title: 探究Android异步消息的处理之Handler详解
date: 2015-09-29 13:19:12
categories: Android Blog
tags: [Android,源码解析]
---
在学习Android的路上，大家肯定会遇到异步消息处理，Android提供给我们一个类来处理相关的问题，那就是Handler。相信大家大多都用过Handler了，下面我们就来看看Handler最简单的用法：
``` java
public class FirstActivity extends AppCompatActivity {
public static final String TAG = "FirstActivity";
private static Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case 0:
Log.i(TAG, "handler receive msg.what = " + msg.what);
break;
default:
break;
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_first);
new Thread(new Runnable() {
@Override
public void run() {
//这里做相关操作
handler.sendEmptyMessage(0);
}
}).start();
}
}
```
上面代码实现了在子线程中发出一个消息，然后在主线程中接收消息。Handler其他类似的用法在这就不过多叙述了。下面我们来看看Handler到底是怎么实现异步消息处理的吧！
先来看看我们new一个Handler的对象到底发生了什么（只截取了关键源码）：
``` java
public Handler() {
this(null, false);
}
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;
}
```
可以看到我们平常写的 new Handler()；其实是调用了另外一个构造方法，并且判断了mLooper是不是为空，为空则抛出一个异常**"Can't create handler inside thread that has not called Looper.prepare()"**，mLooper其实是一个Looper类的成员变量，官方文档上对Looper类的解释是 **Class used to run a message loop for a thread.**也就是说Looper用于在一个线程中传递message的。 然后我们根据异常的提示知道要在new一个Handler的对象之前必须
先调用Looper.prepare()。那接下来就只能先去看看Looper.prepare()方法了：
``` java
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;
/** 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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
/**
* 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.");
}
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 (;;) {
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(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);
}
msg.recycleUnchecked();
}
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
```
prepare()方法就是将一个sThreadLocal和新建的Looper对象相绑定，同时mQueue成员变量也创建了新的MessageQueue对象，MessageQueue这个类就是用于存储Message的队列。在prepare()方法的注释上写着在调用prepare()方法之后还要调用loop()方法，我们再看loop方法，可以看到方法里写了一个for的死循环，主要用于在MessageQueue里不断地去取Message，如果msg为空，则阻塞；不然会调用msg.target.dispatchMessage(msg)这个方法。dispatchMessage()这个方法我会在后面讲解，先暂时放一边不管。
好了，捋一捋思路，当你在新建一个Handler对象时，要先确保调用了Looper.prepare()方法，然后调用Looper.loop()方法让MessageQueue这个队列“动”起来。这样你就成功地创建了一个Handler的对象。然后我们再使用Handler的sendMessage系列方法来发送一个消息。下面我们就来看看sendMessage系列方法里到底干了什么：
``` java
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);
}
```
为什么我就贴出一个方法呢？这是因为Handler一系列的sendMessage方法基本上最后都是调用了sendMessageAtTime这个方法。从源码中我们看到主要就是干了把Message加入队列这个事,并把当前的Handler对象赋给了msg的target。再联系上面的Looper.loop方法，我们大概就懂了。好了，我们回过头来看看上面的msg.target.dispatchMessage(msg)主要的功能。其实就是调用了Handler的dispatchMessage方法：
``` java
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
```
我们看到了一行熟悉的代码：handleMessage(msg)，这不正是我们再创建Handler对象时重写的那个方法么！好了，这一切的逻辑我们似乎已经搞清了：首先调用Looper.prepare()创建一个Looper对象，然后handler发送消息后把消息加入到MessageQueue里，因为之前调用了Looper.loop(),所以MessageQueue在不断地做出队的操作，然后再根据message的target变量分发消息，回到handler的handleMessage()方法。
也许有人会有疑问了，为什么在主线程中创建Handler对象可以直接使用而不需要调用Looper.prepare()和Looper.loop()两个方法呢？这是因为在ActivityThread里面已经调用了，下面附上ActivityThread的源码：
``` java
/**
* This manages the execution of the main thread in an
* application process, scheduling and executing activities,
* broadcasts, and other operations on it as the activity
* manager requests.
*
* {@hide}
*/
public final class ActivityThread {
static ContextImpl mSystemContext = null;
static IPackageManager sPackageManager;
// 创建ApplicationThread实例，以接收AMS指令并执行
final ApplicationThread mAppThread = new ApplicationThread();
final Looper mLooper = Looper.myLooper();
final H mH = new H();
final HashMap<IBinder, ActivityClientRecord> mActivities
= new HashMap<IBinder, ActivityClientRecord>();
// List of new activities (via ActivityRecord.nextIdle) that should
// be reported when next we idle.
ActivityClientRecord mNewActivities = null;
// Number of activities that are currently visible on-screen.
int mNumVisibleActivities = 0;
final HashMap<IBinder, Service> mServices
= new HashMap<IBinder, Service>();
Application mInitialApplication;
final ArrayList<Application> mAllApplications
= new ArrayList<Application>();
static final ThreadLocal<ActivityThread> sThreadLocal = new ThreadLocal<ActivityThread>();
Instrumentation mInstrumentation;
static Handler sMainThreadHandler; // set once in main()
static final class ActivityClientRecord {
IBinder token;
int ident;
Intent intent;
Bundle state;
Activity activity;
Window window;
Activity parent;
String embeddedID;
Activity.NonConfigurationInstances lastNonConfigurationInstances;
boolean paused;
boolean stopped;
boolean hideForNow;
Configuration newConfig;
Configuration createdConfig;
ActivityClientRecord nextIdle;
String profileFile;
ParcelFileDescriptor profileFd;
boolean autoStopProfiler;
ActivityInfo activityInfo;
CompatibilityInfo compatInfo;
LoadedApk packageInfo; //包信息，通过调用ActivityThread.getPapckageInfo而获得
List<ResultInfo> pendingResults;
List<Intent> pendingIntents;
boolean startsNotResumed;
boolean isForward;
int pendingConfigChanges;
boolean onlyLocalRequest;
View mPendingRemoveWindow;
WindowManager mPendingRemoveWindowManager;
...
}
private class ApplicationThread extends ApplicationThreadNative {
private void updatePendingConfiguration(Configuration config) {
synchronized (mPackages) {
if (mPendingConfiguration == null ||
mPendingConfiguration.isOtherSeqNewer(config)) {
mPendingConfiguration = config;
}
}
}
public final void schedulePauseActivity(IBinder token, boolean finished,
boolean userLeaving, int configChanges) {
queueOrSendMessage(
finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,
token,
(userLeaving ? 1 : 0),
configChanges);
}
// we use token to identify this activity without having to send the
// activity itself back to the activity manager. (matters more with ipc)
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
Bundle state, List<ResultInfo> pendingResults,
List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profileFile = profileName;
r.profileFd = profileFd;
r.autoStopProfiler = autoStopProfiler;
updatePendingConfiguration(curConfig);
queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
}
...
}
private class H extends Handler {
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
ActivityClientRecord r = (ActivityClientRecord)msg.obj;
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
...
}
if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
}
...
}
public static ActivityThread currentActivityThread() {
return sThreadLocal.get();
}
public static void main(String[] args) {
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());
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
// 创建ActivityThread实例
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
}
```
可以看到上面的main方法里的181行和198行已经调用了prepare和loop的方法。因此在主线程中使用Handler不需要再调用prepare和loop方法了。
好了，今天该讲的差不多了，就到这吧。
由于第一次写讲解源码的博客，不便之处请大家多多包涵。有问题的可以在下面评论。