一 概述
Handler主要被用来在子线程中访问UI线程，在ViewRootImpl中有一个checkThread()方法，对UI的操作都会有此验证。所以操作UI只能在主线程中进行。
概念：
Handler的运行由底层MessageQueue和Looper支撑。
MessageQueue消息队列，单链表存储消息列表。它是消息的存储单元。
Looper循环，它会无限循环查找是否有消息，有就处理掉。Looper创建一般被保存在TheadLoacal中。
创建：
Handler创建采用当前线程的Looper来构造消息循环系统。线程默认没有Looper
主线程(UI线程)即：ActivityThread,主线程创建时会初始化Looper,所以主线程可直接使用Handler
二 运行机制
2.1 TheadLoacal
TheadLoacal的用来保存数据，以线程作用域，不同线程具有不同的数据副本。
以一个例子来简单粗暴的说明：
//定义一个Integer泛型的ThreadLocal
private ThreadLocal mIntTheadLocal = new ThreadLocal();
mIntTheadLocal.set(0);
Log.d("yink","UI Thead, mIntTheadLocal = " + mIntTheadLocal.get());//UI线程
new Thread("thead 1") {
@Override
public void run() {
mIntTheadLocal.set(1);
Log.d("yink","thead 1, mIntTheadLocal = " + mIntTheadLocal.get());//线程1
}
}.start();
new Thread("thead 2") {
@Override
public void run() {
Log.d("yink","thead 2, mIntTheadLocal = " + mIntTheadLocal.get());//线程2
}
}.start();
mIntTheadLocal是同一个对象，我们分别在UI线程、线程1和线程2中对其进行set,get并打印Log如下：
07-06 23:24:33.151 12661-12661/com.example.android.myapplication D/yink: UI Thead, mIntTheadLocal = 0
07-06 23:24:33.153 12661-16454/com.example.android.myapplication D/yink: thead 1, mIntTheadLocal = 1
07-06 23:24:33.154 12661-16455/com.example.android.myapplication D/yink: thead 2, mIntTheadLocal = null
由于TheadLoacal的特性，它其中之一的功能就是被用来保存Looper对象
2.2 MessageQueue
MessageQueue,单链表数据结构，主要包含两个操作：插入(enqueueMessage)和读取(next)
先看enqueueMessage，它之做了单链表插入数据操作
boolean enqueueMessage(Message msg, long when) {
...
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;
//单链表插入数据操作。
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;
}
...
}
再看next，查询到msg后，移除这个msg,并返回这个msg
Message next() {
...
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
//被阻塞，在队列中查询下一个消息
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;
//msg不为空，移除msg
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
//返回msg
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
...
}
2.3 Looper
Looper作用：不停的从MessageQueue中查询是否有消息，有就处理，没有就一直阻塞。
我们先来一个demo,为子线程创建Looper，来理解Looper的作用：
//主线程中定义
private Handler mHandler;
new Thread("Thread 1") {
@Override
public void run() {
//创建Looper
Looper.prepare();
//在Thread 1中创建Handler
mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
Bundle b = new Bundle();
b = msg.getData();
Log.d("yink","receive the number = " + b.getInt(KEY));
switch (b.getInt(KEY)) {
case 3:
//子线程处理完事情后，退出循环
//Looper还提供一个quitSafely()，这个方法是设置标记位，把队列中消息处理完后再退出。
getLooper().quit();
break;
default:
break;
}
super.handleMessage(msg);
}
};
//循环,关键函数
Looper.loop();
}
}.start();
new Thread("Thread 2") {
@Override
public void run() {
for (int i = 0 ; i < 5 ; i++) {
Message msg = new Message();
Bundle b = new Bundle();
b.putInt(KEY,i);
msg.setData(b);
//如果线程1的run没执行的话，mHandler还是null,所以加个判断
if (mHandler != null) {
Log.d("yink","send key i = " + i);
//发送消息
mHandler.sendMessage(msg);
} else {
Log.d("yink","mHandler is null, i = " + i);
}
SystemClock.sleep(1000);
}
}
}.start();
结果如下：
07-07 02:29:24.219 1151 12404 D yink : send key i = 0
07-07 02:29:24.220 1151 12403 D yink : receive the number = 0
07-07 02:29:25.221 1151 12404 D yink : send key i = 1
07-07 02:29:25.223 1151 12403 D yink : receive the number = 1
07-07 02:29:26.223 1151 12404 D yink : send key i = 2
07-07 02:29:26.223 1151 12403 D yink : receive the number = 2
07-07 02:29:27.225 1151 12404 D yink : send key i = 3
07-07 02:29:27.226 1151 12403 D yink : receive the number = 3
07-07 02:29:28.227 1151 12404 D yink : send key i = 4
07-07 02:29:29.231 1151 12404 D yink : send key i = 5
ActivityThread中的Looper比较特殊，它的创建是：prepareMainLooper(),获得：getMainLooper,可在任何地方获取到主线程的Looper
接下来我们来看Looper中最重要的loop()方法：
public static void loop() {
...
for (;;) {
//无限循环获取queue.next(),由前面描述的MesageQueue的next方法，只有获取到消息才会返回，否则一直会阻塞，所以looper也会在这里阻塞
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
//只有当Looper调用退出后，queue.next()才会返回null,因为不退出，MesageQueue会阻塞在那儿一直查询
//此无限循环中，只有当msg == null才退出循环
return;
}
...
try {
//target实际就是Handler,所以最终调用到创建Handler线程中去。
msg.target.dispatchMessage(msg);
}
....
2.4 Handler
Handler主要也是两个动作：发送和接收
先看发送：
public final boolean sendMessage(Message msg){
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
发送过程最后调用到了enqueueMessage，最后是向队列中插入了一条消息。
最后通过Looper返回给了Handler的dispatchMessage，转向接收处理。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
第一种接收if (msg.callback != null) ：
如果msg.callback != null成立，则调用handleCallback(msg)来处理
private static void handleCallback(Message message) {
message.callback.run();
}
这个callback其实就是Handler.post中的runable
post过程如下：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);//调用getPostMessage
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;//把post中的Runnable赋值给msg.callback
return m;
}
证实:
new Thread("Thead 1") {
@Override
public void run() {
Looper.prepare();
mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
Log.d("yink","handleMessage ");
super.handleMessage(msg);
}
@Override
public void dispatchMessage(Message msg) {
Log.d("yink","dispatchMessage ");
super.dispatchMessage(msg);
}
};
Looper.loop();
}
}.start();
new Thread("Thead 2") {
@Override
public void run() {
mHandler.post(new Runnable() {
@Override
public void run() {
Log.d("yink","post runable");
}
});
}
}.start();
log如下，先是调用那个dispatchMessage然后走到调用了message.callback.run();
07-07 03:42:43.862 10475 10515 D yink : dispatchMessage
07-07 03:42:43.862 10475 10515 D yink : post runable
第二种接收else：
如果 if (mCallback != null) 则调用mCallback.handleMessage(msg)
这里的mCallBack为：
public interface Callback {
public boolean handleMessage(Message msg);
}
//Handler的一个构造函数
public Handler(Callback callback) {
this(callback, false);
}
可以在创建Handler的时候传递进来，这样就可以回调监听。就是一个接口回调，这里就不举例了。
如果mCallback == null就调用handleMessage，在源码中handleMessage是一个空函数，可以继承Handler实现handleMessage方法来进行监听。Looper中的demo就是实现handleMessage方法实现监听的。也可继承Handler实现，道理相同
//Handler中的handleMessage方法，一个空函数
public void handleMessage(Message msg) {
}
总结
Android的消息机制整体来说也就是Handler的消息机制，Handler、Looper和MessageQueue三者结合，实现了线程间的通信。明白这种机制，使用更加如鱼得水。
Read the fucking source code!