Handler實現線程之間的通信下載文件動態更新進度條

1. 原理

每一個線程對應一個消息隊列MessageQueue，實現線程之間的通信，可通過Handler對象將數據裝進Message中，再將消息加入消息隊列，而後線程會依次處理消息隊列中的消息。

2. Message

初始化：一般使用Message.obtain()方法獲取一個消息對象，該方法會檢查Message對象池中是否存在可重復利用的對象，若無，才會new一個新對象。

what：相當於Message的標識符，區別於其它消息。

arg1、arg2：int類型，可傳遞整數。

obj：object類型，可傳遞任意對象。

3. 發送消息

在子線程中可調用主線程的handler.sendMessage(msg)進行發送消息，經過一系列方法調用，會觸發handler的handleMessage方法，從而進行消息處理。

發送消息的主要方法：

handler.sendMessage(Message msg);
handler.sendMessageAtTime(Message msg, int time);
handler.sendMessageDelayed(Message msg, int time);

sendMessageAtTime()和sendMessageDelayed()區別在於前者是在指定時間發送消息，可配合SystemClock.uptimeMillis()使用；而後者則是延時發送消息。

除了SendMessage()方法以外，還可以通過post()方法發送消息：

handler.post(Runnable r);
handler.postDelayed(Runnable r, int time);

sendMessage()與post()的區別：http://www.jb51.net/article/120624.htm

4. 內存洩漏

http://www.jb51.net/article/120627.htm

5. 通過Handler對象實現下載文件動態更新進度條

AndroidManifest加入權限聲明：

<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"
 
 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教輔%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) {
 //開啟子線程
 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();//獲取文件總大小
 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;//記錄已經下載的大小
 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的值為0到100，因此得到的百分數要乘以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);//以弱引用的形式傳遞Activity，避免內存洩漏
 }
 @Override
 public void handleMessage(Message msg) {
 super.handleMessage(msg);
 DownloadActivity activity = weakReference.get();
 //消息處理
 switch (msg.what) {
 case DOWNLOAD_MESSAGE_CODE:
  activity.mProgressBar.setProgress((Integer) msg.obj);
  break;
 case DOWNLOAD_MESSAGE_FAIL_CODE:
  Toast.makeText(activity, "下載失敗！", Toast.LENGTH_SHORT).show();
  break;
 }
 }
 }
}

總結

以上所述是小編給大家介紹的Handler實現線程之間的通信下載文件動態更新進度條，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對本站網站的支持！