Handler實現線程之間的通信-下載文件動態更新進度條

<ProgressBar
android:id="@+id/progress_bar"
stylex="?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" />

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，避免內存泄漏
}