这篇文章讲的Android SDK版本是API 23。
Android分为UI线程(主线程)，和普通的线程(自己写的线程)。做Android的都知道，需要启动线程请求网络或者做
耗时的操作，否则会报ANR异常。另外不能在子线程中去更新UI，不然又会报异常Only the original thread that created a view hierarchy can touch its views.所以需要使用
消息循环。子线程没有消息循环需要自己添加。UI线程默认有消息循环，这是项目启动的时候系统添加进去的，看ActivityThread.java类源码：
public static void main(String[] args) {
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);
<span style="background-color: rgb(255, 0, 0);">Looper.loop();</span>
throw new RuntimeException("Main thread loop unexpectedly exited");
}这个类是项目启动的时候自己调用的，为我们创建了消息循环红色这行Looper.prepareMainLooper();是Looper里的，后面会讲到。所以在UI线程 中创建Handler的时候不需要创建消息循环。但是在子线程中创建Handler就需要创建消息循环，也就是调用Looper.prepare();Looper.loop(); 为子线程创建消息循环。
例如：
class LooperThread extends Thread {
public void run() {
Looper.prepare();
Log.d("MainActivity", "after prepare");
Handler mHandler=new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.d("MainActivity", msg.what"");
}
};
mHandler.sendMessage(new Message());
mHandler.sendEmptyMessage(0);
Looper.loop();
Log.d("MainActivity", "after loop");
}
}两行关键的代码，Looper.prepare()和Looeper.loop()。看下Looper.prepare();的源代码：
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));
}
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
上面代码其实就是把一个Looper实例添加到了当前线程的本地变量里，也就是调用了红色的这么一句代码：sThreadLocal.set(new Looper(quitAllowed)); 想必大家看到了prepareMainLooper()，这里面也是调用了prepare方法，这个方法是有ActivityThread自己调用的，所以UI线程默认会有消息循环！！！每一个线程只有一个Looper，否则会抛出throw new RuntimeException("Only one Looper may be created per thread");
如果不懂ThreadLocal，可以去网上查查。所以说Looper与线程的关联就是通过ThreadLocal来实现的。这样就给线程创建了一个消息循环
而且在创建Looper实例的时候初始化了一个MessageQueue实例，看Looper的构造方法：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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);
}
<span style="color:#FF0000;">msg.target.dispatchMessage(msg);</span>
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();
}
}那么到底是怎么分发出去的呢？又分发到哪里去了呢？重点看这一行代码： msg.target.dispatchMessage(msg); Message的target是Handler类型，Handler把消息添加到消息队列的时候把自己赋值给了Message的target，看下面的代码：private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }Looper.loop()是通过循环不断的把消息队列中的消息分发给当前的Handler，所以Handler既是消息的发送者，又是消息的处理者。它是怎么做的呢？看Handler的创建过程：
public Handler() {
this(null, false);
}
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;
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}创建Handler的时候调用: mLooper=Looper.myLooper(); 也就是Looper里的方法从当前Handler所在的线程中取出了属于当前线程的Looper，并且mQueue = mLooper.mQueue;取出了消息队列mQueue。 所以Handler发送消息就是把消息添加到了MessageQueue中，看Handler发送消息的一步步调用：
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
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;//这里刚才已经说过，Handler把自己赋值给了Message的target
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}这里是通过层层调用的，不过没关系，只是一些方法的封装。最后一个方法最后一行： queue.enqueueMessage(msg, uptimeMillis); 就是调用消息队列的方法把消息添加进入了，自己可以去看看。在添加消息的过程中有这么一句msg.target = this;这里就是给target赋值。 this当然是指的Handler，所以在loop方法中才可以调用Handler的dispatchMessage(Message msg)方法。 看下dispatchMessage(Message msg)源码：
/**
* 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);</span>
}
}在这个方法里我们看到了熟悉的方法： handleMessage(msg); 这就是在重写Handler的时候自己实现的方法，这是个空的没有实现的方法，需要我们自己实现，这里仅仅是回调。到此Handler就完成了发送消息到 接收处理消息的过程。
总的来说就是，在创建Handler的时候取得了Handler所在线程的Looper实例，和Looper中的MessageQueue实例，这个过程是通过ThreadLocal实现。然后把消息添加到该MessageQueue中，当调用loop方法的时候，取出消息，然后调用消息中Handler的dispatchMessage方法，把消息又交给了Handler的HandleMessage处理。
Handler还有post方法，handler.post(Runnable r),它的参数是一个线程实例，内部也是创建了一个消息，看源码：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}在getPostMessage中获取了一个消息： Message.obtain();这也是官方推荐的获取消息的方法，效率高。并把post的线程实例赋值给了消息的callBack变量：Runnable callback;当然， 在后面的消息分发时候做了判断，再次看Handler的dispatchMessage方法：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}这里是做了判断的，当msg.callback不为空，也就是post的时候回调用：handleCallback(msg);这里面调用了Runnable的run方法，也就是我们自己写的：
private static void handleCallback(Message message) {
message.callback.run();
}至此，消息机制讲完了，其中它们的关联实现，ThreadLocal起了很大作用。其实通过Handler更新UI用的比较少了。可以用回调，也可以用Android的AsynTask类。不过AsynTask效率不高，不常用，下一篇讲AsynTask类！！！
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