Handler原理的大致描述
一、APP启动
在ActivityThread 类的main方法（这是APP启动入口方法），会有下面代码
ActivityThread.java
public static void main(String[] args) {
//调用了prepare 方法 将当前线程与创建的Looper对象关联起来
Looper.prepareMainLooper();
//...删除多余的部分
ActivityThread thread = new ActivityThread();
thread.attach(false);
//...删除多余的部分
Looper.loop();
}
Looper.java
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
上面的代码解释一下，就是创建了一个Looper对象（Looper构造方法中一共做了两件事，创建了MessageQueue和获取到当前线程实例），然后与当前线程也就是主线程绑定，开启了一个loop()循环。
ThreadLocal 就是一个保存线程数据的容器，任何位置只要在同一个线程中，获取到的Looper都是同一个。
二、创建Handler 发送消息
private static class MyHandler extends Handler{
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.d("Handler","Message msg.what = "+msg.what);
}
}
new Thread(new Runnable() {
@Override
public void run() {
MyHandler myHandler = new MyHandler();
Message message = Message.obtain();
message.what = 100;
message.obj = " XXXxxxx";
myHandler.sendMessage(message);
}
}).start();
通常我们使用Handler 是上面的这种方式，上面主要就做了一件事 sendMessage(message)，这个方法主要完成了一个将 message消息添加到Message队列或者说Message消息池当中。
Handler.java
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;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
最终他会调用Handler类中的这个方法将 MessageQueue实例传递给enqueueMessage方法。
MessageQueue.java
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;
}
//核心部分的代码， 仔细理解发现他就只做了一件事，就是想消息串联起来，并且是按照when的大小从小打到排序
// 怎么理解 就是
// msg1.next = msg2
// msg2.next = msg3
// msg3.next = msg4
// msg4.next = mMessages = msg1
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;
// 这里有个for循环，很重要， 上面的操作就是在这个 for循环完成的
for (;;) {
prev = p;
p = p.next;
// 当当前消息的when 小于上一个消息的when
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
// 将两个消息变换一下
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
//唤醒挂起 或者延时
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
上面的代码比较难理解，我也是在 本子上比划了半天才勉强看懂一些，至于最后一步 msg4.next = mMessages = msg1这个结果是我计算出来的， 但是不太确定msg4.next = msg1 最后会这样联系起来。
简单概括sendMessage 就是完成了一个Message 加入到消息队列的一个操作，并且按照when从小到大的顺序
三、处理消息
回到第一步 Looper.loop()方法中开启了一个无线循环 不停的从消息队列中获取消息处理。
Looper.java
public static void loop() {
final Looper me = myLooper();
final MessageQueue queue = me.mQueue;
for (;;) {
//难点在 next中做了什么东西
Message msg = queue.next(); // might block
final long end;
try {
msg.target.dispatchMessage(msg);
}
}
}
上面的方法是删除了一些多余的方法之后的结果， 看上去非常好理解，就是queue.next() 获取一个对象，然后去msg.target.dispatchMessage(msg); 至于target是什么，以前博客中说道过就是Handler对象引用。 最后就是调用
第二步中的handleMessage(Message msg) 处理消息。
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;
//这里又有一个无限for循环
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//native 阻塞方法 nextPollTimeoutMillis = 0 表示不阻塞
// nextPollTimeoutMillis = -1 表示阻塞
// nextPollTimeoutMillis > 0 表示延时
// nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
//第二步中 说道过mMessages 最后是指向了 第一个msg1
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) {
// 如果获取到的消息大于当前时间 nextPollTimeoutMillis > 0 到时间以后就会唤醒执行
// 有一个问题 这个机制的前提就是保证 消息队列是已经按照when的大小排序好的
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;
}
}
这个方法就是 取出一个消息 计算出nextPollTimeoutMillis 值 判断是否需要阻塞。 然后释放CPU 等待被唤醒。
