Android初級開發（十）——服務—下載實例

一、效果圖
三個按鈕，分別控制下載任務的開始、暫停和結束

下載任務在後臺運行，下載狀態在通知裏可以查看

下載完成後在SD卡中找到已下載的文件查看：

二、代碼
1、新建DownloadListener回調接口

/**
 * 定義一個回調接口，用於對於下載過程中的各種狀態進行監聽和回調
 */


public interface DownloadListener  {
    //用於通知當前的下載進度
    void onProgress(int progress);
    //用於通知下載成功事件
    void onSuccess();
    //用於通知下載失敗事件
    void onFailed();
    //用於通知下載暫停事件
    void onPaused();
    //用於同事下載取消事件
    void onCanceled();
}

2、新建一個DownloadTask類用於實現下載功能

/**
 * 下載功能的實現
 */

/**
 * AsyncTask中傳入3個泛型參數
 * 第一個指定爲String，表示在執行AsyncTask的時候需要傳入一個字符串參數給後臺任務
 * 第二個指定爲Integer，表示使用整型數據來作爲進度顯示單位
 * 第三個泛型參數指定爲Integer，則表示使用整型數據來反饋執行結果
 */
public class DownloadTask extends AsyncTask<String,Integer,Integer> {
    //表示下載成功
    public static final int TYPE_SUCCESS = 0;
    //表示下載失敗
    public static final int TYPE_FAILED = 1;
    //表示暫停下載
    public static final int TYPE_PAUSED = 2;
    //表示取消下載
    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;
    }

    /**
     * 這個方法用於在後臺執行具體的下載邏輯
     * @param strings
     * @return
     */
    @Override
    protected Integer doInBackground(String... strings) {
        InputStream is = null;
        RandomAccessFile savedFile = null;
        File file = null;
        try{
            long downloadLength = 0;//記錄已下載的文件長度
            String downloadUrl = strings[0];//根據參數獲取到下載的URL地址
            String fileName = downloadUrl.substring(downloadUrl.lastIndexOf("/"));//根據URL地址解析出下載的文件名
            //指定將文件下載到SD卡的Download目錄
            String directory = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();
            file = new File(directory + fileName);
            //判斷目錄中是否已經存在要下載的文件了
            if(file.exists()){
                //已存在，讀取已下載的字節數
                downloadLength = file.length();
            }
            //調用getContentLength（）方法來獲取待下載的文件的總長度
            long contentLength = getContentLength(downloadUrl);
            //文件長度等於0則說明文件有問題
            if(contentLength == 0 ){
                return  TYPE_FAILED;
                //已下載字節和文件總字節相等，說明已經下載完成了
            }else if(contentLength == downloadLength){
                return TYPE_SUCCESS;
            }
            //使用OkHttp發送一條網絡請求
            //在請求中添加一個header，用於告訴服務器我們想要從哪個字節開始下載
            OkHttpClient client = new OkHttpClient();
            Request request = new Request.Builder()
                    //斷點下載，指定從那個字節開始下載
                    .addHeader("RANGE","bytes="+downloadLength+"-")
                    .url(downloadUrl)
                    .build();
            //讀取服務器相應的數據
            Response response = client.newCall(request).execute();
            //使用Java的文件流方式，不斷從網絡上讀取數據，寫到本地
            if(response != null){
                is = response.body().byteStream();
                savedFile = new RandomAccessFile(file,"rw");
                savedFile.seek(downloadLength); //跳過已下載的字節
                byte[] b = new byte[1024];
                int total = 0;
                int len;
                while ((len = is.read(b)) != -1){
                    //判斷用戶有沒有觸發暫停或者取消的操作
                    if (isCanceled){
                        return TYPE_CANCELED;
                    }else if(isPaused){
                        return TYPE_PAUSED;
                    }else {
                        total += len;
                        savedFile.write(b,0,len);
                        //計算已下載的百分比
                        int progress = (int)((total + downloadLength) * 100 /contentLength);
                        //調用publishProgress（）方法進行通知
                        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;
    }

    /**
     * 用於在界面上更新當前的下載進度
     * @param values
     */
    @Override
    protected void onProgressUpdate(Integer... values) {
        //從參數中獲取到當前的下載進度
        int progress = values[0];
        //和上一次的下載進度進行對比
        if(progress > lastProgress){
            listener.onProgress(progress);
            lastProgress = progress;
        }
    }

    /**
     * 用於通知最終的下載結果
     * @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、新建一個下載服務DownloadService類

**
 * 創建一個下載的服務，保證下載任務一直在後臺運行
 */

public class DownloadService extends Service {

    private DownloadTask downloadTask;

    private String downloadUrl;

    //創建一個DownloadListener的匿名類實例
    private DownloadListener listener = new DownloadListener() {

        //調用getNotification（）方法構建了一個用於顯示下載進度的通知
        //調用getNotificationManager（）的notify()方法去觸發這個通知。
        @Override
        public void onProgress(int progress) {
            getNotificationManager().notify(1,getNotification("Downloading...",progress));
        }

        @Override
        public void onSuccess() {
            downloadTask = null;
            //下載成功時將前臺服務通知關閉，並創建一個下載成功的通知
            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;
            //下載失敗時將前臺服務通知關閉，並創建一個下載失敗的通知
            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 {
        //開始下載
        public void startDownload(String url){
            if(downloadTask == null){
                downloadUrl = url;
                //創建一個DownloadTask實例，
                downloadTask = new DownloadTask(listener);
                //，將下載文件的URL地址傳入execute（）並開啓下載
                downloadTask.execute(downloadUrl);
                //在系統狀態欄中創建一個持續運行的通知
                startForeground(1,getNotification("Downloading...",0));
                Toast.makeText(DownloadService.this, "Downloading...", Toast.LENGTH_SHORT).show();
            }
        }

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

        //取消下載
        public void cancelDownload(){
            if(downloadTask != null){
                downloadTask.cancelDownload();
            }else {
                if(downloadUrl != null) {
                    //取消下載時需要將文件刪除，並將通知關閉
                    String fileName = downloadUrl.substring(downloadUrl.lastIndexOf("/"));
                    String directory = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();
                    File file = new File(directory + fileName);
                    //將正在下載的文件刪除掉
                    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 ){
            //當progress大於或等於0時才需顯示下載進度
            builder.setContentTitle(progress + "%");
            //setProgress接受3個參數，第一個參數傳入通知的最大進度；
            // 第二個從參數傳入通知的當前進度
            //第三個參數表示是否使用模糊進度條
            builder.setProgress(100,progress,false);
        }
        return builder.build();
    }
}

4、寫佈局文件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="開始下載"/>
    <Button
        android:id="@+id/pause_dowmload"
        android:layout_width="match_parent"
        android:layout_height="80dp"
        android:textSize="25dp"
        android:text="暫停下載"/>
    <Button
        android:id="@+id/cancel_dowmload"
        android:layout_width="match_parent"
        android:layout_height="80dp"
        android:textSize="25dp"
        android:text="停止下載"/>

</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); //啓動服務
        bindService(intent,connection,BIND_AUTO_CREATE);//綁定服務
        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, "拒絕權限將無法使用程序", Toast.LENGTH_SHORT).show();
                    finish();
                }
                break;
            default:
        }
    }

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

6、聲明權限

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

然後，運行看效果吧~~