前言
我相信，用过Android的人基本都会使用Handler，或者多多少少会听到这个东西，在安卓里面，这东西太重要了，如果你还不会基本用法，那应该是需要反省一下。当然，用过它的人也不必沾沾自喜，我们真的很了解Handler吗，还是说只会使用？你有看过他的每一行代码？仔细思考过吗？对于我来说，确实没有，所以我带着问题，想全面了解 Handler。
是什么东西？
handler，英文有（信息）处理机的意思，这里我们暂且可以认为他有处理消息的功能，当然，这也是他最主要的功能，他还有其他的功能，我们可以根据源码一谈究竟。用最通俗的总结一下，他是用来接收从各个线程发过来的消息，然后集中处理的东西。你可以在主线程（MainThread）处理消息，在子线程（WorkerThread）发送消息，当然，这个关系也可以转换，下面我手写一小段代码展示一下（只为了演示，实际中不会这么写）
// 写在Activity中作为实例变量
private Handler mHandler = new Handler() {
@Override
public void handleMesssage(Message msg) {
ui.setText("msg received");
}
}
// 启动一个线程发一个消息
new Thread(() -> mHandler.sendEmptyMessage());
上面简单的代码演示了如何从一个子线程发送消息到主线程，然后更新UI，至于Android不能再主线程更新UI，我想这个道理大家多少都知道原因，以后有机会再好好分析。
前世今生
很干净，只有自己
public class Handler {}
源码分析
属性
这里个都是类变量，MAIN_THREAD_HANDLER 顾名思义，主线程的 Handler
/*
* Set this flag to true to detect anonymous, local or member classes
* that extend this Handler class and that are not static. These kind
* of classes can potentially create leaks.
*/
private static final boolean FIND_POTENTIAL_LEAKS = false;
private static final String TAG = "Handler";
private static Handler MAIN_THREAD_HANDLER = null;
接下来这几个很重要，一个是Looper对象，MessageQueue，Callback，mAsynchronous，还有IMessager，集体有什么用，后面用到了再慢慢说
final Looper mLooper;
final MessageQueue mQueue;
final Callback mCallback;
final boolean mAsynchronous;
IMessenger mMessenger;
这个东西大家用的可能不多，是一个内部接口，主要是说你可以不用写一个Handler子类就可以做到消息处理的功能。比如说上面的代码，我就是实现了自己的子类才能做到消息处理，你也可以实现这个Callback，然后 new Handler（Callback）就可以了。
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public boolean handleMessage(Message msg);
}
函数
构造函数
4个参数，都有默认值，分别是 Looper.myLooper() 获取到的，如果你在主线程初始化，那默认就是MainLooper，否则就是其他线程自己的Looper，这里有一个要注意的地方，就是说，当前线程必须执行过 Looper.prepare()，一般你在子线程都需要先执行这个方法，但在主线程是不需要的，因为早在应用启动的时候，ActivityThread 的 main 方法以及帮你执行了，这是我们程序比较早的一个入口。然后绑定MessageQueue，默认是Looper的Queue，还有Callback，默认为空，还有异步mAsynchronous ，默认为false。
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
// 静态方法获取Looper，使用到了ThreadLocal
// 这个强大的变量，从当前线程取值，
// 前提是你之前有放入过东西，不然怎么取
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
// 静态方法，往ThreadLocal存在Looper实例
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));
}
然后是我们通常需要重写的函数，里面是消息处理的逻辑
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
这个函数原则上你也可以重写，但是不推荐，我们可以看到，它首先判断msg是否带有callback，如果不为空，则执行处理逻辑，否则判断mCallback是否为空，这两个Callback室友区别的，后面这个就是上面那个接口的实现类，而msg的callback是背后自己封装的，稍后我们在看这个细节。如果没有传进去callback，那么则调用handleMessage（），就到了我们重写的那个地方了，这些是在主线程执行的。
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
发送消息的函数，默认是直接发送，注意，虽然是 EmptyMessage，但还是在内部封装了一个 Message，以后获取Message，不要再傻傻的new Message了，通过Message.obtain 或者 Handler.obtain。这一步可以在子线程调用
/**
* Sends a Message containing only the what value.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessage(int what) {
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
// 最后还是调用了atTime
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
获取MessageQueue，将Message入队，注意，这里msg.target = this，将handler绑定到message上，到时候就能找到消息对应的handler实例了，强调一下，一个线程对应一个queue，一个queue可以有多个handler，所以有需要区分多个handler，就是这里设置target，绑定实例，然后入队，注意，异步也是在这里配置
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);
}
MessageQueue 的源码，使用synchronised保证顺序性，至于MessageQueue的种种细节，下次在分析
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;
}
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;
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;
}
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();
}
// poll 消息
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;
}
}
获取消息，调用MessageQueue的next，然后看到了msg.target.dispatchMessage(msg)这个是在主线程的
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 获取 Queue 通过 MessageQueue
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
// 获取消息，调用MessageQueue的next
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);
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// 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();
}
}
当然，handler还有其他用法，比如post，但需要注意，这个虽然提交了一个Runnable，但跟随者消息链，你会发现，它的执行是直接在主线程调用run方法，也就是说运行在主线程的，所以使用时候需要注意了，别用错了
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
// callback message 的callback，与之前Callback有很大的区别
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
小结
- 使用方法，继承Handler，或者实现Callback，又或者post
- 各部分运行在什么地方，主线程和子线程要区分
- MessageQueue，Looper，Handler 之间的关系，在什么地方初始化
欢迎讨论
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