这篇文章是跟踪源码看handler是如何发消息的。
从sendMessage（Message msg）开始吧
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}点入看看
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}点入看看
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
//到这里我们发现了一个成员变量mQueue
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);
}在这里我们发现了一个成员变量mQueue，让我们来看看这个不速之客
final Looper mLooper;
final MessageQueue mQueue;
final Callback mCallback;
final boolean mAsynchronous;
IMessenger mMessenger;来看看什么时候初始化的
*
* @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()");
}
//原来是在这个地方初始化的
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}也就是说我们在创建Handler对象的时候初始化的
我们看先具体流程
1. 创建Handler对象
public Handler() {
this(null, false);
}点进去看看以后发现就是我们上一段代码，也试试先获取了mLooper对象（与当前线程相关联的Looper对象）然后获取了该Looper对象的MessageQueue对象
进入Looper代码发现
private Looper(boolean quitAllowed) {
//创建MessageQueue对象
mQueue = new MessageQueue(quitAllowed);
//获取当前线程
mThread = Thread.currentThread();
}那这个方法什么时候被调用的呢
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");
}
//看这里
sThreadLocal.set(new Looper(quitAllowed));
}看到了吧，也即是在调用Looper.prepare()的时候这MessageQueue初始化的，对于ThreadLocal 我们讲完这一章节在接着细说
那Looper.prepare()在主线程中什么时候被调用的呢，让我们来看看ActivityThread的源码
public static void main(String[] args) {
...
//看这里
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
...
} /**
* 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();
}
}真相就是这样
好了，言归正传我们接着看enqueueMessage(queue, msg, uptimeMillis) 方法
进入该方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//记住这里很重要，msg.target是当前的Handler对象
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}接着走
boolean enqueueMessage(Message msg, long when) {
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.");
}
synchronized (this) {
if (mQuitting) {
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;
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;
}这就是Message加入消息队列的过程，感兴趣的可以看看Message的数据结构
// sometimes we store linked lists of these things
/*package*/ Message next;加入队列后，到此也就结束了。
接下来我们要看看如何从消息队列中取Message对象了，那让我们的目光转向Looper的loop() 方法。
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
//获取当前线程关联的looper对象
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//获取当前looper维护的消息队列
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();
// 一个死循环
for (;;) {
//从消息队列中获取消息
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
//还记的我们的msg.target吧就是我们发消息的时候使用的那个Handler对象
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
}
}让我们接着看看 msg.target.dispatchMessage(msg);
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//看到这里也就觉悟了，这不是我们创建Handler的时候要覆写的方法吗
handleMessage(msg);
}
}Handler + Message + Looper 消息分发机制到此结束