AsyncTask nullPointerException按下后退按钮 [英] AsyncTask nullPointerException pressing back button

问题描述

这是我的情况.我有这个AsyncTask:

private class Logo extends AsyncTask<Void, Void, Void> {
        @Override
        protected void onPreExecute() {
            super.onPreExecute(); 
        }

        @Override
        protected Void doInBackground(Void... params) {

            try {
                // Connect to the web site
                Document document = Jsoup.connect(BLOG_URL).get();
                // Using Elements to get the class data 
                // Locate the src attribute
                for(Element img : document.select("div.col-1-1 .image img[src]")) {
                    String ImgSrc = img.attr("src");
                    // Download image from URL
                    InputStream is = new java.net.URL(ImgSrc).openStream();
                    //add Bitmap to an array
                    bitmap.add(BitmapFactory.decodeStream(is));
                 }
            } catch (IOException e) {
                e.printStackTrace();
                Log.e("ESEMPIO", "ERRORE NEL PARSING DELLE IMMAGINI");
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {

            ParsingAdapterCategorie adapter = new ParsingAdapterCategorie(getActivity(), titoli, bitmap, data);
            lista.setAdapter(adapter);

        }
    }

如果我按下后退"按钮，它将崩溃，并显示日志错误，例如:

FATAL EXCEPTION: main
java.lang.NullPointerException
at android.widget.ArrayAdapter.init(ArrayAdapter.java:310)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:166)

我认为这是因为活动结果为null.我该怎么办?

解决方案

基本上，您必须将onPostExecute()的代码放入if(getActivity()！= null)中，以防止出现诸如旋转屏幕之类的情况.

但是您从哪里开始AsyncTask execute()?您必须尊重片段的生命周期...例如，应在 Fragment.onActivityCreated()或 Fragment.onResume() .. NOT上调用它 Fragment.onCreate() 的原因，因为此时尚未进行活动.因此，getActivity()始终为 NULL .

This is my situation. I have this AsyncTask:

private class Logo extends AsyncTask<Void, Void, Void> {
        @Override
        protected void onPreExecute() {
            super.onPreExecute(); 
        }

        @Override
        protected Void doInBackground(Void... params) {

            try {
                // Connect to the web site
                Document document = Jsoup.connect(BLOG_URL).get();
                // Using Elements to get the class data 
                // Locate the src attribute
                for(Element img : document.select("div.col-1-1 .image img[src]")) {
                    String ImgSrc = img.attr("src");
                    // Download image from URL
                    InputStream is = new java.net.URL(ImgSrc).openStream();
                    //add Bitmap to an array
                    bitmap.add(BitmapFactory.decodeStream(is));
                 }
            } catch (IOException e) {
                e.printStackTrace();
                Log.e("ESEMPIO", "ERRORE NEL PARSING DELLE IMMAGINI");
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {

            ParsingAdapterCategorie adapter = new ParsingAdapterCategorie(getActivity(), titoli, bitmap, data);
            lista.setAdapter(adapter);

        }
    }

If I press the back button it crashes with a log error like:

FATAL EXCEPTION: main
java.lang.NullPointerException
at android.widget.ArrayAdapter.init(ArrayAdapter.java:310)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:166)

This is because the activity result is null, I think. What can I do?

解决方案

Basically you have to put the code of the onPostExecute() inside a if(getActivity() != null)...to prevent cases like rotating screen..

BUT where do you start your AsyncTask execute()? You have to respect the lifecycle of the fragment...for instance, you should call it on the Fragment.onActivityCreated() OR Fragment.onResume()..NOT on the Fragment.onCreate() because at this point the activity is not attached yet..therefore getActivity() will always be NULL.

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