Handler

Handler

Android 异步消息处理机制 ,Handle机制其实也为我们提供了异步消息处理机制代码的参考。

由于Android系统规定主线程不能被阻塞,所以耗时操作必须放在子线程中进行。但是子线程中又不能访问UI。

Handler解决了在子线程中无法访问UI的矛盾。

使用

public void onClick(View v){
        new Thread(new Runnable() {
            @Override
            public void run() {
                //拿到Message对象
                Message msg = Message.obtain();
                msg.arg1 = 1;
                mHandler.sendMessage(msg);
            }
        }) .start();
    }
  private Handler mHandler =  new Handler(){
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        //更新ui
        TextView.setText("msg = " + msg.arg1);
    }
};

实现消息驱动有几个要素:

  • 消息的表示:Message
  • 消息队列:MessageQueue
  • 消息循环,用于循环取出消息进行处理:Looper
  • 消息处理,消息循环从消息队列中取出消息后要对消息进行处理:Handler

源码分析

使用Handler之前,我们都是通过new Handler()初始化一个实例,同时会获取Looper和messageQueue的实例。

 public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            //检测扩展此Handler类并且不是静态的匿名,本地或成员类。 这些类可能会产生泄漏。
            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());
            }
        }

        //默认将关联当前线程的looper
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        //直接把关联looper的MQ作为自己的MQ,因此它的消息将发送到关联looper的MQ上
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

Handler的send或者post类方法被调用时,最终会调用MessageQueue的enqueueMessage方法,将消息放入消息队列中。(post(Runnable r)中的Runnable对象会被封装成message对象)

    //发送消息
    public final boolean sendMessage(Message msg){
        return sendMessageDelayed(msg, 0);
    }
    
    //发送消息
    public final boolean sendMessageDelayed(Message msg, long delayMillis){
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
    
    //最终都是调用sendMessageAtTime()方法
    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;//meg.target赋值为当前handler
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        //调用enqueueMessage()将Message送到MessageQueue中去,该MessageQueue的实例是looper在初始化的时候创建的
        return queue.enqueueMessage(msg, uptimeMillis);
    }

当Looper发现有新消息来时,就会处理这个消息(Android 在进程的入口函数 ActivityThread.main()方法中,会调用 Looper.prepareMainLooper(), 为应用的主线程创建Looper,然后调用Looper.loop()就启动了进程的消息循环。所以我们在activity中创建的Handler默认是运行在ui线程中的,可以直接更新ui。我们也可以在自线程中去调用Looper.prepare()方法去创建该线程的Looper)

//
public static final void prepare() {  
        //一个线程中只有一个Looper实例
        if (sThreadLocal.get() != null) {  
            throw new RuntimeException("Only one Looper may be created per thread");  
        }  
        sThreadLocal.set(new Looper(true));  
}

//轮询处理调用handler.
public static void loop() {
        final Looper me = myLooper();//获取ThreadLocal中存储的Looper实例
        //需要先调用prepare()创建Looper实例
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;//获取该looper实例中的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();

        //无限循环
        for (;;) {
            Message msg = queue.next(); // might block
            //取出一条消息,如果没有消息则阻塞
            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);
            }

            //把消息交给msg.target的dispatchMessage方法去处理
            //msg.target 是handler中enqueueMessage()方法中msg.target = this赋值的handler实例
            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);
            }

            //释放资源
            msg.recycleUnchecked();
        }
    }

looper轮询处理消息调用dispatchMessage()方法

 // 处理消息,该方法由looper调用
 public void dispatchMessage(Message msg) {
        // 如果message设置了callback,即runnable消息,处理callback!
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            // 如果handler本身设置了callback,则执行callback
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            //调用handleMessage()方法,内部是空实现,交给用户复写,处理消息。
            handleMessage(msg);
        }
    }
    
    
     // 处理runnable消息
    private final void handleCallback(Message message) {
        message.callback.run();  //直接调用run方法
    }
    // 由用户复写
    public void handleMessage(Message msg) {
    }

Message
在整个消息处理机制中,message封装了任务携带的信息和处理该任务的handler。

  1. 尽管Message有public的默认构造方法,但是推荐通过Message.obtain()来从消息池中获得空消息对象,以节省资源。
  2. 如果你的message只储存int信息,优先使用Message.arg1和Message.arg2来传递信息,这比用Bundle更省内存
  3. 擅用message.what来标识信息,以便用不同方式处理message。

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