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本文以Android 9.0 版本进行分析，当然你也可以在线看源码
在线源码查看
Android源码下载编译
在此特别说明，我这篇主要分析流程和注释。我把英语注释也粘贴了，大家自己去翻译自己消化，个人意见重点是流程+注释，流程+注释，流程+注释。
产生Handler原因：
- 主线程不能做耗时操作
- 子线程不能更新UI
- 多个线程并发更新UI的同时保证线程安全
为什么不能在子线程更新UI
详细请参考 架构设计分析（二）Android消息机制之为什么不能在子线程中更新UI
framework/base/core/java/android/view/ViewRootImpl.java
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
- mThread是UI线程，这里会检查当前线程是不是UI线程。那么为什么onCreate()方法内没有进行这个检查呢？因为Activity的生命周期内，在onCreate()方法中，UI处于创建过程，对用户来说界面还不可见，直到onStart()方法之后界面可视，在到onResume()方法后界面开始交互。在某种程度来讲，在onCreate()方法中不能算是更新UI,只能说是适配了UI或者是设置UI的属性。这个时候不会调用ViewRootImpl.checkThread()方法，因为ViewRootImpl还没创建。而在onResume()方法之后，ViewRootImpl才被创建，这个时候去交互UI才算是更新UI。
private void performTraversals() {
// cache mView since it is used so much below...
final View host = mView;
if (DBG) {
System.out.println("======================================");
System.out.println("performTraversals");
host.debug();
}
if (host == null || !mAdded)
return;
mIsInTraversal = true;
mWillDrawSoon = true;
boolean windowSizeMayChange = false;
boolean newSurface = false;
boolean surfaceChanged = false;
WindowManager.LayoutParams lp = mWindowAttributes;
int desiredWindowWidth;
int desiredWindowHeight;
final int viewVisibility = getHostVisibility();
final boolean viewVisibilityChanged = !mFirst
&& (mViewVisibility != viewVisibility || mNewSurfaceNeeded
// Also check for possible double visibility update, which will make current
// viewVisibility value equal to mViewVisibility and we may miss it.
|| mAppVisibilityChanged);
mAppVisibilityChanged = false;
final boolean viewUserVisibilityChanged = !mFirst &&
((mViewVisibility == View.VISIBLE) != (viewVisibility == View.VISIBLE));
WindowManager.LayoutParams params = null;
if (mWindowAttributesChanged) {
mWindowAttributesChanged = false;
surfaceChanged = true;
params = lp;
}
- setContentView只是建立了View树并没有进行渲染工作，真正的渲染工作是在onResume()方法之后也就是建立了VIew树。因此我们可以通过findViewById()来获取View对象，但是由于没有进行渲染视图的工作，也就是没有执行ViewRootImpl.performTraversals()。同样View也不会执行onMeasure()。如果在onResume()方法中直接获取View.getHeight()和View.getWidth()得到的结果都是0。
- zhangbin: onResume getHeight:0 getWidth：0
Android消息机制
Android 的消息机制主要是指Handler得运行机制
消息机制流程图.png
以上模型的解释：
1.以Handler的sendMessage方法为例，当发送一个消息后，会将此消息加入消息队列MessageQueue中。
2.Looper负责去遍历消息队列并且将队列中的消息分发给对应的Handler进行处理。
3.在Handler的handleMessage方法中处理该消息，这就完成了一个消息的发送和处理过程。
这里从图中可以看到参与消息处理有四个对象，它们分别是 Handler, Message, MessageQueue，Looper。
为什么Handler构造方法里面的Looper不是直接new呢？
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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
代码也很好解释，如果Handler构造方法中new Looper无法保证Looper唯一。目前使用的
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
为什么MessageQueue要放在Looper私有构造方法中初始化？
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
因为一个线程对应一个Looper,所有在Looper构造方法初始化可以保证mQueue也是唯一的，也就是一个Thread对应一个Looper对应一个MessageQueue对象。
分析入口ActivityThread的main方法
ActivityThread就是我们常说的主线程或UI线程，ActivityThread的main方法是整个APP的入口，这个(main里面的attach这里) 也是Activity启动分析的入口。有童鞋问这个文件目录在哪，frameworks/base/core/java/android/app/ActivityThread.java
public static void main(String[] args) {
... ...
Looper.prepareMainLooper();
// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
// It will be in the format "seq=114"
long startSeq = 0;
if (args != null) {
for (int i = args.length - 1; i >= 0; --i) {
if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
startSeq = Long.parseLong(
args[i].substring(PROC_START_SEQ_IDENT.length()));
}
}
}
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
1-1、主线程Looper的prepareMainLooper()方法
- 主要看注释，注释看个几遍不为过，先看下Looper的注释。
/**
* Class used to run a message loop for a thread. Threads by default do
* not have a message loop associated with them; to create one, call
* {@link #prepare} in the thread that is to run the loop, and then
* {@link #loop} to have it process messages until the loop is stopped.
*
* <p>Most interaction with a message loop is through the
* {@link Handler} class.
*
* <p>This is a typical example of the implementation of a Looper thread,
* using the separation of {@link #prepare} and {@link #loop} to create an
* initial Handler to communicate with the Looper.
*
* <pre>
* class LooperThread extends Thread {
* public Handler mHandler;
*
* public void run() {
* Looper.prepare();
*
* mHandler = new Handler() {
* public void handleMessage(Message msg) {
* // process incoming messages here
* }
* };
*
* Looper.loop();
* }
* }</pre>
*/
- prepareMainLooper()方法
/**
* 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();
}
}
1-1-1、 prepare()方法，还是主要看注释，注释看个几遍不为过。
false表示主线程不准许退出，主动掉prepare()值为true。这里的quitAllowed参数，最终会传递给MessageQueue，当调用MessageQueue的quit方法时，会判断这个参数，如果是主线程，也就是quitAllowed参数为false时，会抛出异常。
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
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));
}
sThreadLocal的变量定义，继续看注释。
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
1-1-2、ThreadLocal的set和get方法，主要实现线程的单例，上面是Looper的单例。
- 先看下人家写这个类的注释，我特意百度了一下这个人。他是Google 的首席 Java 架构师人物介绍
/**
* This class provides thread-local variables. These variables differ from
* their normal counterparts in that each thread that accesses one (via its
* {@code get} or {@code set} method) has its own, independently initialized
* copy of the variable. {@code ThreadLocal} instances are typically private
* static fields in classes that wish to associate state with a thread (e.g.,
* a user ID or Transaction ID).
*
* <p>For example, the class below generates unique identifiers local to each
* thread.
* A thread's id is assigned the first time it invokes {@code ThreadId.get()}
* and remains unchanged on subsequent calls.
* <pre>
* import java.util.concurrent.atomic.AtomicInteger;
*
* public class ThreadId {
* // Atomic integer containing the next thread ID to be assigned
* private static final AtomicInteger nextId = new AtomicInteger(0);
*
* // Thread local variable containing each thread's ID
* private static final ThreadLocal<Integer> threadId =
* new ThreadLocal<Integer>() {
* @Override protected Integer initialValue() {
* return nextId.getAndIncrement();
* }
* };
*
* // Returns the current thread's unique ID, assigning it if necessary
* public static int get() {
* return threadId.get();
* }
* }
* </pre>
* <p>Each thread holds an implicit reference to its copy of a thread-local
* variable as long as the thread is alive and the {@code ThreadLocal}
* instance is accessible; after a thread goes away, all of its copies of
* thread-local instances are subject to garbage collection (unless other
* references to these copies exist).
*
* @author Josh Bloch and Doug Lea
* @since 1.2
*/
1-1-3、get方法
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
1-1-4、setInitialValue()方法
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
1-1-5、initialValue()方法
/**
* Returns the current thread's "initial value" for this
* thread-local variable. This method will be invoked the first
* time a thread accesses the variable with the {@link #get}
* method, unless the thread previously invoked the {@link #set}
* method, in which case the {@code initialValue} method will not
* be invoked for the thread. Normally, this method is invoked at
* most once per thread, but it may be invoked again in case of
* subsequent invocations of {@link #remove} followed by {@link #get}.
*
* <p>This implementation simply returns {@code null}; if the
* programmer desires thread-local variables to have an initial
* value other than {@code null}, {@code ThreadLocal} must be
* subclassed, and this method overridden. Typically, an
* anonymous inner class will be used.
*
* @return the initial value for this thread-local
*/
protected T initialValue() {
return null;
}
1-1-6、set方法
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
1-1-7、Looper的构造方法。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
1-1-8、MessageQueue的构造方法。
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
1-2、myLooper()分析
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
2-1、ActivityThread中的main方法的getHandler()方法
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
final Handler getHandler() {
return mH;
}
final H mH = new H();
class H extends Handler {
}
3-1、主线程Looper的loop()方法
这个循环取出Message并交给Handler处理msg.target.dispatchMessage(msg);
/**
* 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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
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 traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();
}
}
3-1-1、取出Looper
final Looper me = myLooper();
3-1-2、MessageQueue
final MessageQueue queue = me.mQueue;
3-1-3、Message
Message msg = queue.next(); // might block
3-1-4、Handler
/**
* A Handler allows you to send and process {@link Message} and Runnable
* objects associated with a thread's {@link MessageQueue}. Each Handler
* instance is associated with a single thread and that thread's message
* queue. When you create a new Handler, it is bound to the thread /
* message queue of the thread that is creating it -- from that point on,
* it will deliver messages and runnables to that message queue and execute
* them as they come out of the message queue.
*
* <p>There are two main uses for a Handler: (1) to schedule messages and
* runnables to be executed as some point in the future; and (2) to enqueue
* an action to be performed on a different thread than your own.
*
* <p>Scheduling messages is accomplished with the
* {@link #post}, {@link #postAtTime(Runnable, long)},
* {@link #postDelayed}, {@link #sendEmptyMessage},
* {@link #sendMessage}, {@link #sendMessageAtTime}, and
* {@link #sendMessageDelayed} methods. The <em>post</em> versions allow
* you to enqueue Runnable objects to be called by the message queue when
* they are received; the <em>sendMessage</em> versions allow you to enqueue
* a {@link Message} object containing a bundle of data that will be
* processed by the Handler's {@link #handleMessage} method (requiring that
* you implement a subclass of Handler).
*
* <p>When posting or sending to a Handler, you can either
* allow the item to be processed as soon as the message queue is ready
* to do so, or specify a delay before it gets processed or absolute time for
* it to be processed. The latter two allow you to implement timeouts,
* ticks, and other timing-based behavior.
*
* <p>When a
* process is created for your application, its main thread is dedicated to
* running a message queue that takes care of managing the top-level
* application objects (activities, broadcast receivers, etc) and any windows
* they create. You can create your own threads, and communicate back with
* the main application thread through a Handler. This is done by calling
* the same <em>post</em> or <em>sendMessage</em> methods as before, but from
* your new thread. The given Runnable or Message will then be scheduled
* in the Handler's message queue and processed when appropriate.
*/
/**
* 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);
}
}
final Callback mCallback;
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public boolean handleMessage(Message msg);
}
这个就是使用Handler重写的这个方法
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
4-1、Handler结合源码分析使用
4-1-1、Handler构造方法
Handler handler = new Handler();
重写handleMessage方法
/**
* 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);
}
/**
* 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 respect 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();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
看到此我相信有些童鞋已经看到Looper.myLooper()这个方法了
4-1-1、myLooper()分析
参考1-2、myLooper()分析
4-1-2、发送消息，任意一个方法都可以
mHandler.sendEmptyMessageDelayed(DRAG_SHOW,1500);
4-1-3、最终都会走到sendMessageAtTime()
/**
* 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>
* Time spent in deep sleep will add an additional delay to execution.
* 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);
}
4-1-4、sendMessageAtTime()调用 enqueueMessage() 这个队列是单向链表维护 先进先出
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
4-1-5、MessageQueue的enqueueMessage()这里面根据时间将消息插入合适的链表中
boolean enqueueMessage(Message msg, long when) {
//每一个普通的Message必须有一个target
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) { //正在退出时，回收msg 加入到消息池中
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;
//p为null代表MessageQueue没有消息
if (p == null || when == 0 || when < p.when) {
//when比最新插入对象的时间还要小就替换
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;//进行唤醒
} 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.
// 将消息按照时间插入到MessageQueue 一般不需要唤醒事件队列
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;
}
4-1-6、有消息之后主线程Looper的loop()循环取出消息交给在重写的handleMessage进行处理
分析参考3-1、主线程Looper的loop()方法
MessageQueue # next
Message next() {
final long ptr = mPtr;
if (ptr == 0) {//若消息循环已退出
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration 首次迭代
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// 阻塞操作，当等待nextPollTimeoutMillis时长，或者消息队列被唤醒，都会返回
// 陷入阻塞，等待被唤醒
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;
// 当消息的Handler为空时，则查询异步消息
// msg.target == null表示此消息为消息屏障（通过postSyncBarrier方法发送来的）
// 如果发现了一个消息屏障，会循环找出第一个异步消息（如果有异步消息的话），所有同步消息都将忽略（平常发送的一般都是同步消息）
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) {
//当异步消息触发时间大于当前时间，则设置下一次轮询的超时时长
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
//正常取出消息
//设置mBlocked = false代表目前没有阻塞
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
//设置消息的使用状态，即flags |= FLAG_IN_USE
msg.markInUse();
return msg;//成功地获取MessageQueue中的下一条即将要执行的消息
}
} else {
// 没有消息，会一直阻塞，直到被唤醒
nextPollTimeoutMillis = -1;
}
// 消息正在退出
if (mQuitting) {
dispose();
return null;
}
// 当消息队列为空，或者是消息队列的第一个消息时
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
//没有idle handlers 需要运行，则循环并等待。
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.
//只有第一次循环时，会运行idle handlers，执行完成后，重置pendingIdleHandlerCount为0.
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.
//重置idle handler个数为0，以保证不会再次重复运行
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.
//当调用一个空闲handler时，一个新message能够被分发，因此无需等待可以直接查询pending message.
//重置数量，保证每次 next() 时，只会执行一次 IdleHandler 方法。
nextPollTimeoutMillis = 0;
}
}
