android - 在 Android 中下载背景图片时需要帮助吗？

Question

我有一个 ImageView ，我写了刷卡，在刷卡的时候，图片是从互联网上下载的，所以我想我必须在刷卡之前在后台下载图片，为此我需要使用 asynctask或 Service 或 IntentService，所有这些都将有助于下载和存储在 data/data/mypackages 中，但在我的情况下滑动仍然变慢任何想法，也告诉我哪个是最好的，我是否以正确的方式调用

<强>1。异步任务

<强>2。服务

<强>3。 Intent Service如下图，

我很困惑哪一个是正确的方法，因为我的问题还没有解决

这是异步任务代码示例 fragment

public class Demo extends Activity {

  @Override

  public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.main);

       new FirstTask().execute(); // calling Asynctask here

  }

异步任务代码

private class FirstTask extends AsyncTask<Void, Void, Void> {
        private final ProgressDialog dialog = new ProgressDialog(Catalogue.this);
        int temp = 0;

        // can use UI thread here
        protected void onPreExecute() {
            this.dialog.setMessage("Loading...");
            this.dialog.setCancelable(false);
            //this.dialog.show();
            System.gc();
            Toast.makeText(Catalogue.this, "My Async  Created",
                    Toast.LENGTH_LONG).show();
        }

        @Override
        protected Void doInBackground(Void... params) {
            Looper.prepare();  
            try {

                myddownloadmethod();// calling my download method

            } catch (Exception e) {
                Util.trace("Error in Async"+e.getMessage());

            }
              Looper.loop();

            return null;
        }

        protected void onPostExecute(Void result) {

            if (this.dialog.isShowing()) {
                Toast.makeText(Catalogue.this, "My Async destroyed",
                        Toast.LENGTH_LONG).show();
                Toast.makeText(Catalogue.this, "count" + temp,
                        Toast.LENGTH_LONG).show();
                this.dialog.dismiss();
            }
        }
    }

这是我的服务代码

public class MyService extends Service implements Runnable

{      @Override

        public void onCreate() {

                super.onCreate();

                Thread mythread = new Thread(this);

                mythread.start();

        }



        public void run() {

                Looper.prepare();  

                try {

                        myddownloadmethod();// calling my download method

                } catch (Exception e1) {

                        // TODO Auto-generated catch block

                        e1.printStackTrace();

                }

                Looper.loop();

        }



        @Override
        public IBinder onBind(Intent intent) {

                // TODO Auto-generated method stub

                return null;

        }



}



Invoking Service  

public class ServicesDemo extends Activity {    

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

      startService(new Intent(this, MyService.class));


  }

}

这是 IntentService 代码

public class Downloader extends IntentService {

    public Downloader() {
        super("Downloader");
    }

    @Override
    public void onCreate() {
        super.onCreate();


    }

    @Override
    public void onDestroy() {
        super.onDestroy();

            }

    @Override   
    public void onHandleIntent(Intent i) {

        try {
             myddownloadmethod();// calling my download method
        } catch (Exception e1) {
            // TODO Auto-generated catch block
            Log.d("Error",e1.getMessage());
        }

    }
}

从 MyActivity 调用 IntentService

public class ServicesDemo extends Activity {    
      @Override
      public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Intent i1=new Intent(this, Downloader.class);
        startService(i1);



      }

    }

Answer 1

使用服务下载它的最佳方式就像我从服务器下载文件并放入 SD 卡一样，也使用它的通知。 这是相当长的代码，但我认为是完美的代码，如果有任何不明白的地方，请访问 android 开发者博客获取服务。

public class DownloadService extends Service{

     SharedPreferences preferences;

    private static final String DOCUMENT_VIEW_STATE_PREFERENCES = "DjvuDocumentViewState";

      private Looper mServiceLooper;
      private ServiceHandler mServiceHandler;
      private NotificationManager mNM;
      String downloadUrl;
      public static boolean serviceState=false;

      // Handler that receives messages from the thread
      private final class ServiceHandler extends Handler {
          public ServiceHandler(Looper looper) {
              super(looper);
          }
          @Override
          public void handleMessage(Message msg) {
              downloadFile();
              showNotification(getResources().getString(R.string.notification_catalog_downloaded),"VVS");
              stopSelf(msg.arg1);
          }
      }


    @Override
        public void onCreate() {
            serviceState=true;
            mNM = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);
            HandlerThread thread = new HandlerThread("ServiceStartArguments",1);
            thread.start();

            // Get the HandlerThread's Looper and use it for our Handler 
            mServiceLooper = thread.getLooper();
            mServiceHandler = new ServiceHandler(mServiceLooper);

        }



     @Override
        public int onStartCommand(Intent intent, int flags, int startId) {
             Log.d("SERVICE-ONCOMMAND","onStartCommand");  

               Bundle extra = intent.getExtras();
               if(extra != null){
                   String downloadUrl = extra.getString("downloadUrl");
                   Log.d("URL",downloadUrl);

                   this.downloadUrl=downloadUrl;
               }

              Message msg = mServiceHandler.obtainMessage();
              msg.arg1 = startId;
              mServiceHandler.sendMessage(msg);

              // If we get killed, after returning from here, restart
              return START_STICKY;
          }



     @Override
          public void onDestroy() {

             Log.d("SERVICE-DESTROY","DESTORY");
             serviceState=false;
            //Toast.makeText(this, "service done", Toast.LENGTH_SHORT).show(); 
          }


     @Override
      public IBinder onBind(Intent intent) {
          // We don't provide binding, so return null
          return null;
      }


      public void downloadFile(){

            downloadFile(this.downloadUrl,fileName);


      }



        void showNotification(String message,String title) {
        // In this sample, we'll use the same text for the ticker and the expanded notification
        CharSequence text = message;

        // Set the icon, scrolling text and timestamp
       Notification notification = new Notification(R.drawable.icon, "vvs",
                System.currentTimeMillis());
        notification.flags |= Notification.FLAG_AUTO_CANCEL;
        Intent intent = new Intent(this, HomeScreenActivity.class);
        intent.setFlags (Intent.FLAG_ACTIVITY_CLEAR_TOP);
         //The PendingIntent to launch our activity if the user selects this notification
        PendingIntent contentIntent = PendingIntent.getActivity(this.getBaseContext(), 0,
              intent, PendingIntent.FLAG_CANCEL_CURRENT);

        // Set the info for the views that show in the notification panel.
        notification.setLatestEventInfo(this, title,
                      text, contentIntent);
        // Send the notification.
        // We use a layout id because it is a unique number.  We use it later to cancel.
        mNM.notify(R.string.app_name, notification);
    }

  public void downloadFile(String fileURL, String fileName) {

    StatFs stat_fs = new StatFs(Environment.getExternalStorageDirectory().getPath());
    double avail_sd_space = (double)stat_fs.getAvailableBlocks() *(double)stat_fs.getBlockSize();
    //double GB_Available = (avail_sd_space / 1073741824);
    double MB_Available = (avail_sd_space / 10485783);
    //System.out.println("Available MB : " + MB_Available);
    Log.d("MB",""+MB_Available);
   try {
        File root =new File(Environment.getExternalStorageDirectory()+"/vvveksperten");
        if(root.exists() && root.isDirectory()) {

        }else{
            root.mkdir();
        }
        Log.d("CURRENT PATH",root.getPath());
        URL u = new URL(fileURL);
        HttpURLConnection c = (HttpURLConnection) u.openConnection();
        c.setRequestMethod("GET");
        c.setDoOutput(true);
        c.connect();
          int fileSize  = c.getContentLength()/1048576;
          Log.d("FILESIZE",""+fileSize);
          if(MB_Available <= fileSize ){
               this.showNotification(getResources().getString(R.string.notification_no_memory),getResources().getString(R.string.notification_error));
              c.disconnect();
              return;
          } 

        FileOutputStream f = new FileOutputStream(new File(root.getPath(), fileName));

        InputStream in = c.getInputStream();


        byte[] buffer = new byte[1024];
        int len1 = 0;
        while ((len1 = in.read(buffer)) > 0) {
            f.write(buffer, 0, len1);
        }
        f.close();
        File file = new File(root.getAbsolutePath() + "/" + "some.pdf");
        if(file.exists()){
            file.delete();
            Log.d("FILE-DELETE","YES");
        }else{
            Log.d("FILE-DELETE","NO");
        }
        File from =new File(root.getAbsolutePath() + "/" + fileName);
        File to = new File(root.getAbsolutePath() + "/" + "some.pdf");


        } catch (Exception e) {
        Log.d("Downloader", e.getMessage());

    }