Looper
Looper是Handler和Thread相关联的桥梁,也是APP开发中线程间通信的用的最多的一个.
然而使用率却是异常的低下,因为在向主线程交互的时Looper该做的工作已经做好,使用上只需要在主线程创建Handler对象,然后再需要的地方发送消息即可.
参考博客 Android 异步消息处理机制 让你深入理解 Looper、Handler、Message三者关系 洪洋
Looper的初始化创建
Looper和线程的关系是一一对应的,每一个线程有且只有一个Looper,可以从Looper的使用中看出.
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));
}
在使用子线程中使用Hander时,需要有Looper对象,而Looper的构造函数是private修饰的,想要获得创建一个Looper对象就只能调用静态方法prepare.
在方法中我们可以看到调用了sThreadLocal.get()方法,返回的若非空就会抛出异常.这里就限定了一个线程只能有一个Looper.
Looper的构造函数
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
在构造函数中创建了一个MessageQueue(消息队列).
Looper的使用
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
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.recycleUnchecked();
}
}
当操作完成就可以调用Looper.loop();从代码中可以看出在通过myLooper中拿到了当前线程的Looper对象,获取了对应的MessageQueue之后,该线程会执行一个死循环,在死循环中不停的读取MessageQueue队列中的Message并调用 msg.target.dispatchMessage(msg);去处理该消息.
Handler
在使用Handler时,我们通常都是实例化一个对象去发送消息,然后在handleMessage方法中去处理对应的消息,并作出对应的处理.
Hanlder的初始化
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;
}
在初始化中我们看到了Handler将自己和Looper进行了关联,如若在子线程中未创建Looper就创建Hanlder那么便会抛出异常.并且在下面继续获取了Looper的消息队列.
Handler的使用
Message message = Message.obtain();
message.what = TEXT;
message.obj = (int) (Math.random() * 1000);
mHandler.sendMessage(message);
Handler使用方式十分之简单,在需要使用的地方发送一个Message消息即可,在创建Handler时重写handleMessage方法中处理对应的Message消息即可.
多数人走到这一步就停下了,因为已经知道如何使用Hanlder和Looper了,能满足跨线程交互需求了,也就没有继续探索的愿望了,然后在面试的一个一问Handler为什么能切换到主线程去执行对应的方法就懵逼了(面试问懵逼好几个了....)
Handler跨线程的原因
既然发送消息是在子线程,在handleMessage中处理的时候已经到了主线程,那么线程切换的奥秘一定就在这两个方法调用过程中,我们就从sendMessage方法中一步一步往下看.
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);
}
到这里就可以看出一点端倪了,mQueue出现了,也就是Handler在创建时获取的Looper的MessageQueue对象.
我们继续向下看
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
在这里我们又看到了一点意思的东西,msg.target = this在Looper.loop中我们看到msg.target.dispatchMessage(msg);这么一行代码,现在稍微一推敲就知道最终发生了什么,怎么切换线程的,但是我们还是继续往下看.
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;
}
恩,到这里就是将Message加入到消息队列中去中去,hanlder中消息发送流程已经走完了.
在这里handler将Message加入到了消息队列中,而Looper一直线主线程中阻塞着,所以一收到消息就拿到了Message对象,然后在主线程中调用了Hanlder的处理方法.
Looper对消息的处理
从上面的分析已经了解到了子线程和主线程是如何切换的,我们继续看在主线程中Looper是如何处理发送过来的消息的.
// Looper.loop();中代码
Message msg = queue.next(); // might block
// MessageQueue中next();代码
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;
}
}
瞧,我们发现了什么,又是一个死循环阻塞,原来主线程不是阻塞在Looper中,而是阻塞在MessageQueue对象中.
可以看到当消息队列有消息的时候立马返回消息,没有消息就阻塞(我就偷了个懒,不在继续深入MessageQueue的原理了...)
Looper收到返回的消息会调用msg.target.dispatchMessage(msg)去处理,也就是调用Handler去处理.
我们继续来到Hanlder的处理方法中.
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
msg.callback我们先放一放,就我们普通发送的Message通常是没有callback对象的,下面的mCallback我们来看一看,该对象出现在构造函数之中,通过构造函数来赋值,我们来看看这个对象的定义
public interface Callback {
public boolean handleMessage(Message msg);
}
可以看出就是一个接口,和自己重写handleMessage一样,只不过提供另一种方式来处理.(说实话,在成员对象创建上面还重写其方法,我感觉是看着挺难受的,这样抽取成对象,然后通过构造函数传递进去,看着舒服多了.)
回到正题中来,我们还剩下一个Messaged的callBack对象没整明白.
首先,我们来看一看这究竟是一个什么对象.
/*package*/ Runnable callback;
可以看到就是一个任务而已,而handleCallback(msg)方法点进去再来看一看
private static void handleCallback(Message message) {
message.callback.run();
}
只是单纯的是运行这个任务而已,但是这个任务是哪里来的呢?
答案在Handler,Handler不仅可以发消息给自己在主线程处理,也可以直接发送一个任务去主线程运行.
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
到这里也就看明白了,剩下就和普通消息一样的处理方式了.
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