它高傲,但是宅心仁厚,它低调,但是受万人景仰。他可以把神赐给人类的android,贯穿得出神入化,使用堪称之艺术的回调方式。他究竟是神仙的化身?还是地狱的使者?没人知道,但是可以肯定,每个人都给他一个称号——Handler!
handler是android开发中提到频率比较高的词。那它到底是何方神圣?一切对象都可以归结为三个哲学问题:它是谁?它能干嘛?它是怎么实现的?
Handler主要有两个用途:(1)调度Message和runnable对象在将来某个时间点被执行。(2)在自己的线程里队列一个action以执行。
handler调度message通过post(Runnable r),postAtTime(Runnable r, long uptimeMillis),postAtTime(Runnable r, Object token, long uptimeMillis),postDelayed(Runnable r, long delayMillis)和sendMessage(Message msg),sendEmptyMessage(int what),sendEmptyMessageDelayed(int what, long delayMillis),sendEmptyMessageAtTime(int what, long uptimeMillis),sendMessageDelayed(Message msg, long delayMillis),sendMessageAtTime(Message msg, long uptimeMillis)等方法来完成。其中post开头的方法,用于把runnable对象加入消息队列中(实际上消息列队只接受Message对象的加入,这里实质上是把runnable对象封装成Message再添加到消息队列中,后面的源码分析会清楚地看到),并在取出时执行。而send开头的方法,用于把Message对象提交到消息列队中,Message对象可以携带Bundle数据,在取出消息列队时在handler的handleMessage(Message msg)方法中处理(该方法需要重写)。
当应用启动的时候,主线程就创建了一个专用的消息列队,用于处理和应用级别有关的,比如Activity、broadcast,receivers等,这时我们可以创建自己的线程,并通过handler来用于线程间的交流。
创建Handler实例时,通过其创建时所在的线程获取得到与该线程绑定的Looper对象,并把获取到的Looper对象和Looper对象下的成员变量Message Queue分别赋值给自己的成员变量,自此handler就和消息队列关联上了。请看代码:
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class 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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
看红色代码,Looper.myLooper()返回与该线程绑定的looper实例,别问我怎么知道=.=这我们就去看看这个方法怎么实现以求真相~!打开Looper源码,找到myLooper方法: /**
* 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();
}
只有一行代码~!看源码的时候会经常看到这样很简洁的方法体~谷歌工程师就是这么吊~!从sThreadLocal对象中取出looper返回当前线程绑定的looper,那么sThreadLocal又是何方神圣?// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal sThreadLocal = new ThreadLocal();
可以看到sThreadLocal是ThreadLocal类的实例,那么这个类又是什么?另外注意标注红色注释:当调用prepare()方法后,sThreadLocal.get()方法取到的才不为空。/**
* Implements a thread-local storage, that is, a variable for which each thread
* has its own value. All threads share the same {@code ThreadLocal} object,
* but each sees a different value when accessing it, and changes made by one
* thread do not affect the other threads. The implementation supports
* {@code null} values.
*
* @see java.lang.Thread
* @author Bob Lee
*/
看类介绍可知,该类用于给每一个线程存取取一个值,所有的线程共享该类的实例,每个线程可以访问它取出绑定的值,这个值可以为空。看看其get方法是怎么实现的:
public T get() {
// Optimized for the fast path.
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values != null) {
Object[] table = values.table;
int index = hash & values.mask;
if (this.reference == table[index]) {
return (T) table[index + 1];
}
} else {
values = initializeValues(currentThread);
}
return (T) values.getAfterMiss(this);
}
嗯,首先获得获得当前线程,并通过当前线程作为key取出与之绑定的values!那问题来了,那么它是怎么把looper和线程绑定到一起呢?先看看它set方法的实现: public void set(T value) {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}
先获取当前线程,然后把线程和传进来的值绑定在一起~!现在我们回到上面红色标注的地方,嗯,我们现在就去看看prepare()方法是怎么实现的: 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));
}
prepare()方法里调用了prepare(boolean quitAllowed)方法,所以把两个方法放在一起便于一目了然。在prepare(boolean quitAllowed)中我们可以看到其先判断当前线程是否绑定有looper对象,如果有将抛出异常,因为每个线程只能对应于一个looper对象。然后创建一个looper实例并和当前线程绑定在了一起。那么消息队列呢?不急,我们这就去看看looper创建的时候做了什么: private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
嗯,looper创建的时候,就创建了一个消息队列赋值给自己的成员变量mQueue并获取当前线程同样赋值给自己的成员变量mThread。到此,我们就清晰了,handler包含一个looper对象和MessageQueue对象,looper对象包含一个MessageQueue对象,handler里的MessageQueue对象就是looper对象里的MessageQueue对象,或者换一种说法:handler和looper共用一个消息队列。就这样handler和消息队列绑定上了。 public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
哎,还是看不到它是如何发消息的,但是我们可以看到这里调用了enqueueMessage方法的时候把成员变量mQueue(消息列队)传了进去,那么enqueueMessage这个方法内部又是怎么样的呢?我们继续: private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
看红色代码,把当前handler实例赋值给了要发送的消息的target属性,这个属性是什么呢?我们可以到Message源码去看个究竟!在Message源码中的成员变量中我们可以找到这么一句:/*package*/ Handler target;
也就是handler实例发消息的时候把自身赋值给了该消息的成员变量!这么做有么用?为么这么设计?答案稍后揭晓~ 嗯,回到enqueueMessage这个方法实体来,看最后的那句红色代码,意味着handler发送消息的本质是把消息交给自己的成员变量mQueue(消息队列)的enqueueMessage(Message msg, long when)方法去处理。这个时候handler就实现了发送消息并把消息提交到消息队列中的过程。 boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {//如果消息已经使用
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {//如果没有handler
throw new AndroidRuntimeException("Message must have a target.");
}
synchronized (this) {
if (mQuitting) {
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;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {//是否第一次有消息进来,是否需要马上处理,
// New head, wake up the event queue if blocked. //是否在消息队列最前端的消息先触发
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 (;;) {//无线循环直到找到消息插入的位置
prev = p;
p = p.next;
if (p == null || when < p.when) {//通过比较触发时间找到插入位置,或者找不到直到消息队列列尾
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
//插入消息队列中
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;
}
/**
* 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();//获得与当前线程的looper实例
if (me == null) {//不存在则抛出异常
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;//获得与looper对应的消息队列
// 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();//当前线程标识
for (;;) {//死循环 不断的从消息队列中取出消息
Message msg = queue.next(); // 从消息队列中取出消息,没有消息则阻塞
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);//取出消息让消息本身的成员变量target的dispatchMessage方法去处理,其实就是handler去处理
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) {//判断取出消息之前和之后是否在同一线程
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();//回收该消息
}
}
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {//这里是post头的方法发送runnable对象从消息队列取出后的处理方法
handleCallback(msg);
} else {//否则是send开头方法发送的消息从消息队列取出的处理方法
if (mCallback != null) {//如果在创建handler的时候传入Callback接口不为空
if (mCallback.handleMessage(msg)) {//并且Callback接口方法消耗了该消息则返回,这里我们可以看到,如果我们要拦截消息,就可以让callback
return; //接口的方法返回true,以便消耗了该消息,使得handler的handlerMessage方法无法接收到消息和处理。
}
}
handleMessage(msg);
}
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
getPostMessage方法就是把一个runnable对象封装成一个消息,然后提交到消息队列中去。我们看看这个方法是怎么样的: private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();//从消息池里拿出一个消息,
m.callback = r;//把runnable对象复制给该消息的成员变量
return m;
}
然后在从消息队列中取出的时候,处理方法是handleCallback(Message msg):private static void handleCallback(Message message) {
message.callback.run();
}
这就是我们传进去之前已经实现的runnable.run()方法,至此,至此,至此,handler就像个羞答答的裸体美女站在我们面前无处躲藏!!! Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {//无限循环取出
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// We can assume mPtr != 0 because the loop is obviously still running.
// The looper will not call this method after the loop quits.
nativePollOnce(mPtr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;//消息队列最前端要被取出的消息
if (msg != null && msg.target == null) {//如果消息没有handler处理
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {//循环查找异步消息,直到取到非异步消息或者队列中没有消息为止
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {//要取出的消息不为空
if (now < msg.when) {//如果还没到触发时间,则设置下一次轮询的时间
// 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) {//异步消息不为空,
prevMsg.next = msg.next;//能走到这步说明异步消息的next为非异步消息,也就是prevMsg.next是下一个是要被取出消息列队的消息
} else {//否则把取出消息的next赋值给下一个取出的消息
mMessages = msg.next;
}
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
msg.markInUse();//标志消息已经使用
return msg;//返回取到的消息
}
} 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("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;
}
}
从消息队列中不断地取出消息,如果当前消息还没到触发时间,则设置下次轮询时间。否则取出。代码中已经注释很清楚了~