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带着两个问题看源码：
- 一个线程可以有几个Looper？
- 一个线程可以有几个Handler？
1. Handler
/** 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));
}
Handler的prepare方法会把当前Thread 变成一个Looper。
成员变量
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;
sMainLooper 标识主线程Looper
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
该方法，会被Android 程序主入口，ActivityThread及SystemServer调用
为什么Looper 使用前要先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));
}
/**
* 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 traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
}
}
保证Looper完成初始化，每个Thread只能绑定一个Looper，不能重复创建。
Looper中死循环不断的处理队列中的Message。
为什么代码里，可以不用初始化Looper,直接使用 myHandler.sendMessage(message); 发送消息，然后在自定义的Handler的handleMessage(Message msg)中接收并处理呢？
这是因为这种场景下，当前线上是主线程，所以可以主线程mainThread 的mainLooper 系统已经默认初始化了。
2. Handler
当我们调用new Handler()时（无参构造子），会调用到
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;
}
callback = null，async = false。 可以看出此时Handler 关联了一个当前线程的Looper。
handler的作用：
There are two main uses for a Handler:
(1) to schedule messages and runnables to be executed as some point in the future;
(2) to enqueue an action to be performed on a different thread than your own.
- 调度message ,runnable 延后执行
2.切换线程执行任务
3. Message
Looper.loop()方法中，for循环不停取出Message，然后调用msg.target.dispatchMessage(msg)方法，用来分发传递进Looper的Message。
msg 为Message,target 为Handler
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
没有特殊设计都会执行到 handleMessage(msg);即执行用户设定的分发逻辑中。
常见的用法：
int MSG_START = 1;
private Handler mHandler = new Handler() {
public void handleMessage(Message msg) {
switch(msg.what){
......
}
}
}
class MyThread extends Thread {
@Override
public void run() {
mHandler.sendEmptyMessage(MSG_START);
//或者
Message msg = new Message();
msg.what = MSG_DOWN_SUCCESS;
//msg.arg1 = 111; 可以设置arg1、arg2、obj等参数
//msg.arg2 = 222;
// msg.obj = obj;
mHandler.sendMessage(msg);
}
}
Handler的实现中适及以下对象：
1、Handler本身：负责消息的发送和处理
2、Message：消息对象
3、MessageQueue：消息队列（用于存放消息对象的数据结构）是一个典型的生产者、消费者模式（ enqueueMessage()入队，next()出队 ）
4、Looper：消息队列的处理者（用于轮询消息队列的消息对象，取出后回调handler的dispatchMessage进行消息的分发，dispatchMessage方法会回调handleMessage方法把消息传入，由Handler的实现类来处理）
Message对象的内部实现是链表，最大长度是50，用于缓存消息对象，达到重复利用消息对象的目的，以减少消息对象的创建（享元模式），所以通常我们要使用obtainMessage方法来获取消息对象
安全：Handler的消息处理机制是线程安全的
关系:创建Handler时会创建Looper，Looper对象的创建又创建了MessageQueue
4 回答开头的两个小问题
4.1一个线程可以有几个Looper？
只能有一个，注意Looper.prepare()方法的提示“Only one Looper may be created per thread”！ 这是系统源码来保障的。
4.2 一个线程可以有几个Handler
没有限制，但是他们使用的消息队列都是同一个，也就是同一个Looper。货物再多，传送带只有一条。
同一个Looper是怎么区分不同的Handler的，换句话说，不同的Handler是怎么做到处理自己发出的消息的。
源码在Handler的sendMessage()，最终调用
Handler.enqueueMessage，即开头图示中依次进入队列过程
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以看到这一句msg.target = this;，这里就是将当前的Handler赋值给Message对象的target字段，这样在处理消息的时候通过msg.target就可以区分开不同的Handler了。
处理的方法在Looper.loop中：
依次取出Msg 过程
Looper.loop()
...
msg.target.dispatchMessage(msg);
...
在Message的obtain的各种重载方法里面也有对target的赋值
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