Handler在各个博客中已经被说烂了，但是呢，我觉得只有自己输出出来，才算真正的掌握了，遂记录此文。
背景知识
使用场景
Handler在Android中主要用于线程间的通信，在Android源码中也使用到了。主线程ActivityThread创建名为H的Handler，当ApplicationThread在Binder线程中收到AMS的调度后，通过Handler发送消息到主线程，从而控制应用程序的运行。
体系架构
Handler正常工作需要由三个部分组成：1. Looper；2. MessageQueue；3. Message；
Message
表示一条具体的消息的类，通过Message.obtain创建Message，其中可以指定目的Handler，并且携带一些信息。
public static Message obtain(Handler h, int what, int arg1, int arg2, Object obj)
MessageQueue
顾名思意，MessageQueue是一个消息队列，收发Message消息。在每个线程中，随着Looper的创建而创建，MessageQueue中有很多native的方法，底层用到了epoll机制，确保在队列为空而阻塞时不会占用CPU资源。
Looper
每个线程只能有唯一的一个Looper，Looper负责开启一个永不停止的循环，不停的从MessageQueue中取出Message，交给Handler去处理。
工作过程
我们从一个Google推荐的使用方式入手
Message message = Message.obtain(handler, 123);
message.sendToTarget();
public final class Message implements Parcelable {
public static Message obtain(Handler h, int what) {
Message m = obtain();
m.target = h;
m.what = what;
return m;
}
public void sendToTarget() {
target.sendMessage(this);
}
}
可以看到在创建Message时，需要传入一个作为发送目标的Handler对象，然后调用Handler的sendMessage发送消息。
public class Handler {
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull 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(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
}
可以看到Handler最终调用了MessageQueue.enqueueMessage方法，将Message添加到消息队列中。
public final class MessageQueue {
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;
}
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
}
消息队列是个列表，将新消息添加到队列的末尾，注意到其中mMessage成员变量，该成员表示消息队列的第一个元素，用于遍历消息队列。下面再来看Looper是如何取出消息来执行的。
public final class Looper {
public static void prepare() {
prepare(true);
}
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();
}
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 获取消息队列
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();
boolean slowDeliveryDetected = false;
for (;;) {
// MessageQueue.next方法，采用epoll机制阻塞，不会占用CPU资源
Message msg = queue.next();
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
// 分发到Handler的dispatchMessage方法中
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
msg.recycleUnchecked();
}
}
}
MessageQueue.next方法非常重要，关系着同步屏障，IdleHandler机制的执行，下面就详细分析一下。
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();
}
// 底层采用epoll机制，nextPollTimeoutMillis为0表示立即返回，不会阻塞
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();
// 取出消息，返回到looper中执行
return msg;
}
} else {
// 如果消息队列中没有消息，设置为-1，传入到epoll中表示永久阻塞
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;
}
}
}
next通过nativePollOnce方法来阻赛线程，注意到首次传入到超时时间时0，表示不阻塞，立即返回。如果这时消息队列为空，则开始执行IdleHandler，然后将超时时间设置为-1，表示永远阻塞，等待消息队列不为空。
// timeout为0表示立即返回，-1表示永远阻塞。
int epoll_wait(int epfd, struct epoll_event * events, int maxevents, int timeout);
Looper开启无限循环后，不停的从MessageQueue中取出消息，取出消息后通过Handler.dispatchMessage方法分发。
public class Handler {
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
// 如果是runable类型的msg，则执行runable
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
// 执行自定义的handleMessage回调
handleMessage(msg);
}
}
}
至此，Handler的发送消息并处理消息的流程就说完了。
同步屏障
Handler也提供了一种感觉Message优先级的方式，那就是同步屏障。通过在消息队列中插入同步屏障，来提高异步消息的处理优先级。
异步消息
Handler默认都是同步消息，如果要构造异步消息，可以通过向Handler的构造方法传入参数
public Handler(boolean async);
public Handler(@Nullable Callback callback, boolean async);
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async);
指定为异步类型后，此后通过enqueueMessage发送的Message，都会被设置为异步消息。
public class Handler {
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
// 设置为异步消息
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
}
插入同步屏障
插入同步屏障通过MessageQueue.postSyncBarrier方法，移除通过removeSyncBarrier。
public final class MessageQueue {
public int postSyncBarrier() {
return postSyncBarrier(SystemClock.uptimeMillis());
}
private int postSyncBarrier(long when) {
// Enqueue a new sync barrier token.
// We don't need to wake the queue because the purpose of a barrier is to stall it.
synchronized (this) {
final int token = mNextBarrierToken++;
final Message msg = Message.obtain();
msg.markInUse();
msg.when = when;
msg.arg1 = token;
Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
}
if (prev != null) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
}
可以看到，其中创建了一个Message，但是没有指定msg.target，也就是为null。这点很关键，在next取消息时，如果判断target为null，则会走到同步屏障的逻辑中。
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) {
// target为null，发现同步屏障
do {
prevMsg = msg;
msg = msg.next;
// 在队列中查找异步消息
} 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;
}
}
}
}
如果发现了一个同步屏障，那么就在队列中查找第一个异步消息执行，这时候就相当于异步消息的优先级大于同步消息，直到同步屏障被移除。
移除同步屏障
移除同步屏障很简单，就是从头开始遍历消息队列，找到target为空的Message，然后删除。
public final class MessageQueue {
public void removeSyncBarrier(int token) {
// Remove a sync barrier token from the queue.
// If the queue is no longer stalled by a barrier then wake it.
synchronized (this) {
Message prev = null;
Message p = mMessages;
while (p != null && (p.target != null || p.arg1 != token)) {
prev = p;
p = p.next;
}
if (p == null) {
throw new IllegalStateException("The specified message queue synchronization "
+ " barrier token has not been posted or has already been removed.");
}
final boolean needWake;
if (prev != null) {
prev.next = p.next;
needWake = false;
} else {
mMessages = p.next;
needWake = mMessages == null || mMessages.target != null;
}
p.recycleUnchecked();
// If the loop is quitting then it is already awake.
// We can assume mPtr != 0 when mQuitting is false.
if (needWake && !mQuitting) {
nativeWake(mPtr);
}
}
}
}
IdleHandler
IdleHandler时MessageQueue的内部接口，IdleHandler在MessageQueue队列为空时执行，比如在启动优化时，将一些非必要的操作放到IdleHandler中，提高启动速度。
public final class MessageQueue {
// MessageQueue的内部接口
public static interface IdleHandler {
/**
* Called when the message queue has run out of messages and will now
* wait for more. Return true to keep your idle handler active, false
* to have it removed. This may be called if there are still messages
* pending in the queue, but they are all scheduled to be dispatched
* after the current time.
*/
boolean queueIdle();
}
}
添加IdleHandler
MessageQueue中维护了一个ArrayList用来保存Idlehandler
public final class MessageQueue {
private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
public void addIdleHandler(@NonNull IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
}
执行IdleHandler
在MessageQueue执行next方法，当消息队列为空时，会从mIdleHandlers取出IdleHandler执行，然后继续for循环，等待新消息的到来。
public final class MessageQueue {
Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// 没有消息，或者超时
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 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)];
}
// 将IdleHandlers转换为数组
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// 遍历数组，并执行queueIdle方法。
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;
}
}
}
