Android中的Handler, Looper, MessageQueue和Thread
参照改编 ： http://www.cnblogs.com/xirihanlin/archive/2011/04/11/2012746.html
对于这部分的内容，将分成4小节来描述：
1.职责与关系
2.消息循环
3.线程与更新
4.几点小结
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1） 接下来，我们开始这部分的内容，首先了解一下各自的职责及相互之间的关系。
职责
Message：消息，其中包含了消息ID，消息处理对象以及处理的数据等，由MessageQueue统一列队，终由Handler处理。
Handler：处理者，负责Message的发送及处理。使用Handler时，需要实现handleMessage(Message msg)方法来对特定的Message进行处理，例如更新UI等。
MessageQueue：消息队列，用来存放Handler发送过来的消息。与链表相似，由messge.next属性相连。当然，存放Message并非实际意义的保存，而是将Message以链表的方式串联起来的，等待Looper的抽取。
Looper：消息泵，不断地从MessageQueue中抽取Message执行。因此，一个MessageQueue需要一个Looper。
Thread：线程，负责调度整个消息循环，即消息循环的执行场所。
关系
Handler，Looper和MessageQueue就是简单的三角关系。Looper和MessageQueue一一对应，创建一个Looper的同时，会创建一个MessageQueue。而Handler与它们的关系，只是简单的聚集关系，即Handler里会引用当前线程里的特定Looper和MessageQueue。
这样说来，多个Handler都可以共享同一Looper和MessageQueue了。当然，这些Handler也就运行在同一个线程里。不同线程可以使用同一个mHandler变量，在其他线程中可以 mHandler.sendMessage(msg)，即可将msg压入到定义mHandler的那个线程的MessageQueue中。
2 ） 接下来，我们简单地看一下消息的循环过程： 生成
Message msg = mHandler.obtainMessage();
msg.what = what;
msg.sendToTarget();
In Handler.class
public final Message obtainMessage()
{
return Message.obtain(this);
}
In Message.class
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
In Message.class
public void sendToTarget() {
target.sendMessage(this);
}
In Handler.class
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
/**
* 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);
}
/**
* 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)
{
boolean sent = false;
MessageQueue queue = mQueue;
if (queue != null) {
msg.target = this;
sent = queue.enqueueMessage(msg, uptimeMillis);
}
else {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
return sent;
}
将Message发送到MessageQueue中.
加入MessageQueue，在MessageQueue中的排序
In MessageQueue.class
final boolean enqueueMessage(Message msg, long when) {
if (msg.when != 0) {
throw new AndroidRuntimeException(msg
+ " This message is already in use.");
}
if (msg.target == null && !mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit");
}
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;
} else if (msg.target == null) {
mQuiting = true;
}
msg.when = when;
//Log.d("MessageQueue", "Enqueing: " + msg);
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
this.notify();
} else {
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
this.notify();
}
}
return true;
}
可以看到，排序的方法是：根据msg.when排序，如果现在就要执行的，就排在最前面；否则，就插入到适当的位置，相似与
链表的排序
规则
抽取
In Looper.class
Looper me = myLooper();
MessageQueue queue = me.mQueue;
while (true) {
Message msg = queue.next(); // might block
if (msg != null) {
if (msg.target == null) {
// No target is a magic identifier for the quit message.
return;
}
msg.target.dispatchMessage(msg);
msg.recycle();
}
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static final Looper myLooper() {
return (Looper)sThreadLocal.get();
}
在 Looper.java 的 loop() 函数里，我们看到，这里有一个死循环，不断地从 MessageQueue 中获取下一个（ next 方法） Message ，然后通过 Message 中携带的 target 信息，交由正确的 Handler 处理（ dispatchMessage 方法）。
处理
In Handler.class
/**
* 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);
}
}
在Handler.java的dispatchMessage(Message msg)方法里，其中的一个分支就是调用handleMessage方法来处理这条Message，而这也正是我们在职责处描述使用Handler时需要实现handleMessage(Message msg)的原因。
至于dispatchMessage方法中的另外一个分支handlerCallback(msg)，用方法handler.post(runnable)时，运行这个分支：
In Handler.class
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the Runnable 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 post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private final Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
private final void handleCallback(Message message) {
message.callback.run();
}注意，此时message.callback.run()方法，而不是new Thread(message.callback.).start().因此，还是在
MainThread中运行callback
至此，我们看到，一个Message经由Handler的发送，MessageQueue的入队，Looper的抽取，又再一次地回到Handler的怀抱。而绕的这一圈，也正好帮助我们将同步操作变成了异步操作。
3 ）剩下的部分，我们将讨论一下 Handler 所处的线程及更新 UI 的方式。 在主线程（UI线程）里，如果创建Handler时不传入Looper对象，那么将直接使用主线程（UI线程）的Looper对象（系统已经帮我们创建了）；在其它线程里，如果创建Handler时不传入Looper对象，那么，这个Handler将不能接收处理消息。在这种情况下，通用的作法是：
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();
}
}
原因是：在prepare（）中，为当前线程创建了一个Looper 和 MessageQueue，并保存在 ThreadLocal对象中（ThreadLocal对象保存的是当前线程的局部变量，是与该线程相对应的），再loop()，即可为不断抽取当前线程的messge.
public static final void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
private Looper() {
mQueue = new MessageQueue();
mRun = true;
mThread = Thread.currentThread();
}
sThreadLocal.set(new Looper())，为当前线程创建对应的Looper对象，并存储起来
关于ThreadLocal : http://blog.csdn.net/qjyong/article/details/2158097
在创建Handler之前，为该线程准备好一个Looper（Looper.prepare），然后让这个Looper跑起来（Looper.loop），抽取Message，这样，Handler才能正常工作。
因此，Handler处理消息总是在创建Handler的线程里运行。而我们的消息处理中，不乏更新UI的操作，不正确的线程直接更新UI将引发异常。因此，需要时刻关心Handler在哪个线程里创建的。
如何更新UI才能不出异常呢？SDK告诉我们，有以下4种方式可以从其它线程访问UI线程：
· Activity.runOnUiThread(Runnable)
· View.post(Runnable)
· View.postDelayed(Runnable, long)
· Handler
其中，重点说一下的是View.post(Runnable)方法。在post(Runnable action)方法里，View获得当前线程（即UI线程）的Handler，然后将action对象post到Handler里。在Handler里，它将传递过来的action对象包装成一个Message（Message的callback为action），然后将其投入UI线程的消息循环中。在Handler再次处理该Message时，有一条分支（未解释的那条）就是为它所设，直接调用runnable的run方法。而此时，已经路由到UI线程里，因此，我们可以毫无顾虑的来更新UI。
4） 几点小结
· Handler的处理过程运行在创建Handler的线程里
· 一个Looper对应一个MessageQueue
· 一个线程对应一个Looper
· 一个Looper可以对应多个Handler
· 不确定当前线程时，更新UI时尽量调用post方法
以View.Click事件为例子，其步骤如下：onTouchEvent ----> Action_Up --->handler. post(new PerformClick()) , class PerformClick implements Runnable{ view.click;} ----> messageQueue.enqueue(msg) ----> Looper.loop() ------> msg.target.dispathMessage() -------> performClick.run();
/**
* Implement this method to handle touch screen motion events.
*
* @param event The motion event.
* @return True if the event was handled, false otherwise.
*/
public boolean onTouchEvent(MotionEvent event) {
final int viewFlags = mViewFlags;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
switch (event.getAction()) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PREPRESSED) != 0;
if ((mPrivateFlags & PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (!mHasPerformedLongPress) {
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
mPrivateFlags |= PRESSED;
refreshDrawableState();
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
break;
case MotionEvent.ACTION_DOWN:
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPrivateFlags |= PREPRESSED;
mHasPerformedLongPress = false;
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
break;
case MotionEvent.ACTION_CANCEL:
mPrivateFlags &= ~PRESSED;
refreshDrawableState();
removeTapCallback();
break;
case MotionEvent.ACTION_MOVE:
final int x = (int) event.getX();
final int y = (int) event.getY();
// Be lenient about moving outside of buttons
int slop = mTouchSlop;
if ((x < 0 - slop) || (x >= getWidth() + slop) ||
(y < 0 - slop) || (y >= getHeight() + slop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
// Need to switch from pressed to not pressed
mPrivateFlags &= ~PRESSED;
refreshDrawableState();
}
}
break;
}
return true;
}
return false;
}
private final class PerformClick implements Runnable {
public void run() {
performClick();
}
}
/**
* Call this view's OnClickListener, if it is defined.
*
* @return True there was an assigned OnClickListener that was called, false
* otherwise is returned.
*/
public boolean performClick() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
if (mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
mOnClickListener.onClick(this);
return true;
}
return false;
}
理解了这些其实就不难想象ANR是如何实现的了。当用户执行操作(比如点击了一个按钮)系统会生成一个Message对象，把用户操作的信息写入Message对象，并把这个Message对象压入MessageQueue队列的头部。系统过一段时间(一般是五秒)后会再来检查，刚刚放入的信息是不是已经被处理了，如果信息还在队列中就表明。处理前面信息的过程当中发生的阻塞，用户的操作没有及时得到响应。系统弹出ANR对话框。
作个总结：
因为UI线程需要保持一直运行的状态，所以要有一个循环保持这个线程不会死掉，但这个线程又必需阻塞，以减少cpu的消耗。android中的这个循环就是通过Looper实现的。有了这个 Looper，Looper就占据了整个线程，导致所有的方法想在些线程中运行就必需通过这个Looper，所以要有个方法可以进入这个Looper的内部。MessageQueue就担当了这个通道 的角色。Message担当了集合的角色。所有在UI线程中运行的方法都必需通过MessageQueue进入Looper内部，不管 是用户定义的方法还是系统事件包括onCreate(),onStop(),用户点击事件etc..
final Message next() {
boolean tryIdle = true;
while (true) {
long now;
Object[] idlers = null;
// Try to retrieve the next message, returning if found.
synchronized (this) {
now = SystemClock.uptimeMillis();
Message msg = pullNextLocked(now);
if (msg != null) return msg;
if (tryIdle && mIdleHandlers.size() > 0) {
idlers = mIdleHandlers.toArray();
}
}
// There was no message so we are going to wait... but first,
// if there are any idle handlers let them know.
boolean didIdle = false;
if (idlers != null) {
for (Object idler : idlers) {
boolean keep = false;
try {
didIdle = true;
keep = ((IdleHandler)idler).queueIdle();
} catch (Throwable t) {
Log.wtf("MessageQueue", "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
}
// While calling an idle handler, a new message could have been
// delivered... so go back and look again for a pending message.
if (didIdle) {
tryIdle = false;
continue;
}
synchronized (this) {
// No messages, nobody to tell about it... time to wait!
try {
if (mMessages != null) {
if (mMessages.when-now > 0) {
Binder.flushPendingCommands();
this.wait(mMessages.when-now);
}
} else {
Binder.flushPendingCommands();
this.wait();
}
}
catch (InterruptedException e) {
}
}
}
}
在next()中，在最后的synchronized(this)中，if(mMessages != null){ ..... } else{ ...this.wait();....} ，将UI线程阻塞；
final boolean enqueueMessage(Message msg, long when) {
if (msg.when != 0) {
throw new AndroidRuntimeException(msg
+ " This message is already in use.");
}
if (msg.target == null && !mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit");
}
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;
} else if (msg.target == null) {
mQuiting = true;
}
msg.when = when;
//Log.d("MessageQueue", "Enqueing: " + msg);
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
this.notify();
} else {
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
this.notify();
}
}
return true;
}在enqueueMessage()中，this.notify()，将UI线程唤醒
/**
* Runs the specified action on the UI thread. If the current thread is the UI
* thread, then the action is executed immediately. If the current thread is
* not the UI thread, the action is posted to the event queue of the UI thread.
*
* @param action the action to run on the UI thread
*/
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}又如：
private Button mButton;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mButton = (Button) findViewById(R.id.btn);
mButton.setOnClickListener(new Button.OnClickListener(){
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
Log.i("TAG","button.onclick");
Thread thread1 = new Thread(runnable1);
thread1.start();
}});
}
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
super.handleMessage(msg);
}
@Override
public void dispatchMessage(Message msg) {
// TODO Auto-generated method stub
Thread thread = Thread.currentThread();
Log.i("TAG","dispatchMessage(); threadName = " + thread.getName());
super.dispatchMessage(msg);
}
@Override
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
// TODO Auto-generated method stub
Thread thread = Thread.currentThread();
Log.i("TAG","sendMessageAtTime(); threadName = " + thread.getName());
return super.sendMessageAtTime(msg, uptimeMillis);
}
};
private Runnable runnable1 = new Runnable(){
@Override
public void run() {
Thread thread = Thread.currentThread();
Log.i("TAG","runnable1.run(); threadName = " + thread.getName());
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
mHandler.sendEmptyMessage(1);
}
};
public boolean sendMessageAtTime(Message msg, long uptimeMillis)
{
boolean sent = false;
MessageQueue queue = mQueue;
if (queue != null) {
msg.target = this;
sent = queue.enqueueMessage(msg, uptimeMillis);
}
else {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
return sent;
}
结果：
很显然，在thread-8中，mHandler将msg发送到主线程的Message Queue,主线程被notify,然后取出msg,由mHandler dispatchMessage,然后在主线程 handle msg.
其实在handler中，有一个mLooper,mQueue,其在默认状态下，是对应于定义handler那个的线程的