handler是什么？
handler是Android提供用来更新UI的一套消息机制，也是一套消息处理的机制（发送和处理消息）
handler原理
handler负责消息发送，looper负责接收handler发送过来的消息，并把消息发送给handler，messageQueue存储消息的容器
这里先说明一下ThreadLocal，主要在线程中保存变量信息，主要有两个比较重要的方法，一个是get方法，一个是set方法
public void set(T value) {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}
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set方法设置当前线程的值，使用键值对的形式存储Thread和looper之间的关系，Thread作为key，looper作为value
public T get() {
// Optimized for the fast path.
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values != null) {
Object[] table = values.table;
int index = hash & values.mask;
if (this.reference == table[index]) {
return (T) table[index + 1];
}
} else {
values = initializeValues(currentThread);
}
return (T) values.getAfterMiss(this);
}
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get方法就是取出当前线程对应的looper，也就是说ThreadLocal是负责thread和looper之间的关系的
下面看一下Looper.prepare()方法
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));
}
默认情况下ThreadLocal是没有存储的，所以要创建一个新的looper
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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默认情况下ThreadLocal是没有存储的，所以要创建一个新的looper
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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从looper方法中，创建了一个MessageQueue，在looper中维护着一个消息队列
知道了looper和MessageQueue之后，究竟handler跟这两者有什么关系呢，继续看源码
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;
}
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首先调用Looper.myLooper()
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
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获得当前的looper对象，通过looper拿到MessageQueue，就完成了handler和looper之间的关联
下面继续看handler的消息发送
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);
}
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先获得当前的消息队列，如果队列为空就抛出异常，不为空，向消息队列中插入消息
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
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插入消息之前就指定消息发送给谁（msg.target），默认情况下发送给自己的handler，然后把消息放入队列中，handler就完成了发送message到MessageQueue的过程
那么消息又是如何轮询的呢？
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.recycleUnchecked();
}
}
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通过myLooper()方法获取当前looper，进而获得当前的消息队列，然后通过MessageQueue的next方法获取消息，消息为空时返回，不为空时，调用handler的dispatchMessage(msg)方法，然后这个过程一直循环
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
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首先查看msg.callback是否为空，不为空时去调用handleCallback(msg)，这个方法在handler的构造方法中存在，可以实现消息的拦截；为空只就
调用handleMessage(msg)，这个方法都是大家熟悉的，不在描述，整体的handler的原理就描述到这。
总结
handler在Android中扮演的非常重要的角色，熟悉handler的原理，不仅在面试的时候有用，就连activity的生命周期也是通过handler发送消息，详细请看源码
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