注：这里只是说一下sendmessage的一个过程，post就类似的
如果我们需要发送消息，会调用sendMessage方法
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
这个方法会调用如下的这个方法
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
接下来设定延迟时间，然后继续调用sendMessageAtTime方法
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);
}
这里获得了消息队列，检查队列是否存在，然后返回enqueMessage的方法的执行结果，这个结果是说明消息能否进入队列的一个布尔值
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
这里是对消息进行入队处理，下面就是在MessageQueue中对消息进行入队
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;
}
就是对传递过来的消息进行一些封装然后放到队列中，至此我们的sendMessage处理完毕，返回的结果是进队是否成功的布尔值，那么究竟消息之后是如何被处理的呢？
我们可以看到在Handler构造的时候记录了一个Looper对象，也记录了一个回掉函数
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;
}
这里的myLooper方法返回的是当前线程关联的一个Looper对象
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
当Looper实例化了以后会执行自己的prepare方法然后执行loop方法，loop方法就是不断的读取消息队列中的消息然后执行相应的操作的方法，因为是在其他线程中执行的循环所以不会影响其他线程
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();
}
}
在循环中如果读取到了消息，就会执行dispatchMessage方法，然后分派完消息之后再执行一次recycleUnchecked方法来重用这个Message，我们看到dispatchMessage方法
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
这里看到直接执行了一个handlerMessage方法，这个方法是一个回调方法，我们是必须实现的，否则Handler什么都不会做，为什么呢？还记得刚刚说构造Handler的时候我们记录了一个CallBack的回掉吗？Handler中的这个handlerMessage方法是一个空方法，如果我们重写了这个方法，在回调的时候就会执行我们先写下的代码，也就是接收到消息之后要做什么。
public interface Callback {
public boolean handleMessage(Message msg);
}
public void handleMessage(Message msg) {
}
这里简单说下整个过程：
当我们实例化一个Handler的子类并重写handleMessage方法之后，这个时候系统已经帮我们做了几个事情
1.实例化了一个消息队列MessageQueue
2.实例化了一个关联的Looper对象，并让Looper不断的读取消息队列
3.把我们重写的handleMessage方法记录为我们需要回调的方法
当我们执行Handler的sendMessage方法的时候，系统会把我们传过去的Message对象添加到消息队列，这个时候如果Looper读取到了消息，就会把消息派发出去，然后回调handleMessage方法，执行我们设定的代码。