Handler 是一种异步处理机制，那么他的主要用途是什么呢？ 在项目中我们经常要在子线程中进行一些耗时操作，但是不可以进行更新UI，那么这里就需要用到Handler来传递数据，去解决UI更新的问题。
那么Handler比不可少的三部分分别是：Looper、MessageQueen、ThreadLocal
1.Handler的首要操作是实例Handler拿到Handler对象
private Handler handler=new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};源码: 要我们知道Handler需要Lopper,所以需要在这个Lopper线程中创建Lopper,开启Lopper的循环。
public class LooperThread extends Thread {
@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
// 开始循环处理消息队列
Looper.loop();
}
}源码: 要创造Lopper的时候也会创造出一个MessageQueen,主要是通过prepere的一个构造方法，创造出来的。
public static void prepareMainLooper() {
prepare(false); //创建looper对象及MessageQueue
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper(); //获取looper对象
}
源码: 这里就引发了下一个问题，Threadlocal：他是线程内部的数据存储类，可以进行存储和获取数据，因为我们looper是通过他来获取到的，然而一个Thread只能有一个Looper,他们互不共享。这里就是对Lopper的一个存储的功能。
public class Looper {
// 每个线程中的Looper对象其实是一个ThreadLocal，即线程本地存储(TLS)对象
private static final ThreadLocal sThreadLocal = new ThreadLocal();
// Looper内的消息队列
final MessageQueue mQueue;
// 当前线程
Thread mThread;
//其他属性
// 每个Looper对象中有它的消息队列，和它所属的线程
private Looper() {
mQueue = new MessageQueue();
mRun = true;
mThread = Thread.currentThread();
}
// 我们调用该方法会在调用线程的TLS中创建Looper对象
public static final void prepare() {
if (sThreadLocal.get() != null) {
// 试图在有Looper的线程中再次创建Looper将抛出异常
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
// 其他方法
}源码: 在new Handler的同时，有一个构造方法通过get方法获取到Lopper。这里我们已经拿到了Lopper:
public static final Looper myLooper() {
// 在任意线程调用Looper.myLooper()返回的都是那个线程的looper
return (Looper)sThreadLocal.get();
}现在我们已经拿到了Lopper和MessageQueen，接下来就要进行发送消息了进行MessageQueen处理数据然后就要进行发送消息了，首先要创建一个Message ，那么Handler的底层有一个obtainMessage的方法，设置一些属性，比如下列：
- public int what：变量，用于定义此Message属于何种操作
- public Object obj：变量，用于定义此Message传递的信息数据，通过它传递信息
- public int arg1：变量，传递一些整型数据时使用
- public int arg2：变量，传递一些整型数据时使用
- public Handler getTarget()：普通方法，取得操作此消息的Handler对象。
Message message = myHandler.obtainMessage();
message.what = 1; //消息标识
myHandler.sendMessage(message); //发送消息
handler.sendMessage(message);源码: 在创建好Message使用obtain方法进行发送，再进行判断，放入MessageQueen当中去.
public final Message obtainMessage(int what, Object obj)
{
return Message.obtain(this, what, obj);
}源码: 因为我们的MessageQueen是一个消息存储单元，并不能进行消息的处理，所以Lopper进行不断的循环，是否有新消息，否则就一直处于阻塞的状态，如果有心消息就通过事件分发发送到Lopper。MessageQueen包含两个操作：euqueueMessage和next方法：
euqueueMessage：
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
next：
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
可以发现next是一个无限循环的方法，没有新消息时，一直循环，阻塞在这里，有新消息，就会移除，返回这条消息。
源码: 那么Lopper就会一直检查MessageQueen是否有新消息，有的话就立即去处理，当中有一个next的方法，我们知道Lopper是一个队列，但是他的底层是以个单链表 ，能够与让我们更好的去添加和删除消息，然后把消息就传给了Handler 就可以通过事件分发调用handlerMessage了，这样就完美的完成了Handler的所有过程！
public static final void loop() {
Looper me = myLooper(); //得到当前线程Looper
MessageQueue queue = me.mQueue; //得到当前looper的MQ
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
// 开始循环
while (true) {
Message msg = queue.next(); // 取出message
if (msg != null) {
if (msg.target == null) {
// message没有target为结束信号，退出循环
return;
}
// 日志
if (me.mLogging!= null) me.mLogging.println(
">>>>> Dispatching to " + msg.target + " "
+ msg.callback + ": " + msg.what
);
// 非常重要！将真正的处理工作交给message的target，即后面要讲的handler
msg.target.dispatchMessage(msg);
// 日志
if (me.mLogging!= null) me.mLogging.println(
"<<<<< Finished to " + msg.target + " "
+ msg.callback);
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf("Looper", "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();
}
}
}