本文章基于AndroidAPI28，主要分析messageQueue 单链表实现的意义和用法、messageQueue.next() 以及 Looper.loop()方法
1. 相关调用图
ps：不会画图简单看一下吧
1.1 handler机制调用时序图.png
2. 相关方法介绍
1.1 Handler
使用方式不具体介绍了！（简单说一下）
内部类重写handMessage方法，调用handler.post(sendMessage 等其他方法)，最终都会调用sendMessageAtTime() 方法
// uptimeMillis = SystemClock.uptimeMillis() + delayMillis 下面会用到
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);
}
mQueue 在构造函数中注入，默认构造使用主线程Looper.mQueue (自行阅读构造函数源码)
1.2 MessageQueue
Low-level class holding the list of messages to be dispatched by a
{@link Looper}. Messages are not added directly to a MessageQueue,
but rather through {@link Handler} objects associated with the Looper.
You can retrieve the MessageQueue for the current thread with
{@link Looper#myQueue() Looper.myQueue()}.
低级类，保存由Looper调度的消息列表。消息不是直接添加到MessageQueue，而是通过与Looper关联的Handler对象添加。 您可以使用Looper#myQueue() 检索当前线程的MessageQueue。
1.2.1 enqueueMessage 方法（具体看中文注释部分代码）
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) {
...
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 {
//插入队列中间。通常，除非队列的开头有障碍并且消息是队列中最早的异步消息，否则我们不必唤醒事件队列。
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
// 按照when从小到大的顺序将msg 插入 链表实现的队列中；
for (;;) {
）
prev = p;
p = p.next;
// 依次移动指针，直到找到p==null（队列尾部 ）或者 msg.when需要注意的几点：
- nativeWake可以唤醒由nativePollOnce方法引起的阻塞
- msg.when = SystemClock.uptimeMillis() + delayMillis（类似postDelay等方法中传入的delay参数）
- nativePollOnce 可以阻塞Android 主线程而且不会出现ANR，具体分析看这里
1.2.2 MessageQueue#next()方法
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;
}
// 这里是IdleHandler 取出Msg的逻辑，详见下一节
// 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;
}
}
1.2.3 IdleHandler 简单使用、分析
使用：Looper.myLoop().getQueue().addIdleHandler(),可以用来做一些预加载的操作
public void addIdleHandler(@NonNull IdleHandler handler) {
synchronized (this) {
mIdleHandlers.add(handler);
}
}
Q : next()方法中IdleHandler是怎么被取出然后执行的？
A : 在MessageQueue#next方法死循环中使用pendingIdleHandlerCount（local variable）控制执行次数，可以避免加锁操作！
// 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;
1.3 Looper
1.3.1 loop() 方法
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
for (;;) {
// 调用next方法 取出
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
......
try {
// 回调handler的dispatchMessage方法，形成一个闭环
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "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);
}
msg.recycleUnchecked();
}
}