Android源码分析之HandlerThread

　　HandlerThread是一种特殊的Thread，也就是有Looper的thread，既然有looper的话，那我们就可以用此looper来

创建一个Handler，从而实现和它的交互，比如你可以通过与它关联的Handler对象在UI线程中发消息给它处理。HandlerThread

一般可以用来执行某些background的操作，比如读写文件（在此HandlerThread而非UI线程中）。既然还是一个Thread，那么

和一般的Thread一样，也要通过调用其start()方法来启动它。它只是Android替我们封装的一个Helper类，其源码相当简洁，我们

下面来看看，很简单。

　　和以往一样，我们先来看看字段和ctor：

    int mPriority; // 线程优先级
    int mTid = -1; // 线程id
    Looper mLooper; // 与线程关联的Looper

    public HandlerThread(String name) { // 提供个名字，方便debug
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT; // 没提供，则使用默认优先级
    }
    
    /**
     * Constructs a HandlerThread.
     * @param name
     * @param priority The priority to run the thread at. The value supplied must be from 
     * {@link android.os.Process} and not from java.lang.Thread.
     */
    public HandlerThread(String name, int priority) {
        super(name);
        mPriority = priority; // 使用用户提供的优先级，基于linux优先级，取值在[-20,19]之间
    }

代码很简单，相关的分析都直接写在代码的注释里了，值得注意的是这里的priority是基于linux的优先级的，而不是Java Thread

类里的MIN_PRIORITY，NORM_PRIORITY，MAX_PRIORITY之类，请注意区分（其实认真阅读方法的doc即可）。

　　接下来看看此类的关键3个方法：

   /**
     * Call back method that can be explicitly overridden if needed to execute some
     * setup before Looper loops.
     */
    protected void onLooperPrepared() { // callback方法，如果你愿意可以Override放自己的逻辑；其在loop开始前执行
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare(); // 此方法我们前面介绍过，会创建与线程关联的Looper对象
        synchronized (this) { // 进入同步块，当mLooper变的可用的使用，调用notifyAll通知其他可能block在当前对象上的线程
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority); // 设置线程优先级
        onLooperPrepared(); // 调用回调函数
        Looper.loop(); // 开始loop
        mTid = -1; // reset为invalid值
    }
    
    /**
     * This method returns the Looper associated with this thread. If this thread not been started
     * or for any reason is isAlive() returns false, this method will return null. If this thread 
     * has been started, this method will block until the looper has been initialized.  
     * @return The looper.
     */
    public Looper getLooper() {
        if (!isAlive()) { // 如果线程不是在alive状态则直接返回null，有可能是你忘记调start方法了。。。
            return null;
        }
        
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) { // 进入同步块，当条件不满足时无限等待，
                try {                              // 直到mLooper被设置成有效值了才退出while（当然也可能是线程状态不满足）；
                    wait();                        // run方法里的notifyAll就是用来唤醒这里的
                } catch (InterruptedException e) { // 忽略InterruptedException
                }
            }
        }
        return mLooper; // 最后返回mLooper，此时可以保证是有效值了。
    }

当你new一个HandlerThread的对象时记得调用其start()方法，然后你可以接着调用其getLooper()方法来new一个Handler对象，

最后你就可以利用此Handler对象来往HandlerThread发送消息来让它为你干活了。

　　最后来看2个退出HandlerThread的方法，其实对应的是Looper的2个退出方法：

    /**
     * Quits the handler thread's looper.
     * <p>
     * Causes the handler thread's looper to terminate without processing any
     * more messages in the message queue.
     * </p><p>
     * Any attempt to post messages to the queue after the looper is asked to quit will fail.
     * For example, the {@link Handler#sendMessage(Message)} method will return false.
     * </p><p class="note">
     * Using this method may be unsafe because some messages may not be delivered
     * before the looper terminates.  Consider using {@link #quitSafely} instead to ensure
     * that all pending work is completed in an orderly manner.
     * </p>
     *
     * @return True if the looper looper has been asked to quit or false if the
     * thread had not yet started running.
     *
     * @see #quitSafely
     */
    public boolean quit() {
        Looper looper = getLooper(); // 注意这里是调用getLooper而不是直接使用mLooper，
        if (looper != null) {        // 因为mLooper可能还没初始化完成，而调用方法可以
            looper.quit();           // 等待初始化完成。
            return true;
        }
        return false;
    }

    /**
     * Quits the handler thread's looper safely.
     * <p>
     * Causes the handler thread's looper to terminate as soon as all remaining messages
     * in the message queue that are already due to be delivered have been handled.
     * Pending delayed messages with due times in the future will not be delivered.
     * </p><p>
     * Any attempt to post messages to the queue after the looper is asked to quit will fail.
     * For example, the {@link Handler#sendMessage(Message)} method will return false.
     * </p><p>
     * If the thread has not been started or has finished (that is if
     * {@link #getLooper} returns null), then false is returned.
     * Otherwise the looper is asked to quit and true is returned.
     * </p>
     *
     * @return True if the looper looper has been asked to quit or false if the
     * thread had not yet started running.
     */
    public boolean quitSafely() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quitSafely();
            return true;
        }
        return false;
    }

通过代码我们可以看到其内部都是delegate给了Looper对象，而Looper我们在前面也介绍过了，感兴趣的同学可以翻看前面的分析或者

查看这2个方法的doc，写的都很详细。

　　至此这个简单的Handy class就算分析完毕了。在实际的开发中，如果你只是要做某些后台的操作（短暂的，比如把某些设置文件load

到内存中），而不需要更新UI的话，那你可以优先使用HandlerThread而不是AsyncTask。

　　接下来准备分析下Android提供的数据存储机制SharedPreferences，敬请期待。。。