if (me == null) {
throw new RuntimeException(“No Looper; Looper.prepare() wasn’t called on this thread.”);
}
final MessageQueue queue = me.mQueue;
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + “: " + msg.what);
}
try {
msg.target.dispatchMessage(msg);
} finally {}
if (logging != null) {
logging.println(”<<<<< Finished to " + msg.target + " " + msg.callback);
}
msg.recycleUnchecked();
}
}
public void quit() {
mQueue.quit(false);
}
}
如果主线程发生了异常，就会退出循环，意味着APP崩溃，所以我们我们需要进行try-catch，避免APP退出，我们可以在主线程再启动一个 Looper.loop() 去执行主线程任务，然后try-catch这个Looper.loop()方法，就不会退出。
基于 Handler 实现单线程的线程池
从上面的 Looper.loop() ，我们可以利用 Handler 实现单线程池功能，而且这个线程池和主线程一样拥有立刻执行post()、延迟执行postDelayed()、定时执行postAtTime()等强大功能。
// 错误用法
var handler: Handler? = null
Thread({
handler = Handler()
}).start()
当我们在异步线程执行上面的代码，就会报错 Can't create handler inside thread Thread[Thread-2,5,main] that has not called Looper.prepare()。 这个是因为 Handler 的工作是依靠 Looper ，必须为线程创建 Looper 才能正常功能，正确的用法如下：
// 正确用法
var handler: Handler? = null
Thread({
Looper.prepare()
handler = Handler()
Looper.loop()
}).start()
测试：
button.setOnClickListener {
handler?.post {
println(Thread.currentThread())
}
handler?.post {
println(Thread.currentThread())
}
}
输出结果：
System.out: Thread[Thread-2,5,main]
System.out: Thread[Thread-2,5,main]
HandlerThread
HandlerThread 是 Androi
d 对Thread的封装，增加了Handler的支持，实现就是实现了前面例子的功能
val handlerThread = HandlerThread(“test”)
handlerThread.start()
handler = Handler(handlerThread.looper)
MessageQueue 源码剖析
我们都知道Handler的功能非常丰富，拥有立刻执行post()、延迟执行postDelayed()、定时执行postAtTime()等执行方式。下面就从源码分析是如何实现的。
public final class MessageQueue {
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;
}
}
}
MessageQueue.next() 是一个带有阻塞的方法，只有退出或者有任务才会return，起阻塞的实现是使用Native层的 nativePollOnce() 函数，如果消息队列中没有消息存在nativePollOnce就不会返回，一直处于Native层等待状态。直到调用 quit() 退出或者调用 enqueueMessage(Message msg, long when) 有新的任务进来调用了Native层的nativeWake()函数，才会重新唤醒。 android_os_MessageQueue.cpp
nativePollOnce(long ptr, int timeoutMillis)
nativePollOnce 是一个带有两个参数的Native函数，第一个参数是作为当前任务队列ID；第二个参数是等待时长，如果是-1，就代表无消息，会进入等待状态，如果是 0，再次查找未等待的消息。如果大于0，就等到指定时长然后返回。
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
在这行代码进行延时的赋值，从而实现postDelayed、postAtTime的功能
enqueueMessage()
看到这里我们可能会有一个疑问，既然是队列，先进先出的原则，那么以下代码输出的结果是如何？
handler?.postDelayed({ println(“任务1”) },5000)
handler?.post { println(“任务2”) }
handler?.postDelayed({ println(“任务3”) },3000)
// 输出结果
任务2
任务3
任务1
之所以是如此，是因为在 enqueueMessage(Message msg, long when) 添加任务的时候已经就已经按照执行的时间要求做好了排序。
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException(“Message must have a target.”);
}
if (msg.isInUse()) {
Message(Message msg, long when)` 添加任务的时候已经就已经按照执行的时间要求做好了排序。
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException(“Message must have a target.”);
}
if (msg.isInUse()) {