1. 原理
每一个线程对应一个消息队列MessageQueue，实现线程之间的通信，可通过Handler对象将数据装进Message中，再将消息加入消息队列，而后线程会依次处理消息队列中的消息。
2. Message
初始化：一般使用Message.obtain()方法获取一个消息对象，该方法会检查Message对象池中是否存在可重复利用的对象，若无，才会new一个新对象。
what：相当于Message的标识符，区别于其它消息。
arg1、arg2：int类型，可传递整数。
obj：object类型，可传递任意对象。
3. 发送消息
在子线程中可调用主线程的handler.sendMessage(msg)进行发送消息，经过一系列方法调用，会触发handler的handleMessage方法，从而进行消息处理。
发送消息的主要方法：
handler.sendMessage(Message msg); handler.sendMessageAtTime(Message msg, int time); handler.sendMessageDelayed(Message msg, int time);
sendMessageAtTime()和sendMessageDelayed()区别在于前者是在指定时间发送消息，可配合SystemClock.uptimeMillis()使用；而后者则是延时发送消息。
除了SendMessage()方法以外，还可以通过post()方法发送消息：
handler.post(Runnable r); handler.postDelayed(Runnable r, int time);
sendMessage()与post()的区别：http://www.jb51.net/article/120624.htm
4. 内存泄漏
http://www.jb51.net/article/120627.htm
5. 通过Handler对象实现下载文件动态更新进度条
AndroidManifest加入权限声明：
<uses-permission android:name="android.permission.INTERNET" /> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
布局：
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" android:padding="10dp" tools:context="com.studying.network.DownloadActivity"> <ProgressBar android:id="@+id/progress_bar" style="?android:progressBarStyleHorizontal" android:layout_width="match_parent" android:layout_height="wrap_content" android:max="100" /> <Button android:id="@+id/download" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_marginTop="10dp" android:text="@string/download" /> </LinearLayout>
Activity：
public class DownloadActivity extends Activity { private static final int DOWNLOAD_MESSAGE_CODE = 100001; private static final int DOWNLOAD_MESSAGE_FAIL_CODE = 100002; private static final String APP_URL = "http://clfile.imooc.com/class/assist/119/1328281/Android%20Studio%20教辅%20.pdf"; private MyHandler mHandler; private ProgressBar mProgressBar; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_download); findViewById(R.id.download).setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { //开启子线程 new Thread(new Runnable() { @Override public void run() { download(APP_URL); } }).start(); } }); mProgressBar = (ProgressBar) findViewById(R.id.progress_bar); mHandler = new MyHandler(this); } private void download(String appUrl) { try { URL url = new URL(appUrl); URLConnection conn = url.openConnection(); InputStream in = conn.getInputStream(); int contentLength = conn.getContentLength();//获取文件总大小 String downloadPath = Environment.getExternalStorageDirectory() + File.separator + "imooc" + File.separator; File file = new File(downloadPath); if (!file.exists()) { file.mkdir(); } String fileName = downloadPath + "test.pdf"; File apkFile = new File(fileName); if (apkFile.exists()) { apkFile.delete(); } int downloadSize = 0;//记录已经下载的大小 byte[] bytes = new byte[1024]; int length = 0; OutputStream out = new FileOutputStream(fileName); while ((length = in.read(bytes)) != -1) { out.write(bytes, 0, length); downloadSize += length; Message msg = Message.obtain(); msg.obj = downloadSize / contentLength * 100;//progress的值为0到100，因此得到的百分数要乘以100 msg.what = DOWNLOAD_MESSAGE_CODE; mHandler.sendMessage(msg); } in.close(); out.close(); } catch (IOException e) { notifyDownloadFailed(); e.printStackTrace(); } } private void notifyDownloadFailed() { Message msg = Message.obtain(); msg.what = DOWNLOAD_MESSAGE_FAIL_CODE; mHandler.sendMessage(msg); } private static class MyHandler extends Handler{ private WeakReference<DownloadActivity> weakReference; MyHandler(DownloadActivity activity) { this.weakReference = new WeakReference<>(activity);//以弱引用的形式传递Activity，避免内存泄漏 } @Override public void handleMessage(Message msg) { super.handleMessage(msg); DownloadActivity activity = weakReference.get(); //消息处理 switch (msg.what) { case DOWNLOAD_MESSAGE_CODE: activity.mProgressBar.setProgress((Integer) msg.obj); break; case DOWNLOAD_MESSAGE_FAIL_CODE: Toast.makeText(activity, "下载失败！", Toast.LENGTH_SHORT).show(); break; } } } }
总结
以上所述是小编给大家介绍的Handler实现线程之间的通信下载文件动态更新进度条，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对脚本之家网站的支持！