPostMessage(r)将runnable封装成message的callback，再由handler的enqueueMessage（）调用MessageQueue的enqueueMessage（）方法 将消息添加到消息队列中。
6)handler处理消息
// 处理消息，该方法在looper.loop()中被调用
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
// 如果message设置了callback，即runnable消息，处理callback！
handleCallback(msg);
} else {
// 如果handler本身设置了（Handler.Callback）callback，则执行callback
if (mCallback != null) {
/* 这种方法允许让activity等来实现Handler.Callback接口，避免了自己编写*handler重写handleMessage方法。见http://alex-yang-xiansoftware-*com.iteye.com/blog/850865 */
if (mCallback.handleMessage(msg)) {
return;
}
}
// 如果message没有callback，则调用handler的钩子方handleMessage
handleMessage(msg);
}
}
// 处理runnable消息
private final void handleCallback(Message message) {
message.callback.run(); //直接调用run方法！
}
// 由子类实现的钩子方法
public void handleMessage(Message msg) {
}
public interface Callback {
public boolean handleMessage(Message msg);
}7）ThreadLocal理解http://blog.youkuaiyun.com/qjyong/article/details/2158097
8)总结：
1. handler要设置为静态，否则可能会发生内存泄露
2. 线程创建handler之前必须调用looper.prepare（）对looper初始化
3. looper.loop()开始从消息队列取出队头message交给handler的diapatchMessage（）处理消息
4. handler.post()发出的Runnable对象最后都被封装成message对象
5. 一个线程可以有多个Handler，但是只能有一个Looper对象
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