本文我们讲解的是HandlerThread，可能有部分同学不了解或者没用过这个类。原因很简单，在如今大神们的框架面前，许多的原生类已经不为人所用了，然而早期开发者要想在子线程里更新UI则需要在Thread里创建一个Handler。

class MyThread : Thread() {
    private lateinit var mHandler: Handler

    override fun run() {
        super.run()
        Looper.prepare()

        mHandler = object : Handler() {
            override fun handleMessage(msg: Message?) {
                super.handleMessage(msg)
                if(msg?.what==0){
                    //...处理消息
                }
            }
        }

        Looper.loop()
    }
}

是不是认为很难受？每次要想在子线程里面处理信息必须每次在子线程里创建Handler?

然而贴心的Google工程师为我们提供了一个自带Handler的类，名叫
HandlerThread。

类注释

要想学习一个类的构成，首先要从它的类注释看起。

**
 * Handy class for starting a new thread that has a looper. The looper can then be 
 * used to create handler classes. Note that start() must still be called.
 */
public class HandlerThread extends Thread {
    ...
}

• 由它的注释可以看出，这是一个带有Looper的Thread，这个Looper可以被用于创建Handler，请注意，start()这个方法仍然需要被调用。
• 总结一下，这个类就是自带Handler的线程类。
  
  

构造方法

    int mPriority;//优先级
    int mTid = -1;
    Looper mLooper;//自带的Looper
    private @Nullable Handler mHandler;

    public HandlerThread(String name) {
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT;
    }
    
    /**
     * 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;
    }

• 这里有两个构造方法，一个HandlerThread(String name)，一个HandlerThread(String name, int priority)，我们可以自己设定线程的名字以及优先级。注意！是Process里的优先级而不是Thread的。

    //这是可选的优先级
    public static final int THREAD_PRIORITY_DEFAULT = 0;
    public static final int THREAD_PRIORITY_LOWEST = 19;
    public static final int THREAD_PRIORITY_BACKGROUND = 10;
    public static final int THREAD_PRIORITY_FOREGROUND = -2;
    public static final int THREAD_PRIORITY_DISPLAY = -4;
    public static final int THREAD_PRIORITY_URGENT_DISPLAY = -8;
    public static final int THREAD_PRIORITY_VIDEO = -10;
    public static final int THREAD_PRIORITY_AUDIO = -16;
    public static final int THREAD_PRIORITY_URGENT_AUDIO = -19;
    public static final int THREAD_PRIORITY_MORE_FAVORABLE = -1;
    public static final int THREAD_PRIORITY_LESS_FAVORABLE = +1;

其他方法

    
    /*
        在Looper.loop()之前调用的方法，如需要配置可重写
    */
    protected void onLooperPrepared() {
    }


    /*
        来自Thread的run方法，调用与start()之后    
    */
    @Override
    public void run() {
        mTid = Process.myTid();//线程id
        Looper.prepare();//创建子线程的Looper
        synchronized (this) {
            mLooper = Looper.myLooper();//获取Looper实例
            notifyAll();// native方法，用于唤醒所有等待获取Looper的线程
        }
        Process.setThreadPriority(mPriority);//设置线程优先级
        onLooperPrepared();//上面方法的调用
        Looper.loop();//Looper开始轮询
        mTid = -1;
    }
    
    /*
        获取Looper
    */
    public Looper getLooper() {
        //判断线程是否启动
        if (!isAlive()) {
            return null;
        }
        
        synchronized (this) {
            while (isAlive() && mLooper == null) {//如果线程启动，且Looper为空
                try {
                    wait();//使其等待直至获取到Looper
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

    /*
        获取Handler
    */
    @NonNull
    public Handler getThreadHandler() {
        if (mHandler == null) {
            mHandler = new Handler(getLooper());
        }
        return mHandler;
    }

    
    public boolean quit() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quit();
            return true;
        }
        return false;
    }

    public boolean quitSafely() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quitSafely();
            return true;
        }
        return false;
    }

    
    public int getThreadId() {
        return mTid;
    }

• 注释我已写在代码上，在HandlerThread不使用时记得使用 getLooper().quit()来退出这个线程，但需要注意的是quit和quitSafely的区别，接下来我将用较短的篇幅介绍一下这两个方法的区别。

quit()和quitSafely()的区别

HandlerThread：

public boolean quit() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quit();
            return true;
        }
        return false;
    }

    public boolean quitSafely() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quitSafely();
            return true;
        }
        return false;
    }

Looper在这两个方法中调用了quit()和quitSafely()方法。我们点开源码查看，他们本质都是在调用MessageQueue中quit方法，不同的是quit方法的参数不同：

Looper:

quitSafely:

public void quit() {
        mQueue.quit(true);
    }

quit:

public void quit() {
        mQueue.quit(false);
    }

如果大家对消息机制有所了解，那么一定知道MessageQueue是用于处理Message(消息)的队列，如果不知道的可以去了解一下。话不多说，接下来我们将深入MessageQueue了解一下quit(boolean safe)方法是何方神圣。

MessageQueue:

void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuitting) {
                return;
            }
            mQuitting = true;

            if (safe) {//敲黑板！这就是quitSafely和quit的区别
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            nativeWake(mPtr);
        }
    }

• 当我们在Looper中调用quitSafely()时，在MessageQueue里调用的removeAllFutureMessagesLocked()方法
• 当我们在Looper中调用quit()时，在MessageQueue里调用的是removeAllMessagesLocked()方法
• 而这两个方法本质的区别就是在于remove消息的范围，下面是一段伪代码：

            if (safe) {
                只清空队列中的延时消息(通过postDelay发送的)，非延时消息继续派发，直到完成
            } else {
                移除所有消息，包括延时消息
            }

所以说safe的区别就是是否清除所有消息，这两个方法的实现我们就不深究了，毕竟本文是为了讲解HandlerThread而不是数据结构，若有兴趣的可以去自行研究。

应用

• IntentService的底层就是利用HandlerThread
• 举个栗子
  
  
  
  
  Activity:

class MainActivity : AppCompatActivity() ,Handler.Callback{


    private lateinit var mUIHandler :Handler
    private lateinit var mDownloadThread: DownloadThread

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        init()
    }

    private fun init(){
        mUIHandler = Handler(this)

        val list = mutableListOf<String>()
        list.add("url1")
        list.add("url2")
        list.add("url3")

        mDownloadThread = DownloadThread("Download")
                        .setUrls(list)
                        .setUIHandler(mUIHandler)

        mDownloadThread.start()
        Log.d("test","开始下载")
    }

    override fun handleMessage(msg: Message?): Boolean {
        when(msg?.what){
            DownloadThread.START->{
                Log.d("test","Activity接收到了START信息")
            }
            DownloadThread.FINISH->{
                Log.d("test","Activity接收到了FINISH信息")
            }
        }
        return true
    }
}

DownloadThread:

class DownloadThread(name: String?) : HandlerThread(name), Handler.Callback {
    private lateinit var mWorkHandler: Handler
    private lateinit var mUIHandler: Handler
    private lateinit var urls: List<String>

    companion object {
        const val START = 1
        const val FINISH = 2
        const val KEY = "getUrl"
    }

    fun setUIHandler(mUIHandler: Handler): DownloadThread {
        this.mUIHandler = mUIHandler
        return this
    }

    fun setUrls(urls: List<String>): DownloadThread {
        this.urls = urls
        return this
    }

    override fun onLooperPrepared() {
        super.onLooperPrepared()
        if (looper != null)
            mWorkHandler = Handler(looper, this)

        //在这里发送下载信息
        urls.forEach { url->
            val message = mWorkHandler.obtainMessage()
            val bundle = Bundle()
            bundle.putString(KEY,url)
            message.data = bundle
            mWorkHandler.sendMessage(message)
        }
    }


    //mWorkHandler的handleMessage
    override fun handleMessage(msg: Message?): Boolean {
        if (msg == null || msg.data == null) return false

        val url = msg.data.get(KEY) as String//获取url

        val startMessage: Message = mUIHandler.obtainMessage(START)
        mUIHandler.sendMessage(startMessage)
        Log.d("test","$url :Thread发送START信息")

        Thread.sleep(2000)
        Log.d("test","$url :Thread执行耗时操作中...")

        val finishMessage: Message = mUIHandler.obtainMessage(FINISH)
        mUIHandler.sendMessage(finishMessage)
        Log.d("test","$url :Thread发送FINISH信息")
        return true
    }
}

结果:

2019-01-30 20:47:47.743  D/test: 开始下载

2019-01-30 20:47:47.744  D/test: url1 :Thread发送START信息
2019-01-30 20:47:47.776  D/test: Activity接收到了START信息
2019-01-30 20:47:49.746  D/test: url1 :Thread执行耗时操作中...
2019-01-30 20:47:49.747  D/test: url1 :Thread发送FINISH信息
2019-01-30 20:47:49.747  D/test: Activity接收到了FINISH信息

2019-01-30 20:47:49.747  D/test: url2 :Thread发送START信息
2019-01-30 20:47:49.747  D/test: Activity接收到了START信息
2019-01-30 20:47:51.748  D/test: url2 :Thread执行耗时操作中...
2019-01-30 20:47:51.749  D/test: url2 :Thread发送FINISH信息
2019-01-30 20:47:51.750  D/test: Activity接收到了FINISH信息

2019-01-30 20:47:51.749  D/test: url3 :Thread发送START信息
2019-01-30 20:47:51.750  D/test: Activity接收到了START信息
2019-01-30 20:47:53.750  D/test: url3 :Thread执行耗时操作中...
2019-01-30 20:47:53.751  D/test: url3 :Thread发送FINISH信息
2019-01-30 20:47:53.751  D/test: Activity接收到了FINISH信息

总结

上面就是利用HandlerThread在子线程中执行串行任务，并反馈到主线程的栗子。总的来说HandlerThread的知识点就是这些，若有错误或者遗漏欢迎指出。