android消息处理机制
Android消息处理涉及到的类有Looper,MessageQueue,Handler,Thread。下面看下它们之间的关系。
Looper
默认Thread是没有message loop的，要创建一个，在Thread的run()中执行Looper.prepare(),调用Looper.loop()，使Thread一直运行来处理消息，要让Thread停止运行，调用loop quit。
下面是示例代码
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}看看调用Looper.prepare()做了什么。
/** 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));
}在prepare()中，创建了Looper实例，该实例和当前Thread关联起来，放在ThreadLocal中。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mRun = true;
mThread = Thread.currentThread();
}看Looper.loop()的源码
/**
* 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.recycle();
}
}queue.next()如果没有消息，会block，如果queue quit了其返回Message会是null，loop此时也会结束。拿到Message，则
msg.target.dispatchMessage(msg);看Handler的dispatchMessage():
/**
* 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);
}
}如果Message没有callback,Handler的mCallback返回的是false，最终会执行handleMessage()。
当处理完一条Message后，其会被回收。
MessageQueue
MessageQueue顾名思义，用来存放Messge，Messge不是直接添加到MessageQueue，通过Handler添加的，sendMessage(),sendMessageDelayed()等方法。
通过Looper.myQueue()可以得到MessageQueue的实例。
Handler
先看构造方法
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class 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;
}如果没有指定Looper，会使用当前线程的Looper，如果当前线程没有looper,则报错。
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}指定Looper。常见的使用是，先创建HandlerThread实例，并start，然后再创建Handler，使用HandlerThread实例的Looper，比如在IntentService中。
Handler的sendMessage(),sendMessageDelayed()等方法最终都会调用到sendMessageAtTime，再调用enqueueMessage
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);
}对消息处理的回调，会调用Handler的dispatchMessage()。
/**
* 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);
}
}Message
Message是Handler向MessageQueue发送的消息信息通过该类封装。
重要字段
int what: 用户定义的message code，消息接收者通过该code确定是哪一个消息。
int arg1,arg2：当发送的数据只是简单的1-2个int时，就用这两个变量，而不用setData()，是setData()的low-cost的替代方案。
Object obj: 发给接受者的一个任意对象。
如果要传递更多的数据，就用setData(Bundle bundle)。
获取Message实例的更好的方法是Message.obtain()
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
sPoolSize--;
return m;
}
}
return new Message();
}Message会从Message Pool中获取已经不再使用的Message，不需要再重新创建Message的实例。
/**
* Same as {@link #obtain()}, but sets the values of the target, what, and obj
* members.
* @param h The target value to set.
* @param what The what value to set.
* @param obj The object method to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what, Object obj) {
Message m = obtain();
m.target = h;
m.what = what;
m.obj = obj;
return m;
}
/**
* Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
* creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
* If you don't want that facility, just call Message.obtain() instead.
*/
public final Message obtainMessage()
{
return Message.obtain(this);
}
/**
* Same as {@link #obtainMessage()}, except that it also sets the what member of the returned Message.
*
* @param what Value to assign to the returned Message.what field.
* @return A Message from the global message pool.
*/
public final Message obtainMessage(int what)
{
return Message.obtain(this, what);
}
// sometimes we store linked lists of these things
/*package*/ Message next;
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50; /**
* Return a Message instance to the global pool. You MUST NOT touch
* the Message after calling this function -- it has effectively been
* freed.
*/
public void recycle() {
clearForRecycle();
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
/*package*/ void clearForRecycle() {
flags = 0;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
when = 0;
target = null;
callback = null;
data = null;
} /**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
sPoolSize--;
return m;
}
}
return new Message();
}当有Message调用它的recycle()方法，比如在Looper.loop()中，clearForRecycle()将所有变量置空，sPool将值赋给当前Message对象的next，当前Message对象赋值给sPool，当下一个Message对象m2调用recycle时，m2的next指向之前sPool的值，即上一个Message对象，将m2赋值给sPool。
调用obtain时，如果sPool不为空，则有回收的Message，返回该Message，将该Message的next，即之前的Message赋值给sPool。
Message的类变量sPool，总是指向回收的Message链表的头元素，它相当于一个头指针。
总结
- 在Thread的run()中调用Looper.prepare()给Thread创建looper,调用Looper.loop()使Thread监听MessageQueue，准备处理消息；
- 创建Handler时默认的Looper是当前线程的Looper，也可以指定Looper，sendMessage()即向Looper所持有的MessageQueue发送消息，Thread从MessageQueue取得Handler发来的消息后，回调Handler的callBack或者handleMessage()；
- 当处理完一条Message，调用Message的recycle()，放到Message Pool中备用；
- 当调用Looper的quit，会调用到MessageQueue的quit()，之后Thread的run方法执行完毕。