Handler知其然而不知其所以然，主要遇见的问题，looper如何确定向哪个Handler发送消息？如何保证线程中只有一个looper和MessageQueue？延时消息如何发送之后如何唤醒？除了Callback？？？
第一个问题：首先要查看Looper类的loop方法,因为这个方法的作用就是向Handler发送消息。源码如下
public static void loop() {
//获取本身
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//获取对应的消息队列
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();
//死循环取消息
for (;;) {
//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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
//调用消息target的dispatchMessage方法处理消息
//通过查看得知Message的target是一个Handler对象，查看Message的target赋值的位置
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}
接着应该看msg的target在哪赋值的？首先看Message这个类里面的赋值方法：有两个方法setTarget(Handler target)这个方法直接去赋值，但只在WifiMonitor和WifiP2pMonitor中使用，void recycleUnchecked()这个只是将target赋值为null。所以一般的Hanler并不会直接调用，因此考虑要查看其他类的对target的赋值，Handler有发送Message的作用，查看Handler发送Message的方法。会发现无论使用任何方法会调用boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis)这个方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//这一步将msg.target赋值为this，也就是说在handler向MessageQueue发送消息的时候
//msg.targt就是自己本身,因此looper就会使用msg.target.dispatchMessage(msg)处理就会调到对应的Handler
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
我们可以在看一下Handler的dispatchMessage(Message msg)方法
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
//有回掉接口，则先调用接口，这个接口可自定义返回值，如果为true则不调用handleMessage（），这个是一个空方法，继承后可重写
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
第二个问题首先Looper的创建必须要调用private Looper(boolean quitAllowed)，这个方法为私有方法，因此查看调用这个方法的只有 private static void prepare(boolean quitAllowed)这个方法，这个方法也是私有的，因此只能继续查看调用
prepare(boolean quitAllowed)的方法，调用这个方法的有两个，一个是
public static void prepare() {
prepare(true);
}
这也是我们常用的在子线程中使用Handler的一定要调用方法，另一个是
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
这个方法在两个类中被调用，一个是ActivityThread的main方法，这也解释了在Activity中不需要对Looper做特殊的操作，另一个是SystemServer的run方法。看一下prepare(boolean quitAllowed)的源码：
private static void prepare(boolean quitAllowed) {
//通过ThreadLocal获取当前Looper是否存在，如果存在则抛出运行时异常
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//不存在则设置ThreadLocal，新建Looper
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed)的源码是
private Looper(boolean quitAllowed) {
//建立MessageQueue，因此一个线程中只有一个Looper和MessageQueue
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
第三个问题这个问题需要看一下MessageQueue的next方法
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
//不为0就是说不是第一次赋值，有没到时间的message，去做一个刷新任务命令，是一个nativie方法
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//进行阻塞
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
//获取启动以后的时间
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
//还没到唤起的时间nextPollTimeoutMillis重新设置，将msg设定的时间减去当前的时间
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
第四个问题，除了设置callback还可以使用使用如下方法：
- public Handler()
- public Handler(Callback callback)
- public Handler(Looper looper)
- public Handler(Looper looper, Callback callback)
- public Handler(boolean async)
- public Handler(Callback callback, boolean async)
- public Handler(Looper looper, Callback callback, boolean async)
这几个方法除了public Handler(Looper looper, Callback callback, boolean async)方法之外，其余最终会调用 public Handler(Callback callback, boolean async)。代码如下：
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());
}
}
//设置looper，MessageQueue，Callback
mLooper = Looper.myLooper();
//looper为空，则会抛出去运行异常
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;
}
