记得之前面试时，面试官就问了Handler的工作流程，当时脑子里有MessageQueue、Looper等类名但具体怎么执行的却说不明白。于是乎面试也就失败了。想想还是挺菜鸡的。。以一个简单的例子通过源码来走一遍Handler的执行流程。
Handler handler=new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
Message message=Message.obtain();
message.arg1=1;
handler.sendMessage(message);
}
1.MessageQueue
就是这么一个最简单的例子先进入sendMessage方法,然后又跳到了sendMessageDelayed方法，最后跳到了sendMessageAtTime方法，在sendMessageAtTime方法中看到了熟悉的面孔MessageQueue，初始化了一下MessageQueue，之后进入了enqueueMessage方法。
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;
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);
}
enqueueMessage方法：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
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;
}
在enqueueMessage方法中将msg.target设置为当前的Handler，也就是绑定了Handler，然后进入enqueueMessage中，通过模拟代码执行，有了一个重要的结论，我们每发送一个消息都被保存到了 MessageQueue 消息队列中，消息队列中采用的是单链表的方式。
// 发送 Message1
Message message1 = new Message();
mHandler.sendMessageDelayed(message1, 500);
// 发送 Message2
Message message2 = new Message();
mHandler.sendMessage(message2);
// 发送 Message3
Message message3 = new Message();
mHandler.sendMessageDelayed(message3, 1000);
主要看这段来模拟：
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;
}
模拟如下：
message1.when=500；
Message p = mMessages;
而mMessages=null； 所以Message p =null；
因为p =null，所以进入if
message1.next = null;
mMessages = message1;
发送完message1
然后发送message2：
message2.when=0；
Message p = mMessages; //在发送message1是已经将mMessages赋值为 message1
所以
p=message1；
if (p == null || when == 0 || when < p.when)
因为when=0，所以进入if
message2.next=p；即
message2.next=message1
mMessages =message2;
发送完message2
然后发送message2：
message3.when=1000；
Message p =message2;
if (p == null || when == 0 || when < p.when) 不满足if条件，进入else
Message prev;
进入for（;;）循环
prev = message2;
p = p.next;即
p=message1;
if (p == null || when < p.when) // 不满足if继续循环
prev = p;
即
prev = message1;
p = p.next;即
p=null;
if (p == null || when < p.when) // 满足条件进入if 跳出
msg.next = p;即
message3.next=null；
prev.next = msg; 即
message1.next=message3
所以发送完message3之后应该是这样的：
发送完message3
得出结论消息在队列中是按when和先后顺序排列的。
2. Loop 消息循环
我们始终没有看到 Handler 调用 handleMessage() 方法，到底什么时候会执行这个方法?
书上说在子线程中使用handler要先
Looper.prepare();
Handler handler = new Handler();
Looper.loop();
否则会报错，而在MainActivity即主线程中直接使用不会报错。原因就是在启动Activity过程中在ActivityThread帮我们已经调用过了代码如下：
ActivityThread.java main()
public static void main(String[] args) {
....
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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));
}
public void set(T value) {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}
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();
}
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
在prepare(false)方法中创建一个Looper对象并把它存储到sThreadLocal中并且与当前线程绑定。
当调用 loop()方法再在从sThreadLocal中取出Looper对象。ThreadLocal用来保证一个线程只有一个 Looper 对象，这样就保证了线程的安全。接下来是一个for(;;)死循环调用队列中msg.target.dispatchMessage。这样就找到了在handleMessage中可以收到信息的原因。message的target是在加入队列时设置的。
/**
* 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);
}
}
总结：Looper.prepareMainLooper() 创建了一个 Looper 对象，而且保证一个线程只有一个 Looper；Looper.loop() 里面是一个死循环，不断的从 消息队列 MessageQueue 中取消息，然后通过 Handler 执行。
