android异步消息机制，Handler,Looper,MessageQueue的关系

来源：互联网发布：彩影软件怎么样编辑：程序博客网时间：2024/05/17 03:41

前言：

参考了鸿洋_大神的这篇文章：http://blog.csdn.net/lmj623565791/article/details/38377229

（1）在android中线程间的消息通信是通过handler实现的。handler的常见应用：因为在android中对UI操作只能在主线程中操作，而一些耗时的操作又不能在主线程进行，所以会在子线程做一些网络请求等的耗时操作，然后通过handler把子线程的任务切换到主线程中，在主线程中进行UI操作。如下代码：

 private Handler handler = new Handler(){        @Override        public void handleMessage(Message msg) {            super.handleMessage(msg);        }    };    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        new Thread(new Runnable() {            @Override            public void run() {                Message message = Message.obtain();                message.what = 1;                handler.sendMessage(message);            }        }).start();    }

（2）handler的运行其实需要looper和messageQueue支持，只是当新建Activity已经帮我们新建了looper和messageQueue。如果在子线程使用handler，必须调用Looper.prepare和Looper.loop。如下代码：

    class MyThread implements Runnable{        private Handler handler;        @Override        public void run() {            Looper.prepare();            handler = new Handler(){                @Override                public void handleMessage(Message msg) {                    super.handleMessage(msg);                }            };            Looper.loop();        }    }

（3）handler的应用流程。首先通过Looper.prepare(),将Looper绑定到当前线程，创建Looper实例时会创建一个MessageQueue。当新建一个Handler实例时会获取到当前的线程的Looper和MessageQUeue。当通过handler.sendMessage(message)方法时会把Message插入到MessageQueue中。最后通过Looper.loop()不停遍历MessageQueue，得到Message，并交给msg.target处理，target就是当前handler（handle赋值给msg.target是在handler.sendMessage(message)中实现的）。target会调用msg.target.dispatchMessage(msg)处理，dispatchMessage最后会调用 handleMessage(msg)处理message。handleMessage也是我们需要重写的方法。下面将下各方法的实现原理。

1.Looper.prepare()

Looper.prepare()的作用是创建一个Looper并绑定到当前线程，同时创建一个MessageQueue。Looper绑定到当前线程通过ThreadLocal实现的。

ThreadLocal的作用：为不同线程互不干扰的存储和提供数据（具体原理可以看下singwhatiwanna大神的这篇文章点击打开链接）。

在代码的78行sThreadLocal.set(new Looper(quitAllowed))，新建了一个Looper实例存储到了ThreadLocal中，并在这之前判定了ThreadLocal之前是否存储了Looper,不为null则抛出异常，这样保证了一个线程只有一个Looper。

51 52 public final class More ...Looper {53     private static final String TAG = "Looper";54 55     // sThreadLocal.get() will return null unless you've called prepare().56     static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();57     private static Looper sMainLooper;  // guarded by Looper.class58 59     final MessageQueue mQueue;60     final Thread mThread;61 62     private Printer mLogging;

</pre><pre name="code" class="html"></pre><pre name="code" class="html">70     public static void More ...prepare() {71         prepare(true);72     }73

74     private static void More ...prepare(boolean quitAllowed) {75         if (sThreadLocal.get() != null) {76             throw new RuntimeException("Only one Looper may be created per thread");77         }78         sThreadLocal.set(new Looper(quitAllowed));79     }

下面看下Looper的构造方法。在新建Looper会创建一个MessageQueue。一个就实现了一个线程只有一个Looper和MessageQueue。

private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mRun = true;        mThread = Thread.currentThread();}

2.新建一个Handler对象。

Handler的构造方法的第198行mLooper = Looper.myLooper()获取了当前线程的Looper。再看下Looper.myLooper()的实现,其实是获取了LocalThread存储的Looper， return sThreadLocal.get()第161行，就是在代码78行保存的Looper。
mQueue = mLooper.mQueue（203行）获取了MessageQueu。这样通过新建一个Handler对象，通过ThreadLocal使Handler和Looper，MessageQueue联系到一起。

187188    public More ...Handler(Callback callback, boolean async) {189        if (FIND_POTENTIAL_LEAKS) {190            final Class<? extends Handler> klass = getClass();191            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&192                    (klass.getModifiers() & Modifier.STATIC) == 0) {193                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +194                    klass.getCanonicalName());195            }196        }197198        mLooper = Looper.myLooper();199        if (mLooper == null) {200            throw new RuntimeException(201                "Can't create handler inside thread that has not called Looper.prepare()");202        }203        mQueue = mLooper.mQueue;204        mCallback = callback;205        mAsynchronous = async;206    }

myLooper方法，通过sThreadLocal.get()获取了保存在ThreadLocal当前线程的Looper.

160    public static Looper More ...myLooper() {161        return sThreadLocal.get();162    }

3.handler.sendMessage(message)

先看下代码。不管是sendMessage还是sendEmptyMessage最后都是调用enqueueMessage（626行开始）.我们看下enqueueMessage方法实现。queue.enqueueMessage(msg, uptimeMillis)（631行）把message加入到了MessageQueue中。msg.target = this（627行），把当前Handler赋值给了msg.taget。当Looper遍历MessageQueue，取出Message会交给msg.taget处理，也就是交给handler处理了。

505 public final boolean More ...sendMessage(Message msg)506    {507        return sendMessageDelayed(msg, 0);508    }

516517    public final boolean More ...sendEmptyMessage(int what)518    {519        return sendEmptyMessageDelayed(what, 0);520    }

531    public final boolean More ...sendEmptyMessageDelayed(int what, long delayMillis) {532        Message msg = Message.obtain();533        msg.what = what;534        return sendMessageDelayed(msg, delayMillis);535    }

592    public boolean More ...sendMessageAtTime(Message msg, long uptimeMillis) {593        MessageQueue queue = mQueue;594        if (queue == null) {595            RuntimeException e = new RuntimeException(596                    this + " sendMessageAtTime() called with no mQueue");597            Log.w("Looper", e.getMessage(), e);598            return false;599        }600        return enqueueMessage(queue, msg, uptimeMillis);601    }

626    private boolean More ...enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {627        msg.target = this;628        if (mAsynchronous) {629            msg.setAsynchronous(true);630        }631        return queue.enqueueMessage(msg, uptimeMillis);632    }

4.Looper.loop().

当MessageQueue有消息时，会遍历MessageQueue，获取到Message,然后交给Handler处理。MessageQueue没有消息时则处于阻塞状态。

  public static void More ...loop() {110        final Looper me = myLooper();111        if (me == null) {112            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");113        }114        final MessageQueue queue = me.mQueue;115116        // Make sure the identity of this thread is that of the local process,117        // and keep track of what that identity token actually is.118        Binder.clearCallingIdentity();119        final long ident = Binder.clearCallingIdentity();120121        for (;;) {122            Message msg = queue.next(); // might block123            if (msg == null) {124                // No message indicates that the message queue is quitting.125                return;126            }127128            // This must be in a local variable, in case a UI event sets the logger129            Printer logging = me.mLogging;130            if (logging != null) {131                logging.println(">>>>> Dispatching to " + msg.target + " " +132                        msg.callback + ": " + msg.what);133            }134135            msg.target.dispatchMessage(msg);136137            if (logging != null) {138                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);139            }140141            // Make sure that during the course of dispatching the142            // identity of the thread wasn't corrupted.143            final long newIdent = Binder.clearCallingIdentity();144            if (ident != newIdent) {145                Log.wtf(TAG, "Thread identity changed from 0x"146                        + Long.toHexString(ident) + " to 0x"147                        + Long.toHexString(newIdent) + " while dispatching to "148                        + msg.target.getClass().getName() + " "149                        + msg.callback + " what=" + msg.what);150            }151152            msg.recycleUnchecked();153        }154    }

在代码110到114行获取了Looper和MessageQueue，如果Looper为空则会抛出异常。

代码121到153无限便利MessageQueue.

代码122行（Message msg = queue.next();）获取到了Message，如果没有则处于阻塞状态。

代码135行（msg.target.dispatchMessage(msg);）msg交给了msg.target处理，target就是在新建Handler实例时赋值给它的当前Handler。这里把msg交给了handler.dispatchMessage(msg)处理，我们看下dispatchMessage的代码。

</pre><pre name="code" class="html">93     public void More ...dispatchMessage(Message msg) {94         if (msg.callback != null) {95             handleCallback(msg);96         } else {97             if (mCallback != null) {98                 if (mCallback.handleMessage(msg)) {99                     return;100                }101            }102            handleMessage(msg);103        }104    }

在dispatchMessage方法中如果msg.callback和mCallback为null则交给102行（handleMessage(msg);)处理，handlerMessage就是我们需要重写，去处理返回的消息。
如果msg.callback不为null，看下95行handleCallback(msg)，该方法实现739行（message.callback.run()），其实callback就是一个Runnable。我们在handler.post(runnable)设置了callback。

     private static void More ...handleCallback(Message message) {739        message.callback.run();740    }741

我们看下post方法。其实在post方法中也是调用了sendMessageDelayed（326行），这个和sendMessage，sendEmptyMessage最后调用的方法是一样的。sendMessageDelayed方法的第一个参数类型是Message.我们看下getPostMessage方法。它的实现其实就是创建了一个空的Message，给Message.callback赋值了Runnable，727行(m.callback = r),而 m.callback就是在dispatchMessage方法中最后消息交给其处理（95行）。所以handler.post(runnable)实现了消息的发送(sendMessage)，消息传递过来后的处理(handleMessage(msg);)。发送Message时，只是携带了Runnable的一个空的Message,最后在message.callback.run()（739行）处理消息。所以我们要重写run方法处理消息。

324    public final boolean More ...post(Runnable r)325    {326       return  sendMessageDelayed(getPostMessage(r), 0);327    }

725    private static Message More ...getPostMessage(Runnable r) {726        Message m = Message.obtain();727        m.callback = r;728        return m;729    }730

1 0