AsyncTask onPreExecute() 中未显示进度条

Question

我目前的项目涉及从服务器下载数据 - 这大约需要 20 秒，所以在此期间我想显示一个进度条，以便用户知道发生了什么。下载任务在 DownloadWebpageTask (AsyncTask) 中执行，我在 preExecute() 中设置了进度条可见性，但它没有显示出来。我已经在这里查看了一大堆问题，但我仍然无法让它发挥作用。如果我不隐藏 onPostExecute() 中的进度条，我可以看到发生了什么——即使我在 onPreExecute() 中设置了 progBar.setVisibility(0)，该条实际上直到 doInBackground 之后才显示。我也尝试在 onPreExecute 中设置一个 TextView 以查看它是否只是进度条不起作用，但这也失败了。

请有人帮帮我，我完全被卡住了！！

这调用了我的 AsyncTask：

public void getDTCs(View view) throws URISyntaxException{       

        DtcListSingleton.getInstance().setData(null);
        status = "DTC Parsing Not Attempted";
        noDTCs = 0;

        TextView listTitle = (TextView) findViewById(R.id.DTCtitle);
        String newTitle = "Waiting for DTCs";
        listTitle.setText(newTitle);


        String stringURL = "http://192.168.1.129";
        //Check connection
        ConnectivityManager cMgr = (ConnectivityManager)getSystemService(this.CONNECTIVITY_SERVICE);
        NetworkInfo netInf = cMgr.getActiveNetworkInfo();

        // If connected to network carry on with DTC request
        if(netInf != null && netInf.isConnected()){

            //Download the web page as an InputStream
            new DownloadWebpageTask().execute(stringURL);

这是我的 AsyncTask：

private class DownloadWebpageTask extends AsyncTask<String,Void,String>{
        @Override 
        protected void onPreExecute(){
            super.onPreExecute();
            progBar = (ProgressBar)findViewById(R.id.loadingDTCs);
            progBar.setVisibility(0);
            TextView title = (TextView)findViewById(R.id.DTCtitle);
            title.setText("Loading DTCs");
        }
        @Override
        protected String doInBackground(String...urls){
            //params[0] is the url
            try{
                return downloadUrl(urls[0]);
            }
            catch(IOException e){
                return "Unable to retreieve web page. URL may be invalid.";
            } 

            // DELETE if overcome client.print delay
            catch (InterruptedException e) {
                e.printStackTrace();
                return "Interrupted Exception from Delaying inputstream"; 
            }
        }
        @Override
        protected void onPostExecute(String result){

            AlertDialog.Builder resultURL = new AlertDialog.Builder(ActionReadDTCs.this);

            resultURL.setTitle("Result of DownloadWebPageTask")
            .setMessage(result)

            .setPositiveButton("OK", new DialogInterface.OnClickListener(){
                public void onClick(DialogInterface dialog, int id){
                    progBar = (ProgressBar)findViewById(R.id.loadingDTCs);
                    progBar.setVisibility(8);
                    return;
                }
            })

            .show();
        }
    }

Answer 1

示例代码看看这是否有效？如果这有效，则意味着您在显示和隐藏视图时犯了一些错误

<?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/btnDownload"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_gravity="center"
       android:text="@string/download"
       android:textSize="14sp" />
    <ImageView
       android:id="@+id/imageView"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:contentDescription="@string/your_image_will_appear_here" />
</LinearLayout>

还有你的 java 类

private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
    /** Reference to the view which should receive the image */
    private final WeakReference imageRef;public DownloadImageTask(ImageView imageView) {
        imageRef = new WeakReference(imageView);
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        progressDialog = ProgressDialog.show(DownloadImageActivity.this, "Wait", "Downloading...");
    }

    @Override
    protected Bitmap doInBackground(String... params) {
        InputStream input = null;
        try {
            URL url = new URL(params[0]);
            // We open the connection
            URLConnection conection = url.openConnection();
            conection.connect();
            input = new BufferedInputStream(url.openStream(), 8192);
            // we convert the inputStream into bitmap
            bitmap = BitmapFactory.decodeStream(input);
            input.close();
        } catch (Exception e) {
            Log.e("Error: ", e.getMessage());
        }
        return bitmap;
    }

    /**
    * After completing background task Dismiss the progress dialog
    * **/
    protected void onPostExecute(Bitmap bitmap) {
        progressDialog.dismiss();
        if (isCancelled()) {
            bitmap = null;
        }

        if (imageRef != null) {
            ImageView imageView = imageRef.get();
            if (imageView != null &amp;&amp; bitmap != null) {
                imageView.setImageBitmap(bitmap);
            } else
                Toast.makeText(DownloadImageActivity.this, "Error while downloading the image!", Toast.LENGTH_LONG).show();
        }
    }
}



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

See the result

Answer 2

感谢您的回复，但我已经找到了我的问题。

我还没有发布我的所有代码 - 我实际上是在 AsyncTask 之后调用另一个函数，一旦我将 new DownloadWebpageTask.execute(String url) 之后的所有代码移动到 onPostExecute() 中，我的进度条就显示出来了！