Android消息机制可以说是众人皆知了,作为一个Android开发者没用过是不可能的。其原理相对是一个比较简单的内容。本篇文章我们来进行一个简单的梳理。在消息机制中我们主要有下面几块内容需要掌握。
- Handler：一个发送和处理消息的类，其依赖于Looper构建，如果他所在的线程中没有Looper就会出现异常。而消息的处理也是在looper所在的线程。
- Looper，一个无限循环器，循环去拿MessageQueue中的message。
- MessageQueue，消息队列。单链表形式存储Message。
- ThreadLoacal：此内容可查看另一篇文章：ThreadLocal全面解析。
1、首先我们来看消息队列MessageQueue,它提供了插入消息，和拿出消息的两个方法。分别是enqueueMessage，next
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;
}
根据这个方法我们可以看到，我们的MessageQueue是以单链表形式存储我们的消息的，并且，当我们的消息都是立即发送的时候，会把消息存储在链表第一个位置，当我们发送延时消息的时候，会根据延时的时间大小将我们的消息存放在链表中。时间小的在链表前面。
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 (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
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) {
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;
}
}
通过next方法源码我们可以看出，是一个无线循环的方法，一直去消息队列中取消息，如果没有消息将会阻塞在这里，有消息了就会跳出循环，返回消息并把消息从消息队列中删除。
- Looper消息循环，我们知道如果我们在一个子线程中创建Handler，如果在这个线程中没有调用 Looper.prepare();的话会发生"Can't create handler inside thread that has not called Looper.prepare()"的异常，因为Handler是依赖Looper存在的，但是为什么我们在主线程中创建Handler没有调用 Looper.prepare();就可以呢？那是因为我们的系统在AcitivityThread的Main方法中默认调用了Looper.prepareMainLooper();其原理也是调用了prepare方法。
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));
}
在这个方法中我们可以看到 它创建了一个Looper实例放在了我们的ThreadLocal中。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
而我们创建Looper的同时还创建了我们的MessageQueue.所以我们子线程的消息队列和主线程的消息队列不是同一个。Looper也不是同一个。
我们知道当我们执行了Looper.prepare();方法创建了Looper之后，它并没有真正的开始工作，还没有循环起来去拿队列中的消息进行处理。我们还需要调用Looper.loop();方法。
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
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 {
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();
}
}
从loop方法中我们可以看出loop方法是一个无线循环的方法，在循环里面会调用消息队列的next方法获取消息，并调用msg.target.dispatchMessage(msg);处理消息，而跳出循环的条件是msg==null,但是通过我们之前对消息队列next方法的分析，我们知道，next方法也是一个无线循环的方法，并且在消息队列中没有消息的时候一直循环去拿消息，直到拿到消息。所以loop方法会阻塞在那里，等待消息队列返回消息。那么什么时候next才能返回null呢。其实当我们loop调用quit()或者quitSafely()。就会调用我们的消息队列的quit()方法，这时，消息队列返回消息就会是null。
我们来看一下这两个方法
class Looper
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
class MessageQueue
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
从上面的代码中我们可以看出当我们Looper进行quit或者quitSafely的时候实际上是调用MessageQueue的removeAllMessagesLocked()和removeAllFutureMessagesLocked();
class MessageQueue
private void removeAllMessagesLocked() {
Message p = mMessages;
while (p != null) {
Message n = p.next;
p.recycleUnchecked();
p = n;
}
mMessages = null;
}
private void removeAllFutureMessagesLocked() {
final long now = SystemClock.uptimeMillis();
Message p = mMessages;
if (p != null) {
if (p.when > now) {
removeAllMessagesLocked();
} else {
Message n;
for (;;) {
n = p.next;
if (n == null) {
return;
}
if (n.when > now) {
break;
}
p = n;
}
p.next = null;
do {
p = n;
n = p.next;
p.recycleUnchecked();
} while (n != null);
}
}
}
class Message
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
通过上面的两个方法的源码我们可以看出，Looper的quit方法会直接将我们消息队列中的消息调用recycleUnchecked()方法将所有消息的内容消除，导致我们的消息无法进行处理。并且，不允许再往消息队列中插入新的消息，直到我们消息队列中没有消息之后，MessageQueue的next就会返回null，然后我们的Looper就会停止循环，而我们的removeAllFutureMessagesLocked是把所有的消息和当前时间进行比较，来比较是否是延迟消息如果是延迟消息，将之后的延迟消息的消息内容制空。
- Handler，发送和处理消息的方法。其中有很多可以发送消息的方法。
public final boolean post(Runnable r)；
public final boolean postAtTime(Runnable r, long uptimeMillis)；
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)；
public final boolean postDelayed(Runnable r, long delayMillis)；
public final boolean sendMessage(Message msg)；
public final boolean sendEmptyMessage(int what)；
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) ；
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis)；
public final boolean sendMessageDelayed(Message msg, long delayMillis)；
public boolean sendMessageAtTime(Message msg, long uptimeMillis)；
发送消息的方法有很多，但是通过源码我们可以看出他们是有关系的，比如post方法都会默认调用sendMessageDelayed方法，而sendMessageDelayed方法又会默认去调用sendEmptyMessageAtTime方法。sendMessageDelayed和sendEmptyMessageAtTime方法的区别并不大，sendMessageDelayed的延时事件参数是一个相对时间，相对于现在1000毫秒，而sendEmptyMessageAtTime的参数是一个绝对时间，是以系统开机时间开始（去除休眠时间）的一个绝对时间。
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
而Post系列方法和send系列方法的区别为 post系列方法参数为Runnable他会创建一个Massage并把runnable复制给msg.callback，并且将我们的发送消息的Handler复制给我们的Message的target 参数。通过发送消息的源码我们可以看出最终会调用消息队列的enqueueMessage方法将消息放到消息队列当中，通过上面我们对Looper的分析我们知道，当Looper取到消息的时候会调用msg.target.dispatchMessage(msg);来处理消息，其实就是调用我们发送消息的Handler的dispatchMessage方法来处理消息。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
通过上面的方法我们可以看出，当消息有CallBack时候，就会执行消息的CallBack，如果没有将会查看mCallback,mCallback其实就是我们创建Handler的时候Handler构造方法中的Runnable，如果mCallback返回true的话我们Handler的handleMessage方法就不会执行，如果mCallback返回为false的话我们的handleMessage就会执行了。
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