Android source code(7.1) message-Handler 机制

Message handler处理原理：

1、handler-msg 用Looper维护了一个消息队列，采用for死循环对这个队列进行遍历，当有新的msg消息被压入deque时，将会被及时的处理。

2、Main Thread在创建的时候会为当前thread主动分配一个looper，而子线程需要手动创建looper

从code的角度分析一下 message-handler机制是如何运作的

首先，我们来看一下Looper.java里面的主要code

public final class Looper {
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;
private Printer mLogging;
private long mTraceTag;
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));
}

public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}

public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}

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;
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();

for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
return;
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}

final long newIdent = Binder.clearCallingIdentity();
msg.recycleUnchecked();
}
}

public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}

public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}

private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}

public boolean isCurrentThread() {
return Thread.currentThread() == mThread;
}

public void quit() {
mQueue.quit(false);
}

public void quitSafely() {
mQueue.quit(true);
}
public @NonNull Thread getThread() {
return mThread;
}

public @NonNull MessageQueue getQueue() {
return mQueue;
}
}

1、ThreadLocal sThreadLocal :本地线程变量，ThreadLocal的特点是在不同的thread中保存的值相互独立，所以，它为不同的thread保存thread自己的looper对象，请注意prepare和prepareMainLooper这两个方法

2、mQueue：保存message的消息队列

3、在loop()方法里，msg.target中的target指向的是Handler对象，所以 msg.target.dispatchMessage(msg); 会调用到Handler.dispatchMessage方法

public class Handler {
private static final boolean FIND_POTENTIAL_LEAKS = false;
private static final String TAG = "Handler";
final Looper mLooper;
final MessageQueue mQueue;
final Callback mCallback;
final boolean mAsynchronous;
IMessenger mMessenger;
/**
* 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);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
public interface Callback {
public boolean handleMessage(Message msg);
}

public void handleMessage(Message msg) {
}

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

public Handler() {
this(null, false);
}

public Handler(Callback callback) {
this(callback, false);
}

public Handler(Looper looper) {
this(looper, null, false);
}

public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}

/**
* @hide
*/
public Handler(boolean async) {
this(null, async);
}
/**
* @hide
*/
public Handler(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());
}
}

mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}

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;
return 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);
}

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);
}

public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}

1、Handler里面的mQueue 是Looper里面的mQueue的引用

2、当sendMsg的时候最终是调用enqueueMessage方法将msg进行入队列操作

很多时候，我们会使用HandlerThread来进行Handler-msg操作

public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;

public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}

public HandlerThread(String name, int priority) {
super(name);
mPriority = priority;
}

protected void onLooperPrepared() {
}

@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}

public Looper getLooper() {
if (!isAlive()) {
return null;
}

// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}

public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}

public boolean quitSafely() {
Looper looper = getLooper();
if (looper != null) {
looper.quitSafely();
return true;
}
return false;
}

public int getThreadId() {
return mTid;
}
}

1、HandlerThread是thread

2、HandlerThread在run方法里面为我们构造好了Looper对象已经它的消息队列

下面我们看一下主线程是如何为handler分配一个looper的,下面给出了部分code:

ActivityThread.java
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();

// 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();

// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());

// 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>");

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();
thread.attach(false);

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);
Looper.loop();

throw new RuntimeException("Main thread loop unexpectedly exited");
}

app process启动的时候会调用ActivityThread.main方法，这里会为app分配一个全局的Looper对象

本章介绍结束！