Android HandlerThread使用介绍以及源码解析

1. 前言

首先，看一下官方对HandlerThread的解释：

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.

翻译中文为:HandlerThread 是一个包含Looper的Thread，这个 Looper 可以被用来创建 Handler，start方法必须被调用。

我们先来看看HandlerThread的特点：

HandlerThread继承Thread，本质还是线程类；
HandlerThread内部创建looper对象，这个Looper对象可以被用来创建Handler，通过looper循环，可以在handleMessage方法中执行耗时任务。
HandlerThread必须先调用HandlerThread.start()方法。

2.使用方法

1）创建HandlerThread对象

handlerThread = new HandlerThread("refreash");

2）启动HandlerThread，创建Looper

handlerThread.start();

HandlerThread必须先调用HandlerThread.start()方法，随后Thread会调用run方法，创建Looper对象。

3）HandlerThread内部创建Looper，用来新建handler对象，发送消息

handler = new MyHandler(handlerThread.getLooper());
handler.sendMessage(handler.obtainMessage());

4）handleMessage处理耗时任务

private class MyHandler extends Handler {
        public MyHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            handleIntent();
            sendMessage(handler.obtainMessage());
        }
    }

3.具体使用代码

package com.test.cyf;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Looper;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

    private TextView amount;

    /**
     * 子线程
     */
    private MyHandler handler;

    private int size = 0;

    private HandlerThread handlerThread;

    private class MyHandler extends Handler {
        public MyHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            handleIntent();
            sendMessage(handler.obtainMessage());
        }
    }

    /**
     * 主线程刷新UI
     */
    private Handler uiHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            amount.setText(size + "");
            super.handleMessage(msg);
        }
    };


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        initView();
        loadData();
    }

    /**
     * 初始化view
     */
    private void initView() {
        amount = (TextView) findViewById(R.id.amount);
    }

    /**
     * 初始化数据
     */
    private void loadData() {
        handlerThread = new HandlerThread("refreash");
        handlerThread.start();

        handler = new MyHandler(handlerThread.getLooper());
        handler.sendMessage(handler.obtainMessage());
    }

    /**
     * 子线程处理耗时操作
     */
    private void handleIntent() {
        try {
            //模拟耗时
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        size++;
        Message msg = uiHandler.obtainMessage();
        uiHandler.sendMessage(msg);
    }

    @Override
    protected void onDestroy() {
        handlerThread.quit();
        super.onDestroy();
    }
}

该程序模拟实现如下功能：子线程执行一秒的耗时操作后，数值加1并且刷新界面，如下图所示：

很明显，你也可以自己用Thread+Handler实现，只不过HandlerThread去做了封装，IntentService中使用HandlerThread是比较经典的场景，后面会讲解IntentService。

4.源码讲解

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

/**
 * 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 {
    int mPriority;
    int mTid = -1;
    Looper mLooper;

    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;
    }
    
    /**
     * Call back method that can be explicitly overridden if needed to execute some
     * setup before Looper loops.
     */
    protected void onLooperPrepared() {
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }
    
    /**
     * 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()) {
            return null;
        }
        
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

    /**
     * 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();
        if (looper != null) {
            looper.quit();
            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;
    }

    /**
     * Returns the identifier of this thread. See Process.myTid().
     */
    public int getThreadId() {
        return mTid;
    }
}

1）构造方法HandlerThread（String name）；HandlerThread（String name，int priority）传入线程名和线程等级

2）当调用handlerThread.start（）方法时，会调用HandlerThread的run（）方法，该方法会创建looper对象，设置线程等级，循环looper。

3）当创建handler对象的时候，我们会用HandlerThread的getLooper方法，该方法会返回Looper对象，但是有个wait（），什么时候唤醒呢，在HandlerThread的run（）方法中会看到notifyAll（）方法，就是等到looper创建完成，才能通过getLooper获取到，这样做是为了解决这两个线程的同步问题。

4）上面的例子代码，在ondestory方法中，调用了handlerThread的quit（）方法，quit方法中调用了looper的quit方法，把messageQueue中的message全部移除。如果我们不调用quit（）方法，子线程会一直处于等待状态，所有需要手动调用quit方法。

如有错误欢迎指出来，一起学习。

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