Android-Handler
Android-Looper
Android-MessageQueue
Android-Message
一、消息机制概述
1.消息机制的简介
在Android中使用消息机制，我们首先想到的就是Handler。没错，Handler是 Android消息机制的上层接口。Handler的使用过程很简单，通过它可以轻松地将一 个任务切换到Handler所在的线程中去执行。通常情况下，Handler的使用场景就是 更新UI。
如下就是使用消息机制的一个简单实例:
public class MainActivity extends AppCompatActivity {
private Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
System.out.println(msg.what);
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
new Thread(new Runnable() {
@Override
public void run() {
//...............耗时操作
Message message = Message.obtain();
message.what = 1;
mHandler.sendMessage(message);
}
}).start();
}
}
在子线程中，进行耗时操作，执行完操作后，发送消息，通知主线程更新UI。这便 是消息机制的典型应用场景。我们通常只会接触到Handler和Message来完成消息 机制，其实内部还有两大助手来共同完成消息传递。
2.消息机制的模型
消息机制主要包含:MessageQueue，Handler和Looper这三大部分，以及
Message，下面我们一一介绍。
- Message:需要传递的消息，可以传递数据;
- MessageQueue:消息队列，但是它的内部实现并不是用的队列，实际上是通过一 个单链表的数据结构来维护消息列表，因为单链表在插入和删除上比较有优势。主 要功能向消息池投递消息(MessageQueue.enqueueMessage)和取走消息池的消息 (MessageQueue.next);
- Handler:消息辅助类，主要功能向消息池发送各种消息事件 (Handler.sendMessage)和处理相应消息事件(Handler.handleMessage);
- Looper:不断循环执行(Looper.loop)，从MessageQueue中读取消息，按分发机制 将消息分发给目标处理者。
3.消息机制的架构
- 消息机制的运行流程:在子线程执行完耗时操作，当Handler发送消息时，将会调用MessageQueue.enqueueMessage，向消息队列中添加消息。
- 当通过Looper.loop开启循环后，会不断地从线程池中读取消息，即调
用MessageQueue.next。 - 然后调用目标Handler(即发送该消息的Handler)
的dispatchMessage方法传递消息，然后返回到Handler所在线程，目标Handler收到消息，调用 handleMessage方法，接收消息，处理消息。
每个线程中只能存在一个 Looper，Looper是保存在ThreadLocal中的。主线程(UI线程)已经创建了一个 Looper，所以在主线程中不需要再创建Looper，但是在其他线程中需要创建 Looper。
每个线程中可以有多个Handler，即一个Looper可以处理来自多个Handler 的消息。
Looper中维护一个MessageQueue，来维护消息队列，消息队列中的 Message可以来自不同的Handler。
下面是消息机制的整体架构图，接下来我们将慢慢解剖整个架构。
从中我们可以看出:
- Looper有一个MessageQueue消息队列;
- MessageQueue有一组待处理的Message;
- Message中记录发送和处理消息的Handler;
- Handler中有Looper和MessageQueue。
二、消息机制的源码解析
1.Looper
要想使用消息机制，首先要创建一个Looper。
初始化Looper
无参情况下，默认调用 prepare(true); 表示的是这个Looper可以退出，而对于 false的情况则表示当前Looper不可以退出。
/**
* Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
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));
}
这里看出，不能重复创建Looper，只能创建一个。创建Looper,并保存在 ThreadLocal。其中ThreadLocal是线程本地存储区(Thread Local Storage，简称 为TLS)，每个线程都有自己的私有的本地存储区域，不同线程之间彼此不能访问 对方的TLS区域。
开启Looper
/**
* TODO 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(); //获取TLS存储的Looper对象
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 消息队列
final MessageQueue queue = me.mQueue; //获取Looper对象中的消息 队列
// 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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
// 进入loop的主循环方法
for (; ; ) {
Message msg = queue.next(); // might block 可能会阻塞,因为next()方法可能会无限循环
if (msg == null) {
// No message indicates that the message queue is quitting.
// 消息为空，则退出循环
return;
}
// This must be in a local variable, in case a UI event sets the logger
// 默认为null，可通过setMessageLogging()方法来指定输出，用于debug功能
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
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);
// 下面开始消息处理
try {
// 获取msg的目标Handler， 然后用于分发Message 这个target 就是发送消息的那个 Handler
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);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
}
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);
}
// 回收
msg.recycleUnchecked();
}
}
loop()进入循环模式，不断重复下面的操作，直到消息为空时退出循环: 读取MessageQueue的下一条Message(关于next()，后面详细介绍); 把Message分发给相应的target。
当next()取出下一条消息时，队列中已经没有消息时，next()会无限循环，产生阻 塞。等待MessageQueue中加入消息，然后重新唤醒。
主线程中不需要自己创建Looper，这是由于在程序启动的时候，系统已经帮我们自 动调用了 Looper.prepare() 方法。查看ActivityThread中的 main() 方法，代 码如下所示:
ActivityThread main函数
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// Install selective syscall interception
AndroidOs.install();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
// TODO 开启
Looper.prepareMainLooper();
// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
// It will be in the format "seq=114"
long startSeq = 0;
if (args != null) {
for (int i = args.length - 1; i >= 0; --i) {
if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
startSeq = Long.parseLong(
args[i].substring(PROC_START_SEQ_IDENT.length()));
}
}
}
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
// TODO 启动循环
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
其中 `prepareMainLooper() 方法会调用 prepare(false) 方法。
2.创建Handler
public Handler() {
this(null, false);
}
public Handler(boolean async) {
this(null, async);
}
public Handler(@Nullable Callback callback) {
this(callback, false);
}
public Handler(@Nullable Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
//拿当前线程的Looper构建消息循环系统
//如果当前线程没有 初始化 looper 就会报错
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; // 这个就是消息队列
mCallback = callback; // 回调
mAsynchronous = async; // 异步
}
对于Handler的无参构造方法，默认采用当前线程TLS中的Looper对象，并且 callback回调方法为null，且消息为同步处理方式。只要执行的 Looper.prepare() 方法，那么便可以获取有效的Looper对象。
3.发送消息
发送消息有几种方式，但是归根结底都是调用了 sendMessageAtTime() 方法。
在子线程中通过Handler的post()方式或send()方式发送消息，最终都是调用
了 sendMessageAtTime() 方法。
post方法
public final boolean post(Runnable r){
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postAtTime(Runnable r, long uptimeMillis){
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis) {
return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
public final boolean postDelayed(Runnable r, long delayMillis) {
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
send方法
public final boolean sendMessage(Message msg) {
return sendMessageDelayed(msg, 0);
}
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;
eturn sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis){
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
就连子线程中调用Activity中的runOnUiThread()中更新UI，其实也是发送消息通知 主线程更新UI，最终也会调用 sendMessageAtTime() 方法。
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}
如果当前的线程不等于UI线程(主线程)，就去调用Handler的post()方法，最终会调 用 sendMessageAtTime() 方法。否则就直接调用Runnable对象的run()方法。
下面我们就来一探究竟，到底 sendMessageAtTime() 方法有什么作用? sendMessageAtTime()
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
//其中mQueue是消息队列，从Looper中获取的
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) {
// Handler 保存进来了 looper的时候使用这个Handler 处理消息的
// 为什么使用匿名内部类来创建Handler的方法会有内存泄漏的风险？
msg.target = this;
// 线程数据
msg.workSourceUid = ThreadLocalWorkSource.getUid();
// 异步
if (mAsynchronous) {
msg.setAsynchronous(true);
}
// 调用的是 Looper 的 Queen 的函数
return queue.enqueueMessage(msg, uptimeMillis);
}
MessageQueue#enqueueMessage()
boolean enqueueMessage(Message msg, long when) {
// 每一个Message必须有一个target 这个target就是发送/处理消息的Handler
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.");
}
// 加了锁
synchronized (this) {
if (mQuitting) { //正在退出时，回收msg，加入到消息池
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;
//p为null(代表MessageQueue没有消息) 或者msg的触发时间是队列中最早的， 则进入该该分支
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;
}
MessageQueue是按照Message触发时间的先后顺序排列的，队头的消息是将要最 早触发的消息。当有消息需要加入消息队列时，会从队列头开始遍历，直到找到消 息应该插入的合适位置，以保证所有消息的时间顺序。
4.获取消息
当发送了消息后，在MessageQueue维护了消息队列，然后在Looper中通
过 loop() 方法，不断地获取消息。上面对 loop() 方法进行了介绍，其中最重 要的是调用了 queue.next() 方法,通过该方法来提取下一条信息。下面我们来看一下 next() 方法的具体流程。
MessageQueue#next()
@UnsupportedAppUsage
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) { //当消息循环已经退出，则直接返回
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration // 循环迭代的首次为-1
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//阻塞操作，当等待nextPollTimeoutMillis时长，或者消息队列被唤醒 ，都会返回
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.
//当消息Handler为空时，查询MessageQueue中的下一条异步 消息msg，为空则退出循环。
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
//当异步消息触发时间大于当前时间，则设置下一次轮询的 超时时长
// 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.
// 获取一条消息，并返回
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
//设置消息的使用状态，即flags |= FLAG_IN_USE
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
//成功地获取MessageQueue中的下一 条即将要执行的消息
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
//没有消息
// 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;
}
}
nativePollOnce是阻塞操作，其中nextPollTimeoutMillis代表下一个消息到来前，还 需要等待的时长;当nextPollTimeoutMillis = -1时，表示消息队列中无消息，会一直 等待下去。
可以看出 next() 方法根据消息的触发时间，获取下一条需要执行的消息,队列中 消息为空时，则会进行阻塞操作。
5.分发消息
在loop()方法中，获取到下一条消息后，执行msg.target.dispatchMessage(msg)
，来分发消息到目标Handler对象。 方法的执行流程。
下面就来具体看下dispatchMessage(msg)方法的执行流程。
dispatchMessage()
/**
* Handle system messages here.
*/
public void dispatchMessage(@NonNull Message msg) {
//当Message存在回调方法，回调msg.callback.run()方法;
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
//当Handler存在Callback成员变量时，回调方法handleMessage();
if (mCallback.handleMessage(msg)) {
return;
}
}
//Handler自身的回调方法handleMessage()
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
/**
* Subclasses must implement this to receive messages.
* 子类必须实现它来接收消息。
*/
public void handleMessage(@NonNull Message msg) {
}
分发消息流程:
- 当Message的msg.callback不为空时，则回调方法 msg.callback.run() ;
- 当Handler的mCallback不为空时，则回调方法mCallback.handleMessage(msg);
- 最后调用Handler自身的回调方法handleMessage(msg),该方法默认为空Handler子类通过覆写该方法来完成具体的逻辑。
消息分发的优先级:
- Message的回调方法:message.callback.run();优先级最高。
- Handler中Callback的回调方法:mCallback.handleMessage(msg)，优先级仅次于1。
- Handler的默认方法: Handler.handleMessage(msg) ，优先级最低。
三、总结
