Handler使用方法
1、Handler.sendMessage（）
2、Handler.post（）
//自定义一个handler类
class myHander extends Handler {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
mEtInput.setText(msg.obj.toString());
i++;
if (i == 5) {
return;
}
Message message = new Message();
message.what = i;
message.obj = "test" + i;
myHandler.sendMessageDelayed(message,1000);
}
}
//创建自定义handler实例
myHandler = new myHander();
//thread类实现多线程显示
new Thread() {
@Override
public void run() {
super.run();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
//创建消息对象
Message msg = Message.obtain();
message.obj = "test";
//子线程通过handle发消息到消息队列中
myHandler.sendMessage(msg);
}
}.start();
//或者通过post方法使用
final Handler handler1 = new Handler();
new Thread() {
@Override
public void run() {
super.run();
handler1.postDelayed(new Runnable() {
@Override
public void run() {
mEtInput.setText("handler post");
}
}, 12000);
}
}.start();
handler使用主要涉及Handler、Lopper、MessageQueue。
整体流程可以概述为：handler通过sendMessage()以及post()方法，将消息发送到消息队列中，post方法最终也是调用的sendMessage()方法，然后由Loop.loop()，方法遍历MessageQueue，通过queue.next()，方法去除msg，若有msg就通过msg.target获得由跟msg绑定的handler调用.dispatchMessage(msg)处理，若msg.callback != null就会由重写的run()方法处理，若msg.callback为null则由重写的handleMessage(Message msg)方法处理。
下面一步步跟着源码分析handler的每一步：
/**
*Handler构造方法
*/
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
...
//在构造方法里通过Looper.myLooper()获取mLooper
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//通过mLooper.mQueue获得mLooper 中的mQueue
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
//通过这次代码我们可以得知在handler的构造函数中，handler与lopper绑定同时等于跟looper中的messagequeue绑定，
//一个线程有lopper的前提主线程与子线程不同，在app启动时主线程ActivityThread的main方法会调用Looper.prepareMainLooper()方法，
//子线程则通过自己调用Looper.prepare()生成Looper对象。
}
上述我们可以知道每一个Looper中都有一个MessageQueue与其绑定。下面看一下如何实现的：
/**
*Looper.prepareMainLooper()方法
*在ActivityThread的main()方法里面会调用Looper.prepareMainLooper()方法
*Looper.prepareMainLooper()最终调用prepare(false)方法
*同时创建主线程 开启消息的自动轮询
*/
public static void main(String[] args) {
...
Looper.prepareMainLooper();//这个方法最终调用的还是prepare(false);方法
ActivityThread thread = new ActivityThread();//创建主线程
thread.attach(false);
...
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();//开启消息的自动轮询
throw new RuntimeException("Main thread loop unexpectedly exited");
}
所以主线程的Looper.prepareMainLooper()实现与MessageQueue绑定，直接看Looper.prepare()就好。
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
//sThreadLocal.get()不为null会抛异常 说明prepare()方法只能调用一次
//也就是说一个线程只有一个Looper实例
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
//创建一个消息队列 这里便是Looper与MessageQueue绑定的地方
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
主线程与子线程创建Handler与Lopper以及MessageQueue向绑定的方式大致相同，但是主线程可以自动调用Looper.loop()以实现循环遍历的的功能，子线程需要自行调用，下面看一下Looper.loop()的具体实现：
/**
*主线程消息循环是不允许退出，即无限循环
*子线程消息循环允许退出，调用MessageQueue的quit()方法
*/
public static void loop() {
// 返回sThreadLocal存储的Looper实例
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 获取Looper实例中的消息队列对象（MessageQueue）
final MessageQueue queue = me.mQueue;
...
// 无限循环
for (;;) {
// 从消息队列中取出消息
Message msg = queue.next(); // might block
// 若为空则线程阻塞
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
try {
// msg.target获得handler，把Message派发给对应的handler
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
// 回收释放资源
msg.recycleUnchecked();
}
}
下面对queue.next()以及msg.target.dispatchMessage(msg)两个方法进行分析：
/**
* queue.next()
*/
Message next() {
...
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// nativePollOnce方法在Native层，当nextPollTimeoutMillis==-1时即消息队列中没有消息，消息队列出于等待状态
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
...
// 按照先进先出的顺序取出消息
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;
}
...
}
}
/**
* dispatchMessage(msg)
*/
// msg.callback != null 证明是使用的post()方法发送消息 会回调Runnable对象里复写的run()方法
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
// msg.callback为空，证明使用的sendMessage(Message msg)发送消息，会回调复写的handleMessage(msg)
handleMessage(msg);
}
}
循环next遍历出消息之后，通过dispatchMessage(msg)进行分发，分发给handler处理，那handler是什么时候赋值给msg从而可以通过msg.target获得handler实例的呢？handler在最初始就是sendMessage(msg)，然后msg才会进入消息队列，所以应该是在sendMessage(msg)方法里面，下面我们分析一下sendMessage(msg)方法：
//一路下来 发现最后是调用到MessageQueue的enqueueMessage(Message msg, long when)方法
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;
...
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
// 这时候还在Handler这个类里面所以，this代表Handler
msg.target = this;
...
return queue.enqueueMessage(msg, uptimeMillis);
}
// 这个方法就是MessageQueue里面的方法了
boolean enqueueMessage(Message msg, long when) {
...
synchronized (this) {
...
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// 消息队列没有消息 就在头部加入消息 如果消息队列属于等待状态 则唤醒消息队列
// 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;
// 有消息根据消息创建的时间插入队列
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;
}
上文中提到过post方法
// 这里会将Runnable对象封装成msg对象并且发送到消息队列中
handler1.postDelayed(new Runnable() {
@Override
public void run() {
mEtInput.setText("handler post");
}
}, 12000);
// postDelayed调用sendMessageDelayed 这就与handler的sendMessage方法一样
// sendMessageDelayed(Message msg, long delayMillis) getPostMessage(r)应该获得一个msg的返回
public final boolean postDelayed(Runnable r, long delayMillis){
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
// 这里给msg的callback赋值一个人Runnable对象 把Runnable 对象封装成了一个Message对象
m.callback = r;
return m;
}
最终调用sendMessageDelayed方法就与sendMessage()方法一样了，唯一的区别就在于Handler.post()方法不需要传入Message对象，
而是sendMessageDelayed(getPostMessage(r), delayMillis)的getPostMessage(Runnable r)方法里将Runnable 对象封装成了一个Message对象，
之后回调也就变成了复写Runnable对象的run()方法。
本文主要学习https://www.jianshu.com/p/b4d745c7ff7a 文末再粘贴两站作者总结的图：
Handler.sendMessage()
Handler.Post()
