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[操作系统]Android主线程的消息系统(Handler\Looper)


 前言:

之前的文章写的都是关于Bitmap和内存的优化技术,这一篇文章给大家谈谈Handler。

Handler是Android系统中比较重要的一个知识,在Android多线程面试经常会被问到,在实际项目中的确也经常用到。当然也比较复杂,知识比较多,牵扯到的类有Thread、Looper、Message、MessageQueue。

Android是支持多线程的,通常应用程序中与用户相关的UI事件都是运行在主线程中,比如点击屏幕、按钮等,为了保持主线程顺畅相应用户事件不被阻塞就需要把耗时的操作(主要是联网、操作大文件等)放到子线程中,这个时候你可能会想到Handler(当然还你可以用其他的比如:异步任务,,这个以后再讲),但是Handler又是怎么和Thread联系起来的呢?这个咱们来看一下Android主线程是怎么创建的。

ActivityThread:

在ActivityThread.java中有一个main()函数,这个函数就是在一个应用启动的入口,调用关系是:ActivityManagerService.java中的startProcessLocked函数调用如下代码:

// Start the process. It will either succeed and return a result containing// the PID of the new process, or else throw a RuntimeException.Process.ProcessStartResult startResult = Process.start("android.app.ActivityThread",          app.processName, uid, uid, gids, debugFlags,          app.info.targetSdkVersion, null);

Process.start又做了如下的操作,只看方法注释就行,现在不需要知道具体做了什么:

/**   * Start a new process.   *   * <p>If processes are enabled, a new process is created and the   * static main() function of a <var>processClass</var> is executed there.   * The process will continue running after this function returns.   *   * <p>If processes are not enabled, a new thread in the caller's   * process is created and main() of <var>processClass</var> called there.   *   * <p>The niceName parameter, if not an empty string, is a custom name to   * give to the process instead of using processClass. This allows you to   * make easily identifyable processes even if you are using the same base   * <var>processClass</var> to start them.   *   * @param processClass The class to use as the process's main entry   *           point.   * @param niceName A more readable name to use for the process.   * @param uid The user-id under which the process will run.   * @param gid The group-id under which the process will run.   * @param gids Additional group-ids associated with the process.   * @param debugFlags Additional flags.   * @param targetSdkVersion The target SDK version for the app.   * @param zygoteArgs Additional arguments to supply to the zygote process.   *   * @return An object that describes the result of the attempt to start the process.   * @throws RuntimeException on fatal start failure   *   * {@hide}   */  public static final ProcessStartResult start(final String processClass,                 final String niceName,                 int uid, int gid, int[] gids,                 int debugFlags, int targetSdkVersion,                 String[] zygoteArgs) {    try {      return startViaZygote(processClass, niceName, uid, gid, gids,          debugFlags, targetSdkVersion, zygoteArgs);    } catch (ZygoteStartFailedEx ex) {      Log.e(LOG_TAG,          "Starting VM process through Zygote failed");      throw new RuntimeException(          "Starting VM process through Zygote failed", ex);    }  }

通过注释也能看到上面的函数会找到ActivityThread的main函数并且执行。main函数中创建了Looper,Looper的作用就是利用线程创建一个消息处理队列,并且维护这个消息队列:

 public static void main(String[] args) {    Looper.prepareMainLooper();//创建Looper    if (sMainThreadHandler == null) {      sMainThreadHandler = new Handler();    }    ActivityThread thread = new ActivityThread();    thread.attach(false);//应用所有的逻辑都在这个方法中    Looper.loop();//开启一个消息循环,不断的读取MessageQueue中的Message。  }

Looper:

Looper.prepareMainLooper()的代码如下:

/**   * Initialize the current thread as a looper, marking it as an   * application's main looper. The main looper for your application   * is created by the Android environment, so you should never need   * to call this function yourself. See also: {@link #prepare()}   */  public static void prepareMainLooper() {    prepare();    setMainLooper(myLooper());    myLooper().mQueue.mQuitAllowed = false;  }

上面的方法注释已经说的很明白,创建了主线程的Looper,这段代码是系统调用的。先看prepare方法做了什么操作。

/** Initialize the current thread as a looper.   * This gives you a chance to create handlers that then reference   * this looper, before actually starting the loop. Be sure to call   * {@link #loop()} after calling this method, and end it by calling   * {@link #quit()}.   */  public static void prepare() {    if (sThreadLocal.get() != null) {      throw new RuntimeException("Only one Looper may be created per thread");    }    sThreadLocal.set(new Looper());  }
  private Looper() {    mQueue = new MessageQueue();    mRun = true;    mThread = Thread.currentThread();//获取当前线程  }

创建主线程的Looper,每一个Looper对应一个Thread、一个MessageQueue,创建Looper的时候会创建一个MessageQueue。到目前位置创建了应用的主线程(Thread)、Looper、MessageQueue,调用Looper.loop(),开始不断的从MessageQueue中读取Message并处理,如果没有消息则等待。现在有了消息循环,有了管理消息循环的Looper就差发送消息和处理消息的Handler了。

Handler:

这个时候你在你的应用中创建一个Handler,一般都是下面的代码:

 private static final Handler mHandler = new Handler() {    public void handleMessage(Message msg) {      ..........      }    }  };

这个Handler是在主线程中创建的,Handler的构造函数如下:

 /**   * Default constructor associates this handler with the queue for the   * current thread.   *   * If there isn't one, this handler won't be able to receive messages.   */  public Handler() {    mLooper = Looper.myLooper();//获取上面在主线程创建的Looper    if (mLooper == null) {      throw new RuntimeException(        "Can't create handler inside thread that has not called Looper.prepare()");    }    mQueue = mLooper.mQueue;//获取Looper的MessageQueue    mCallback = null;//默认为null在后面处理msg时会就行检查  }

创建完Handler你就可以用了,比如你发一个消息:

mHandler.sendEmptyMessage(MSG_WHAT);

在系统中会走最终走到Handler.java下面的方法:

/**   * Enqueue a message into the message queue after all pending messages   * before the absolute time (in milliseconds) <var>uptimeMillis</var>.   * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>   * You will receive it in {@link #handleMessage}, in the thread attached   * to this handler.   *   * @param uptimeMillis The absolute time at which the message should be   *     delivered, using the   *     {@link android.os.SystemClock#uptimeMillis} time-base.   *       * @return Returns true if the message was successfully placed in to the   *     message queue. Returns false on failure, usually because the   *     looper processing the message queue is exiting. Note that a   *     result of true does not mean the message will be processed -- if   *     the looper is quit before the delivery time of the message   *     occurs then the message will be dropped.   */  public boolean sendMessageAtTime(Message msg, long uptimeMillis)  {    boolean sent = false;    MessageQueue queue = mQueue;    if (queue != null) {      msg.target = this;//注意这行代码后面会用,把Handler赋值给Msg的target对象      sent = queue.enqueueMessage(msg, uptimeMillis);//把msg放到MsgQueue中    }    else {      RuntimeException e = new RuntimeException(        this + " sendMessageAtTime() called with no mQueue");      Log.w("Looper", e.getMessage(), e);    }    return sent;  }

上面的方法第二个是延时毫秒数,queue.enqueueMessage把消息发送到MessageQueue后剩下的就是等待消息被处理,前面不是说了Looper.loop()方法开始轮询消息队列吗,你发送的消息就是在loop方法中读取到的,读取到后谁去处理呢?在loop()方法中有一句代码:

msg.target.dispatchMessage(msg);

msg就是你发送到MessageQueue的消息,被读取后调用target.dispatchMessage(),这个target就是上面Handler发送消息是赋值的,就是发送消息的Handler本身,然后Handler调用自己的下面方法就行消息处理:

 /**   * Handle system messages here.   */  public void dispatchMessage(Message msg) {    if (msg.callback != null) {      handleCallback(msg);    } else {      if (mCallback != null) {        if (mCallback.handleMessage(msg)) {          return;        }      }      handleMessage(msg);//在这会调用到上面重写的handleMessage方法。    }  }

因为在new Message的时候callback为空,并且Handler的mCallback = null,所以会调用到你上面new Handler时重写的handleMessage方法。

总结:

每一个线程中都对应一个Looper,每一个Looper都对应一个MessageQueue,这个Looper是用来管理消息队列的,主要是读取消息队列和把消息发送给Message的target去处理。到这你应该清除Thread、Handler、Message、MessageQueue和Looper他们之间的关系了吧。

 

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