跟消息机制相关的API一共有这么几个：①Looper轮询器②MessageQueue消息队列③Handler④Message 消息
这个消息实际上是放在消息队列里。写Handler的时候并没有写跟MessageQueue相关的代码。
消息是放在消息队列里面的，消息队列什么时候把这个消息取出来？实际上就用到了这么一个东西叫做Looper.Looper也叫做消息泵，能把这个消息不断地从消息队列里给它取出来。Looper消息泵和MessageQueue消息队列是消息机制里面不需要咱们手动去写的这个代码。在主线程不需要手动去写。在子线程你也想用消息队列，那你就要去写相应的代码。
主线程为什么不需要手动去写？因为框架已经帮咱们写好了。
/**
* 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();
}
}
实际上应用一跑起来的时候它首先就会给当前的线程调一下这个prepareMainLooper，准备一个主要的消息轮询器。sThreadLocal是线程级的单例。就是我这一个线程用这个sThreadLocal.只要是这个线程里保存的这个对象只有一个，sThreadLocal实现一个线程对应着唯一一个对象。
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {//我当前这个线程里已经有了一个对象了
throw new RuntimeException("Only one Looper may be created per thread");//如果之前的线程已经有了一个Looper这个代码就走不下去了.马上就抛异常
//所以这里就保证了我的一个线程对应着唯一的一个Looper.
}
sThreadLocal.set(new Looper(quitAllowed));//把创建好的Looper通过线程级的单例进行保存,这就实现了一个线程对应着唯一的一个Looper.
}
Looper的构造是一个私有的，不允许你去调，所以说如果你直接去new出来这个东西是不行的。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);//创建这个Looper先创建了一个MessageQueue.new了一个MessageQueue,通过mQueue保存起来
mRun = true;
mThread = Thread.currentThread();
}
想创建新的Looper就得调prepare这个方法。prepareMainLooper和prepare方法它们俩是public static void。
public static void prepare() {
prepare(true);
}
/**
* 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();
}
}
final MessageQueue mQueue;
mQueue是一个final类型.final类型只要赋值了之后无法修改了.由于一个线程通过线程级的单例对应起了一个唯一的Looper.在创建这个Looper的时候又搞了一个MessageQueue,而且MessageQueue还是final的.所以一个Looper又对应起了唯一的MessageQueue.这就保证了一个线程对应一个消息队列.一个消息队列用一个消息泵取消息.所以说只要你这个消息队列创建好了之后,它就跟当前的线程对应起来了.只要是你在这一个线程里进行操作，不管是通过谁去丢消息，肯定丢不错。你丢到的都是同一个消息队列里。因为一个线程只有这么一个消息队列。
通过构造创建好了Looper还不够，下一步得让Looper转起来。真正的得不断地从消息队列里面去取消息，为什么子线程一sendMessage()这边主线程就取出来了？实际上有一段代码在不断地取消息。怎么实现不断地取消息？
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
final Looper me = myLooper();//实际上就是获取一下这个Looper.
if (me == null) {//如果没有Looper,就抛出异常.所以这个方法不能先调,得先准备好一个Looper,然后才能去调它的Looper.loop().
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;//拿出Looper之后把消息队列拿出来了.
// 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 (;;) {//死循环,相当于while(true)
Message msg = queue.next(); // 拿下一条消息,一条一条拿消息.might block 有可能会阻塞.当前消息队列是空的没消息那它就一直在那儿等.一旦有消息进来就马上把消息取出来.
if (msg == null) {//取出一条消息就是Message.
// 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);//Message还有一个属性是target.
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.recycle();
}
}
运行当前线程的消息队列。
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();//通过线程级的单例保存这个Looper.直接从这个对象里面就可以把它获取到.
}
实际上消息一取出来它就调了Handler的dispatchMessage.来看看Handler的dispatchMessage.
/**
* 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);//重写了handleMessage方法
}
}
我的主线程一运行起来就准备了这么一个Looper,并且准备了一个消息队列.然后调了Looper的loop方法.loop方法不断地调从消息队列里不断地去取消息.一旦有消息取出来马上去调dispatchMessage.一调dispatchMessage实际上就走到了这个handleMessage这个方法.所以说咱们就实现了这个地儿你重写handleMessage往这儿一丢马上就能取出来.
发消息是怎么搞的？
/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);//发消息调的是sendMessageDelayed.sendMessageDelayed(msg,0)表示是马上执行的.
}
/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);//SystemClock.uptimeMillis()获取系统当前的时间.拿着开机开多长时间+延迟多长的时间.
}
Open Declaration long android.os.SystemClock.uptimeMillis() Returns milliseconds since boot, not counting time spent in deep sleep. Returns: milliseconds of non-sleep uptime since boot.
since boot从开机之后.机器开了多久那就获取一下这个时间.
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
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);
}
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @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();// 把当前线程的Looper给它获取到.
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;
}
当Handler一构造起来之后,为什么说Handler发消息发不错？我在主线程创建了Handler,一定是丢给了主线程。首先在构造Handler的时候调的是无参的构造，
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
无参构造调的是两个参数的构造。两个参数的有参构造首先是从 Looper.myLooper()开始.刚才已经看了myLooper()的源码.
Open Declaration Looper android.os.Looper.myLooper() Return the Looper object associated with the current thread. Returns null if the calling thread is not associated with a Looper.
把当前线程的Looper给它获取到.所以Handler一创建起来首先通过成员变量mLooper拿到了当前线程的Looper.拿到之后通过mLooper又拿到了mQueue.所以说Handler拿到了,你在哪个线程创建的Handler,最终你的消息就发到哪个线程对应的消息队列。最终对这个线程搞的Looper进行轮询。
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;//拿到消息队列
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);//把消息队列传给enqueueMessage
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);//queue就是当前线程的消息队列,Handler是在主线程创建的,那么它就是主线程的消息队列.
}
boolean enqueueMessage(Message msg, long when) {//when=当前系统开机的时间+延迟的时间
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}
boolean needWake;
synchronized (this) {
if (mQuiting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
}
msg.when = when;//when是消息要执行的时间.通过消息要执行的时间进行排序.
Message p = mMessages;//只记住了mMessages,只有一条消息
if (p == null || when == 0 || when < p.when) {//p=null说明当前的消息队列是空的,when=0说明我这条消息需要立即执行.when<p.when都说明我这条新加进来的消息应该优先执行.
它应该排在消息队列的第一条.// New head, wake up the event queue if blocked.
msg.next = p;//每一条消息都有一个成员变量next
mMessages = msg;//mMessages等于传进来的msg
needWake = mBlocked;
} else {//当前传进去的消息不是优先执行的.比当前消息队列记住的第一条消息这个when要大.
// 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 (;;) {//for循环是给这个消息找到一个合适的位置
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;
}
}
if (needWake) {
nativeWake(mPtr);
}
return true;
}
虽然MessageQueue是消息队列，但是并没有什么队列的东西在里面。没有什么集合HashMap之类的。
Message mMessages;//只记住了一条消息.通过Message类型的成员变量mMessages把消息队列里的第一条消息给它记住了
// sometimes we store linked lists of these things
/*package*/ Message next;//每一条消息都有一个成员变量next
这个消息队列怎么去排的？就是这么去排的。
sendMessageAtTime最终调的是enqueueMessage(把消息进行入队)。把消息进行一下排序,怎么去排序呢？就是通过要执行的时间when进行排序。谁先执行谁就排在消息队列的前面，谁后执行谁就排在后面。怎么去排？实际上就是按照这个时间来。
最终就按照消息要执行的先后时间把这个消息就入队了。
丢到这里这个Looper就不断地在转，
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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 Looper不断地在转
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.recycle();
}
}
一旦有消息丢进来了，检查一下这个时间，如果这个时间需要立即执行，这就涉及到一个next()方法。next()就是从消息队列里去取消息。
Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(mPtr, nextPollTimeoutMillis);//nativePollOnce后面还会介绍，是通过C来实现的.
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) {//拿到消息之后比一下时间,与第一条消息比一下时间,是不是当前的时间.如果不是当前时间的话说明我这个消息不需要立即执行.now>msg.when是需要立即执行的.立即执行就给它拽出来.如果now<msg.when的话说明还得接着等,接着等的话记一下时间然后就当前的这个Looper就接着顺下去了.
// 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 (false) Log.v("MessageQueue", "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 (mQuiting) {
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("MessageQueue", "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;
}
}
究竟这个消息什么时候执行，是取决于时间的.
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