Android中使用AsyncTask实现下载文件动态更新进度条功能

1. 泛型

AysncTask<Params, Progress, Result>

Params：启动任务时传入的参数，通过调用asyncTask.execute(param)方法传入。

Progress：后台任务执行的进度，若不用显示进度条，则不需要指定。

Result：后台任务结束时返回的结果。

2. 重要方法

doInBackground(Params... params)：必须重写的方法，后台任务就在这里执行，会开启一个新的线程。params为启动任务时传入的参数，参数个数不定。

onPreExecute()：在主线程中调用，在后台任务开启前的操作在这里进行，例如显示一个进度条对话框。

onPostExecute(Result result)：当后台任务结束后，在主线程中调用，处理doInBackground()方法返回的结果。

onProgressUpdate(Progress... values)：当在doInBackground()中调用publishProgress(Progress... values)时，返回主线程中调用，这里的参数个数也是不定的。

onCancelled()：取消任务。

3. 注意事项

（1）execute()方法必须在主线程中调用；

（2）AsyncTask实例必须在主线程中创建；

（3）不要手动调用doInBackground()、onPreExecute()、onPostExecute()、onProgressUpdate()方法；

（4）注意防止内存泄漏，在doInBackground()方法中若出现对Activity的强引用，可能会造成内存泄漏。

4. 下载文件动态更新进度条（未封装）

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="20dp"
  tools:context="com.studying.asynctaskdemo.MainActivity">
  <ProgressBar
    android:id="@+id/progressBar"
    style="?android:progressBarStyleHorizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:progress="0" />
  <Button
    android:id="@+id/download"
    android:layout_width="match_parent"
    android:layout_height="50dp"
    android:layout_marginTop="20dp"
    android:text="@string/start_btn" />
  <TextView
    android:id="@+id/status"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="20dp"
    android:text="@string/waiting" />
</LinearLayout>

Activity：

public class MainActivity extends Activity {
  private static final String FILE_NAME = "test.pdf";//下载文件的名称
  private static final String PDF_URL = "http://clfile.imooc.com/class/assist/118/1328281/AsyncTask.pdf";
  private ProgressBar mProgressBar;
  private Button mDownloadBtn;
  private TextView mStatus;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    initView();
    setListener();
  }
  private void initView() {
    mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
    mDownloadBtn = (Button) findViewById(R.id.download);
    mStatus = (TextView) findViewById(R.id.status);
  }
  private void setListener() {
    mDownloadBtn.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        //AsyncTask实例必须在主线程创建
        DownloadAsyncTask asyncTask = new DownloadAsyncTask();
        asyncTask.execute(PDF_URL);
      }
    });
  }
  /**
   * 泛型：
   * String：传入参数为文件下载地址
   * Integer：下载过程中更新ProgressBar的进度
   * Boolean：是否下载成功
   */
  private class DownloadAsyncTask extends AsyncTask<String, Integer, Boolean> {
    private String mFilePath;//下载文件的保存路径
    @Override
    protected Boolean doInBackground(String... params) {
      if (params != null && params.length > 0) {
        String pdfUrl = params[0];
        try {
          URL url = new URL(pdfUrl);
          URLConnection urlConnection = url.openConnection();
          InputStream in = urlConnection.getInputStream();
          int contentLength = urlConnection.getContentLength();//获取内容总长度
          mFilePath = Environment.getExternalStorageDirectory() + File.separator + FILE_NAME;
          //若存在同名文件则删除
          File pdfFile = new File(mFilePath);
          if (pdfFile.exists()) {
            boolean result = pdfFile.delete();
            if (!result) {
              return false;
            }
          }
          int downloadSize = 0;//已经下载的大小
          byte[] bytes = new byte[1024];
          int length = 0;
          OutputStream out = new FileOutputStream(mFilePath);
          while ((length = in.read(bytes)) != -1) {
            out.write(bytes, 0, length);
            downloadSize += length;
            publishProgress(downloadSize / contentLength * 100);
          }
          in.close();
          out.close();
        } catch (IOException e) {
          e.printStackTrace();
          return false;
        }
      } else {
        return false;
      }
      return true;
    }
    @Override
    protected void onPreExecute() {
      super.onPreExecute();
      mDownloadBtn.setText("下载中");
      mDownloadBtn.setEnabled(false);
      mStatus.setText("下载中");
      mProgressBar.setProgress(0);
    }
    @Override
    protected void onPostExecute(Boolean aBoolean) {
      super.onPostExecute(aBoolean);
      mDownloadBtn.setText("下载完成");
      mStatus.setText(aBoolean ? "下载完成" + mFilePath : "下载失败");
    }
    @Override
    protected void onProgressUpdate(Integer... values) {
      super.onProgressUpdate(values);
      if (values != null && values.length > 0) {
        mProgressBar.setProgress(values[0]);
      }
    }
  }
}

5. 下载文件动态更新进度条（封装）

Activity：

public class MainActivity extends Activity {
  private static final String FILE_NAME = "test.pdf";
  private static final String PDF_URL = "http://clfile.imooc.com/class/assist/118/1328281/AsyncTask.pdf";
  private ProgressBar mProgressBar;
  private Button mDownloadBtn;
  private TextView mStatus;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    initView();
    setListener();
  }
  private void initView() {
    mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
    mDownloadBtn = (Button) findViewById(R.id.download);
    mStatus = (TextView) findViewById(R.id.status);
  }
  private void setListener() {
    mDownloadBtn.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        String localPath = Environment.getExternalStorageDirectory() + File.separator + FILE_NAME;
        DownloadHelper.download(PDF_URL, localPath, new DownloadHelper.OnDownloadListener() {
          @Override
          public void onStart() {
            mDownloadBtn.setText("下载中");
            mDownloadBtn.setEnabled(false);
            mStatus.setText("下载中");
            mProgressBar.setProgress(0);
          }
          @Override
          public void onSuccess(File file) {
            mDownloadBtn.setText("下载完成");
            mStatus.setText(String.format("下载完成：%s", file.getPath()));
          }
          @Override
          public void onFail(File file, String failInfo) {
            mDownloadBtn.setText("开始下载");
            mDownloadBtn.setEnabled(true);
            mStatus.setText(String.format("下载失败：%s", failInfo));
          }
          @Override
          public void onProgress(int progress) {
            mProgressBar.setProgress(progress);
          }
        });
      }
    });
  }
}

DownloadHelper：

class DownloadHelper {
  static void download(String url, String localPath, OnDownloadListener listener) {
    DownloadAsyncTask task = new DownloadAsyncTask(url, localPath, listener);
    task.execute();
  }
  private static class DownloadAsyncTask extends AsyncTask<String, Integer, Boolean> {
    private String mFailInfo;
    private String mUrl;
    private String mFilePath;
    private OnDownloadListener mListener;
    DownloadAsyncTask(String mUrl, String mFilePath, OnDownloadListener mListener) {
      this.mUrl = mUrl;
      this.mFilePath = mFilePath;
      this.mListener = mListener;
    }
    @Override
    protected Boolean doInBackground(String... params) {
        String pdfUrl = mUrl;
        try {
          URL url = new URL(pdfUrl);
          URLConnection urlConnection = url.openConnection();
          InputStream in = urlConnection.getInputStream();
          int contentLength = urlConnection.getContentLength();
          File pdfFile = new File(mFilePath);
          if (pdfFile.exists()) {
            boolean result = pdfFile.delete();
            if (!result) {
              mFailInfo = "存储路径下的同名文件删除失败！";
              return false;
            }
          }
          int downloadSize = 0;
          byte[] bytes = new byte[1024];
          int length;
          OutputStream out = new FileOutputStream(mFilePath);
          while ((length = in.read(bytes)) != -1) {
            out.write(bytes, 0, length);
            downloadSize += length;
            publishProgress(downloadSize / contentLength * 100);
          }
          in.close();
          out.close();
        } catch (IOException e) {
          e.printStackTrace();
          mFailInfo = e.getMessage();
          return false;
        }
      return true;
    }
    @Override
    protected void onPreExecute() {
      super.onPreExecute();
      if (mListener != null) {
        mListener.onStart();
      }
    }
    @Override
    protected void onPostExecute(Boolean aBoolean) {
      super.onPostExecute(aBoolean);
      if (mListener != null) {
        if (aBoolean) {
          mListener.onSuccess(new File(mFilePath));
        } else {
          mListener.onFail(new File(mFilePath), mFailInfo);
        }
      }
    }
    @Override
    protected void onProgressUpdate(Integer... values) {
      super.onProgressUpdate(values);
      if (values != null && values.length > 0) {
        if (mListener != null) {
          mListener.onProgress(values[0]);
        }
      }
    }
  }
  interface OnDownloadListener{
    void onStart();
    void onSuccess(File file);
    void onFail(File file, String failInfo);
    void onProgress(int progress);
  }
}

总结

以上所述是小编给大家介绍的Android中使用AsyncTask实现下载文件动态更新进度条功能，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小牛知识库网站的支持！