JSONParser到ListView需要双倍

Question

我有一个ListView，显示了从OMDBAPI搜索电影的标题，当点击搜索btn时，我需要点击两次而不是一次。

请帮帮忙。

代码如下：

public class JSONParser extends AsyncTask<String, String, String> {

    static ArrayList<String> movieList = new ArrayList<>();

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

        HttpURLConnection httpURLConnection = null;
        BufferedReader reader = null;
        StringBuilder builder = new StringBuilder();

        try {

            URL url = new URL(params[0]);
            httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.connect();
            InputStream stream = httpURLConnection.getInputStream();
            reader = new BufferedReader(new InputStreamReader(stream));
            String line = "";
            while ((line = reader.readLine()) != null){
                builder.append(line);

            }
            return builder.toString();


        } catch (IOException e) {
            e.printStackTrace();
        }

        finally {
            if(httpURLConnection != null){
                httpURLConnection.disconnect();
            }
            try {
                if (reader != null){
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return "Failed Connection";
    }



    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);

        try {
            JSONObject jsonObject = new JSONObject(result);
            JSONArray jsonArray = jsonObject.getJSONArray("Search");
            StringBuilder stringBuilder = new StringBuilder();
            for (int i = 1; i<jsonArray.length(); i++){
                JSONObject finalJsonObject = jsonArray.getJSONObject(i);
                String titleOfMovie = finalJsonObject.getString("Title");
                movieList.add(titleOfMovie);
                stringBuilder.append(movieList);
                finalJsonObject.get("Title");

            }


        } catch (JSONException e) {
            e.printStackTrace();
        }

    }
}

下面是mainActivivty：

public class Main2ActivityWeb extends AppCompatActivity {

    EditText searchBar;
    ListView listOfMovies;
    Button searchBtn;
    Button deleteBtn;
    ArrayAdapter<String> arrayAdapter;
    String url = "http://www.omdbapi.com/?s=";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2_activity_web);

        searchBar = (EditText) findViewById(R.id.etsearch);
        listOfMovies = (ListView) findViewById(R.id.lvMovies);
        searchBtn = (Button) findViewById(R.id.btnSearch);
        deleteBtn = (Button) findViewById(R.id.btnClear);

        searchBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                arrayAdapter = null;
                final String search = url + searchBar.getText().toString();
                new JSONParser().execute(search);
                ArrayList<String> list = new ArrayList<>(JSONParser.movieList);
                arrayAdapter = new ArrayAdapter<>(Main2ActivityWeb.this,
                        android.R.layout.simple_list_item_1, list);
                listOfMovies.setAdapter(arrayAdapter);
                if (arrayAdapter !=null){
                    JSONParser.movieList.clear();
                }



            }
        });

        deleteBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                listOfMovies.setAdapter(null);
                JSONParser.movieList.clear();
                searchBar.setText("");
            }
        });


    }
}

Answer 1

您的问题是，您似乎不理解AsyncTask是异步的概念，所以

new JSONParser().execute(search);

在onPostExecute完成之前，不会将任何内容存储到JSONParser.movieList中。

同时，您可以继续同步运行

ArrayList<String> list = new ArrayList<>(JSONParser.movieList);

当您第一次单击该按钮时，它很可能是空的，因为您仍在等待AsyncTask在后台完成对JSON的解析。

在第二次单击该按钮时，您将启动一个对JSON的新请求，但是您看到的结果来自第一个请求。

解决这个问题的一种方法是回调，就像Volley，Retrofit/OkHttp一样，但你也可以移动你使用的对象，以便它们以正确的顺序执行。例如，给AsyncTask提供ArrayAdapter，而不是让它维护自己的ArrayList

private ArrayList<String> list;

...

onCreate() {
    listOfMovies = (ListView) findViewById(R.id.lvMovies);
    list = new ArrayList<>();
    arrayAdapter = new ArrayAdapter<>(Main2ActivityWeb.this,
                        android.R.layout.simple_list_item_1, list);
    listOfMovies.setAdapter(arrayAdapter);

...
    onClick() {
        ...
        new JSONParser(arrayAdapter).execute(search);
    }
}

在AsyncTask中

public class JSONParser extends AsyncTask<String, String, String> {

    private ArrayAdapter<String> adapter;

    public JSONParser(ArrayAdapter<String> adapter) {
        this.adapter = adapter;
    }

    ...

    onPostExectute() {
        for (...) {
           adapter.add(titleOfMovie);
        }
        apdater.notifyDataSetChanged();
    }
}