使用 AsyncTask 一一下载多个文件？

Question

我正在尝试一个一个下载多个文件（文件已下载，我们开始下载下一个文件）。使用这种方法，我可以跟踪正在下载的文件。问题是我正在使用以下代码来执行任务：

                File file = null;                
                for(int i=0; i< videos.length; i++) {
                     file = new File(root.getPath() + videos[i]);
                     boolean exists = file.exists();
                     if(exists){
                         //tv.append("\n\n"+fileNames[i]+" already exists");
                         continue;
                     }
                     else {
                         currentFile = videos[i];
                         new DownloadFileAsync().execute(videoURL+videos[i],videos[i]);            

                     }
                     file = null;
                }

如您所见，我调用 new DownloadFileAsync().execute(videoURL+videos[i],videos[i]); 在一个循环中，显然为每个文件启动一个任务并同时下载它们。

我的问题是：如何为特定文件运行执行任务，检查它是否已下载 - 如果是，则通过为其执行任务来继续下一个文件？

Answer 1

package com.example.androidhive;

import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.net.URLConnection;

import android.app.Activity;
import android.app.Dialog;
import android.app.ProgressDialog;
import android.graphics.drawable.Drawable;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

public class AndroidDownloadFileByProgressBarActivity extends Activity {

    // button to show progress dialog
    Button btnShowProgress;

    // Progress Dialog
    private ProgressDialog pDialog;
    ImageView my_image;
    // Progress dialog type (0 - for Horizontal progress bar)
    public static final int progress_bar_type = 0;

    // File url to download
    String[] ur = {
            "http://www.funrocker.com/blog/wp-content/uploads/2010/04/Animals-Wild-Life-Jungle-FunRocker.Com-03.jpg",
            "http://2.bp.blogspot.com/-j56yaqpfjVE/TnzTjcqnCjI/AAAAAAAAGPM/MzqmczFkC30/s1600/natural-pictures.jpg",
            "http://www.fantasy-and-art.com/wp-content/gallery/nature-wallpapers/red-tree-wallpaper-hd.jpg" };

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        // show progress bar button
        btnShowProgress = (Button) findViewById(R.id.btnProgressBar);
        // Image view to show image after downloading
        my_image = (ImageView) findViewById(R.id.my_image);
        /**
         * Show Progress bar click event
         * */
        btnShowProgress.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // starting new Async Task
                new DownloadFileFromURL().execute(ur);
            }
        });
    }

    /**
     * Showing Dialog
     * */
    @Override
    protected Dialog onCreateDialog(int id) {
        switch (id) {
        case progress_bar_type:
            pDialog = new ProgressDialog(this);
            pDialog.setMessage("Downloading file. Please wait...");
            pDialog.setIndeterminate(false);
            pDialog.setMax(100);
            pDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            pDialog.setCancelable(true);
            pDialog.show();
            return pDialog;
        default:
            return null;
        }
    }

    /**
     * Background Async Task to download file
     * */
    class DownloadFileFromURL extends AsyncTask<String, Integer, String> {

        /**
         * Before starting background thread Show Progress Bar Dialog
         * */
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            showDialog(progress_bar_type);
        }

        /**
         * Downloading file in background thread
         * */
        @Override
        protected String doInBackground(String... f_url) {
            int count;
            try {

                for (int i = 0; i < f_url.length; i++) {
                    URL url = new URL(f_url[i]);
                    URLConnection conection = url.openConnection();
                    conection.connect();
                    // getting file length
                    int lenghtOfFile = conection.getContentLength();

                    // input stream to read file - with 8k buffer
                    InputStream input = new BufferedInputStream(
                            url.openStream(), 8192);
                    System.out.println("Data::" + f_url[i]);
                    // Output stream to write file
                    OutputStream output = new FileOutputStream(
                            "/sdcard/downloaded" + i + ".jpg");

                    byte data[] = new byte[1024];

                    long total = 0;

                    while ((count = input.read(data)) != -1) {
                        total += count;
                        // publishing the progress....
                        // After this onProgressUpdate will be called
                        publishProgress((int) ((total * 100)/lenghtOfFile));

                        // writing data to file
                        output.write(data, 0, count);
                    }

                    // flushing output
                    output.flush();

                    // closing streams
                    output.close();
                    input.close();
                }
            } catch (Exception e) {
                Log.e("Error: ", e.getMessage());
            }

            return null;
        }

        /**
         * Updating progress bar
         * */
        protected void onProgressUpdate(Integer... progress) {
            // setting progress percentage
            pDialog.setProgress(progress[0]);
        }

        /**
         * After completing background task Dismiss the progress dialog
         * **/
        @Override
        protected void onPostExecute(String file_url) {
            // dismiss the dialog after the file was downloaded
            dismissDialog(progress_bar_type);

            // Displaying downloaded image into image view
            // Reading image path from sdcard
            String imagePath = Environment.getExternalStorageDirectory()
                    .toString() + "/downloaded.jpg";
            // setting downloaded into image view
            // my_image.setImageDrawable(Drawable.createFromPath(imagePath));
        }

    }
}

Answer 2

如果我理解正确，您不想同时（同时）下载所有文件，而是一个接一个（串行）下载。为此，构建一个包含要下载的 URL 的字符串数组，并execute()使用该数组进行调用。

示例：假设您DownloadFileAsync希望 String 作为 it's 的参数doInBackground method，您将调用：

new DownloadFileAsync().execute(url1, url2, url3, url4, video1, video2, video3, video4);