Android初级开发（十）——服务—下载实例

一、效果图
三个按钮，分别控制下载任务的开始、暂停和结束

下载任务在后台运行，下载状态在通知里可以查看

下载完成后在SD卡中找到已下载的文件查看：

二、代码
1、新建DownloadListener回调接口

/**
 * 定义一个回调接口，用于对于下载过程中的各种状态进行监听和回调
 */


public interface DownloadListener  {
    //用于通知当前的下载进度
    void onProgress(int progress);
    //用于通知下载成功事件
    void onSuccess();
    //用于通知下载失败事件
    void onFailed();
    //用于通知下载暂停事件
    void onPaused();
    //用于同事下载取消事件
    void onCanceled();
}

2、新建一个DownloadTask类用于实现下载功能

/**
 * 下载功能的实现
 */

/**
 * AsyncTask中传入3个泛型参数
 * 第一个指定为String，表示在执行AsyncTask的时候需要传入一个字符串参数给后台任务
 * 第二个指定为Integer，表示使用整型数据来作为进度显示单位
 * 第三个泛型参数指定为Integer，则表示使用整型数据来反馈执行结果
 */
public class DownloadTask extends AsyncTask<String,Integer,Integer> {
    //表示下载成功
    public static final int TYPE_SUCCESS = 0;
    //表示下载失败
    public static final int TYPE_FAILED = 1;
    //表示暂停下载
    public static final int TYPE_PAUSED = 2;
    //表示取消下载
    public static final int TYPE_CANCELED = 3;

    private DownloadListener listener;
    private boolean isCanceled =  false;
    private boolean isPaused = false;
    private int lastProgress;

    DownloadTask(DownloadListener listener){
        this.listener = listener;
    }

    /**
     * 这个方法用于在后台执行具体的下载逻辑
     * @param strings
     * @return
     */
    @Override
    protected Integer doInBackground(String... strings) {
        InputStream is = null;
        RandomAccessFile savedFile = null;
        File file = null;
        try{
            long downloadLength = 0;//记录已下载的文件长度
            String downloadUrl = strings[0];//根据参数获取到下载的URL地址
            String fileName = downloadUrl.substring(downloadUrl.lastIndexOf("/"));//根据URL地址解析出下载的文件名
            //指定将文件下载到SD卡的Download目录
            String directory = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();
            file = new File(directory + fileName);
            //判断目录中是否已经存在要下载的文件了
            if(file.exists()){
                //已存在，读取已下载的字节数
                downloadLength = file.length();
            }
            //调用getContentLength（）方法来获取待下载的文件的总长度
            long contentLength = getContentLength(downloadUrl);
            //文件长度等于0则说明文件有问题
            if(contentLength == 0 ){
                return  TYPE_FAILED;
                //已下载字节和文件总字节相等，说明已经下载完成了
            }else if(contentLength == downloadLength){
                return TYPE_SUCCESS;
            }
            //使用OkHttp发送一条网络请求
            //在请求中添加一个header，用于告诉服务器我们想要从哪个字节开始下载
            OkHttpClient client = new OkHttpClient();
            Request request = new Request.Builder()
                    //断点下载，指定从那个字节开始下载
                    .addHeader("RANGE","bytes="+downloadLength+"-")
                    .url(downloadUrl)
                    .build();
            //读取服务器相应的数据
            Response response = client.newCall(request).execute();
            //使用Java的文件流方式，不断从网络上读取数据，写到本地
            if(response != null){
                is = response.body().byteStream();
                savedFile = new RandomAccessFile(file,"rw");
                savedFile.seek(downloadLength); //跳过已下载的字节
                byte[] b = new byte[1024];
                int total = 0;
                int len;
                while ((len = is.read(b)) != -1){
                    //判断用户有没有触发暂停或者取消的操作
                    if (isCanceled){
                        return TYPE_CANCELED;
                    }else if(isPaused){
                        return TYPE_PAUSED;
                    }else {
                        total += len;
                        savedFile.write(b,0,len);
                        //计算已下载的百分比
                        int progress = (int)((total + downloadLength) * 100 /contentLength);
                        //调用publishProgress（）方法进行通知
                        publishProgress(progress);
                    }
                }
                response.body().close();
                return TYPE_SUCCESS;
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            try {
                if(is!=null){
                    is.close();
                }if(savedFile != null){
                    savedFile.close();
                }if(isCanceled && file!=null){
                    file.delete();
                }
            }catch (Exception e){
                e.printStackTrace();
            }
        }
        return TYPE_FAILED;
    }

    /**
     * 用于在界面上更新当前的下载进度
     * @param values
     */
    @Override
    protected void onProgressUpdate(Integer... values) {
        //从参数中获取到当前的下载进度
        int progress = values[0];
        //和上一次的下载进度进行对比
        if(progress > lastProgress){
            listener.onProgress(progress);
            lastProgress = progress;
        }
    }

    /**
     * 用于通知最终的下载结果
     * @param status
     */
    @Override
    protected void onPostExecute(Integer status) {
        switch (status){
            case TYPE_SUCCESS:
                listener.onSuccess();
                break;
            case TYPE_FAILED:
                listener.onFailed();
                break;
            case TYPE_PAUSED:
                listener.onPaused();
                break;
            case TYPE_CANCELED:
                listener.onCanceled();
                break;
            default:
                break;
        }
    }

    public void pauseDownload(){
        isPaused = true;
    }

    public void cancelDownload(){
        isCanceled = true;
    }

    private long getContentLength(String downloadUrl) throws IOException {
       OkHttpClient client = new OkHttpClient();
        Request request = new Request.Builder().url(downloadUrl).build();
        Response response = client.newCall(request).execute();
        if(response != null && response.isSuccessful()){
            long contenLength = response.body().contentLength();
            response.close();
            return contenLength;
        }

        return 0;
    }
}

3、新建一个下载服务DownloadService类

**
 * 创建一个下载的服务，保证下载任务一直在后台运行
 */

public class DownloadService extends Service {

    private DownloadTask downloadTask;

    private String downloadUrl;

    //创建一个DownloadListener的匿名类实例
    private DownloadListener listener = new DownloadListener() {

        //调用getNotification（）方法构建了一个用于显示下载进度的通知
        //调用getNotificationManager（）的notify()方法去触发这个通知。
        @Override
        public void onProgress(int progress) {
            getNotificationManager().notify(1,getNotification("Downloading...",progress));
        }

        @Override
        public void onSuccess() {
            downloadTask = null;
            //下载成功时将前台服务通知关闭，并创建一个下载成功的通知
            stopForeground(true);
            getNotificationManager().notify(1,getNotification("Download Success",-1));
            Toast.makeText(DownloadService.this, "Download Success", Toast.LENGTH_SHORT).show();

        }

        @Override
        public void onFailed() {
            downloadTask = null;
            //下载失败时将前台服务通知关闭，并创建一个下载失败的通知
            stopForeground(true);
            getNotificationManager().notify(1,getNotification("Download Failed",-1));
            Toast.makeText(DownloadService.this, "Download Failed", Toast.LENGTH_SHORT).show();

        }

        @Override
        public void onPaused() {
            downloadTask = null;
            Toast.makeText(DownloadService.this, "Paused", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onCanceled() {
            downloadTask = null;
            stopForeground(true);
            Toast.makeText(DownloadService.this, "Canceled", Toast.LENGTH_SHORT).show();
        }
    };


    private DownloadBinder mBinder = new DownloadBinder();

    @Override
    public IBinder onBind(Intent intent) {
       return mBinder;
    }

    class DownloadBinder extends Binder {
        //开始下载
        public void startDownload(String url){
            if(downloadTask == null){
                downloadUrl = url;
                //创建一个DownloadTask实例，
                downloadTask = new DownloadTask(listener);
                //，将下载文件的URL地址传入execute（）并开启下载
                downloadTask.execute(downloadUrl);
                //在系统状态栏中创建一个持续运行的通知
                startForeground(1,getNotification("Downloading...",0));
                Toast.makeText(DownloadService.this, "Downloading...", Toast.LENGTH_SHORT).show();
            }
        }

        //暂停下载
        public void pauseDownload(){
            if(downloadTask != null){
                downloadTask.pauseDownload();
            }
        }

        //取消下载
        public void cancelDownload(){
            if(downloadTask != null){
                downloadTask.cancelDownload();
            }else {
                if(downloadUrl != null) {
                    //取消下载时需要将文件删除，并将通知关闭
                    String fileName = downloadUrl.substring(downloadUrl.lastIndexOf("/"));
                    String directory = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();
                    File file = new File(directory + fileName);
                    //将正在下载的文件删除掉
                    if(file.exists()){
                        file.delete();
                    }
                    getNotificationManager().cancel(1);
                    stopForeground(true);
                    Toast.makeText(DownloadService.this, "Canceled", Toast.LENGTH_SHORT).show();
                }
            }
        }
    }


    private NotificationManager getNotificationManager(){
        return (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
    }

    private Notification getNotification(String title,int progress){
        Intent intent = new Intent(this,MainActivity.class);
        PendingIntent pi = PendingIntent.getActivity(this,0,intent,0);
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this);
        builder.setSmallIcon(R.mipmap.ic_launcher);
        builder.setLargeIcon(BitmapFactory.decodeResource(getResources(),R.mipmap.ic_launcher));
        builder.setContentIntent(pi);
        builder.setContentTitle(title);
        if(progress > 0 ){
            //当progress大于或等于0时才需显示下载进度
            builder.setContentTitle(progress + "%");
            //setProgress接受3个参数，第一个参数传入通知的最大进度；
            // 第二个从参数传入通知的当前进度
            //第三个参数表示是否使用模糊进度条
            builder.setProgress(100,progress,false);
        }
        return builder.build();
    }
}

4、写布局文件activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <Button
        android:id="@+id/start_dowmload"
        android:layout_width="match_parent"
        android:layout_height="80dp"
        android:textSize="25dp"
        android:text="开始下载"/>
    <Button
        android:id="@+id/pause_dowmload"
        android:layout_width="match_parent"
        android:layout_height="80dp"
        android:textSize="25dp"
        android:text="暂停下载"/>
    <Button
        android:id="@+id/cancel_dowmload"
        android:layout_width="match_parent"
        android:layout_height="80dp"
        android:textSize="25dp"
        android:text="停止下载"/>

</LinearLayout>

5、MainActivity.java

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    private DownloadService.DownloadBinder downloadBinder;

    private ServiceConnection connection = new ServiceConnection() {
        @Override
        public void onServiceDisconnected(ComponentName componentName) {
        }

        @Override
        public void onServiceConnected(ComponentName componentName,IBinder iBinder) {
            downloadBinder = (DownloadService.DownloadBinder)iBinder;
        }


    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button starDonload = (Button) findViewById(R.id.start_dowmload);
        Button pauseDownload = (Button) findViewById(R.id.pause_dowmload);
        Button cancelDownload = (Button) findViewById(R.id.cancel_dowmload);

        starDonload.setOnClickListener(this);
        pauseDownload.setOnClickListener(this);
        cancelDownload.setOnClickListener(this);

        Intent intent = new Intent(this,DownloadService.class);
        startService(intent); //启动服务
        bindService(intent,connection,BIND_AUTO_CREATE);//绑定服务
        if(ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.WRITE_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED){
            ActivityCompat.requestPermissions(MainActivity.this,new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},1);
        }
    }

    @Override
    public void onClick(View view) {
        if(downloadBinder == null){
            return;
        }
        switch (view.getId()) {
            case R.id.start_dowmload:
                //百度logo的地址
                String url = "http://a2.att.hudong.com/10/96/300000931099127952960461732_950.jpg";
                downloadBinder.startDownload(url);
                break;
            case R.id.pause_dowmload:
                downloadBinder.pauseDownload();
                break;
            case R.id.cancel_dowmload:
                downloadBinder.cancelDownload();
                break;
            default:
                break;
        }


    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode){
            case 1:
                if(grantResults.length>0 &&grantResults[0]!=PackageManager.PERMISSION_GRANTED){
                    Toast.makeText(this, "拒绝权限将无法使用程序", Toast.LENGTH_SHORT).show();
                    finish();
                }
                break;
            default:
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        unbindService(connection);
    }
}

6、声明权限

 <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

然后，运行看效果吧~~