Handler想必在大家写Android代码过程中已经运用得炉火纯青,特别是在做阻塞操作线程到UI线程的更新上.Handler用得恰当,能防止很多多线程异常.
而Looper大家也肯定有接触过,只不过写应用的代码一般不会直接用到Looper.但实际Handler处理Message的关键之处全都在于Looper.
以下是我看了<深入理解Android>的有关章节后,写的总结.
Handler
先来看看Handler的构造函数.
public Handler() {
this(null, false);
}
public Handler(Looper looper) {
this(looper, null, false);
}
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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
主要关注Handler的2个成员变量mQueue,mLooper
mLooper可以从构造函数传入.如果构造函数不传的话,则直接取当前线程的Looper:mLooper = Looper.myLooper();
mQueue就是mLooper.mQueue.
把Message插入消息队列
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
上面两个正是把Message插入消息队列的方法.
从中能看出,Message是被插入到mQueue里面,实际是mLooper.mQueue.
每个Message.target = this,也就是target被设置成了当前的Handler实例.
到此,我们有必要看看Looper是做一些什么的了.
Looper
这是Looper一个标准的使用例子.
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
......
Looper.loop();
}
}
我们再看看Looper.prepare()和Looper.loop()的实现.
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));
}
public static Looper myLooper() {
return sThreadLocal.get();
}
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();
}
}
prepare()方法给sThreadLocal设置了一个Looper实例.
sThreadLocal是Thread Local Variables,线程本地变量.
每次调用myLooper()方法就能返回prepare()设置的Looper实例.
Looper()方法里面有一个很显眼的无限For循环,它就是用来不断的处理messageQueue中的Message的.
最终会调用message.target.dispatchMessage(msg)方法.前面介绍过,target是handler的实例.下面看看handler.dispatchMessage()方法的实现.
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
实现非常简单,如果callback不为空则用handleCallback(msg)来处理message.
而大多数情况下,我们实例化Handler的时候都没有传callback,所以都会走到handler.handleMessage()方法了.这方法用过Handler的人,都在再熟悉不过了.
这就是Handler和Looper协同工作的原理.消息队列的实现都在Looper,Handler更像是一个辅助类.
HandlerThread
多数情况下,我们都是用Handler来处理UI界面的更新,这时我们要保证handler的Looper是UI线程的Looper.
只需要这样子实例化Handler就能保证在UI线程处理Message了:Handler handler = new Handler(Looper.getMainLooper());
而当我们不希望Handler在UI线程去处理Message时候,就需要新建一个线程然后把线程的Looper传给Handler做实例化.
也许我们会写出下面类似的代码(样例代码引用<深入理解Android>)
class LooperThread extends Thread {
public Looper myLooper = null;
// 定义一个public 的成员myLooper，初值为空。
public void run() {
// 假设run 在线程2 中执行
Looper.prepare();
// myLooper 必须在这个线程中赋值
myLooper = Looper.myLooper();
Looper.loop();
}
}
// 下面这段代码在线程1 中执行，并且会创建线程2
{
LooperThread lpThread= new LooperThread;
lpThread.start();//start 后会创建线程2
Looper looper = lpThread.myLooper;//<====== 注意
// thread2Handler 和线程2 的Looper 挂上钩
Handler thread2Handler = new Handler(looper);
//sendMessage 发送的消息将由线程2 处理
threadHandler.sendMessage(...)
}
细心的你们可能已经一眼看穿,new Handler(looper);传进来的looper可能为空.
原因是Looper looper = lpThread.myLooper时候,lpThread.myLooper可能为空,因为lpThread还没有开始执行run()方法.
那要怎么样才能保证handler实例化时候,looper不为空呢.
Android给我们提供了完美的解决方案,那就是HandlerThread.
public class HandlerThread extends Thread{
// 线程1 调用getLooper 来获得新线程的Looper
public Looper getLooper() {
......
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();// 如果新线程还未创建Looper，则等待
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
// 线程2 运行它的run 函数，looper 就是在run 线程里创建的。
public void run() {
mTid = Process.myTid();
Looper.prepare(); // 创建这个线程上的Looper
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();// 通知取Looper 的线程1，此时Looper 已经创建好了。
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
}
HandlerThread.getLooper()方法会等待mLooper被赋值了才返回.
在handler实例化调用handlerThread.getLooper()方法的时候,就能保证得到的Looper一定不为空了.
HandlerThread handlerThread = new HandlerThread(); handlerThread.start(); Handler handler = new Handler(handlerThread.getLooper());