Handler、Looper、Message源码解析

在Android中我们在使用异步消息处理时经常用到handler，looper，message。

1. Handler的介绍

一个Handler允许发送和处理线程消息队列中的消息和Runnable对象关联的进程。每个Handler实例都与一个单线程和该线程的消息队列关联。在创建一个新的Handler时，会绑定到创建它的线程的线程/消息队列上，它将传递message和runnable对象到消息队列并且能够执行消息队列中的消息。

下面我们看看源码

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;
}

从上边代码我们可以看出,我们一般使用new Handler()实例化handler对象时,我们实际调用的是public Handler(Callback callback, boolean async)这个构造方法;

从14行中我们可以看到,通过Looper.myLooper()获取Looper实例,同时获取到MessageQueue实例.

值得注意的是:Can’t create handler inside thread that has not called Looper.prepare();提示我们在实例化Handler前,必须能调用Looper.prepa();

重写handleMessage(Message msg);

public void handleMessage(Message msg) {
// 对MessageQueue中取出的Message做处理
}

sendMessage()流程:

public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}

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;
// 如果当前MessageQueue为空抛出异常
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);
}

在这个方法中我们可以看到msg.target = this,就是将当前handler对象赋值给msg.target,下面的Looper中我们再次提到msg.target;

最后返回queue.enqueueMessage(msg, uptimeMillis),应该是将message对象和uptimeLillis延迟毫秒数放入当前线程的messagequeu队列中

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

我们经常使用handler.post(Runnable r)来更新UI界面,实际上它和sendMessage(Message msg)方法差不多,下面看看源码:

public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
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);
}
......

从源码中我们可以看出post(Runnable r)中的Runnable经过getPostMessage(r)转换成了Message,所以post(Runnable r)实际上还是和上边的sendMessage()相同

下面我们看一下dispatchMessage(Message msg)这个方法,因为它会在Looper.loop()中被msg.target(就是handler对象)调用,同时将Message对象传入,最终在handleMessage(Message msg)这个方法中我们获取到Message对象,执行我们所需要的操作

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

2. Looper的介绍

每个线程都有一个唯一的looper和MessageQueue,如果MessageQueue不为空,looper将会循环执行消息队列中的内容,否则阻塞.

prepare() 初始化当前线程中的一个looper,在这个方法后初始化handler,同时将该handler引用该looper,然后在调用Looper.loop()方法循环消息队列.

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));
}

但是我们一般使用handler时,都是在类中直接定义使用,如下:

private Handler handler = new Handler(){
handleMessage(Message msg){
// 处理更新UI或者是....
}
};

这是怎么回事呢?让我们来看一下这个方法:

/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself.  See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}

/**
* Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}

从prepareMainLooper()的注释总我们可以看出,这个main Looper是由Android environment为application创建的,我们不需要创建就可以直接使用.但是我们在子线程中如果要初始化handler时,就必须先调用Looper.prepare()方法,在初始化handler之后调用looper.loop()方法来执行循环:

Handler mHandler = null;

public void get() {
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
}
};
Looper.loop();
}
}).start();
}

下面我们介绍一下loop()方法:

/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
// 获取当前线程中的looper对象
final Looper me = myLooper();
//如果looper为空,抛出异常
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 获取该looper中的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();
// 进入消息队列的循环中
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是handler对象,调用handler中的dispatchMessage(msg)方法,在dispatchMessage()中调用handleMessage(msg)来执行我们需要的操作
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();
}
}

3.Message

Message是MessageQueue消息队列中所包含的对象,下面简单介绍一下:

/*package*/ Handler target; //在上边我们介绍过
/*package*/ Runnable callback; // 在getPostMessage(Runnable r)个方法中用到过

我们在获取Message对象时最好通过Message.obtain()这个方法,因为这个方法可以避免多次new Message(),从而减少内存损耗.

关系图: