Question

MainActivity无法识别我的xml文件中的gridview

这是mainActivity onCreate（）：

protected void onCreate(Bundle savedInstanceState) {
    mContext = this;
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mMovieFragment = new MovieFragment();
    if(savedInstanceState == null){
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, mMovieFragment)
                .commit();
    }
    if(this == null){
        Log.v(null, "THIS is empty");
    }
    GridView gridView = (GridView) findViewById(R.id.gridview);
    if(gridView == null){
        Log.v(null, "GridView is empty");
    }
    /*gridView.setAdapter(new ImageAdapter(this, mMovieFragment.getArrayImages()));

    gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v,
                                int position, long id) {
            Toast.makeText(MainActivity.this, "" + position,
                    Toast.LENGTH_SHORT).show();
        }
    });*/
}

然后我有2个xml文件，activity_main.xml：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/container"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     tools:context=".MainActivity"
     tools:ignore="MergeRootFrame" />

和fragment_main.xml：

<?xml version="1.0" encoding="utf-8"?>
<GridView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/gridview"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:columnWidth="90dp"
    android:numColumns="auto_fit"
    android:verticalSpacing="10dp"
    android:horizontalSpacing="10dp"
    android:stretchMode="columnWidth"
    android:gravity="center"
    tools:context=".MainActivity$MovieFragment">

</GridView>

gridView = findViewById（R.id.gridview）为空

MovieFragment.java：

public class MovieFragment extends Fragment{

public ArrayAdapter<String> mMovieAdapter;
public String[] mArrayImages;

public MovieFragment(){
    mArrayImages = new String[20];
}

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

}

@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
    super.onCreateOptionsMenu(menu, inflater);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    return super.onOptionsItemSelected(item);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    new FetchMoviesTask().execute("http://api.themoviedb.org/3/movie/popular?api_key=37068e0a72b2cc1751b4246899923ba7");

}

public String[] getArrayImages() {
    return mArrayImages;
}

public class FetchMoviesTask extends AsyncTask<String, Void, Void> {

    private final String LOG_TAG = FetchMoviesTask.class.getSimpleName();

    private Void getMovieImagesFromJson(String movieJSONstr) throws JSONException {

        final String OWM_RESULTS = "results";
        final String OWM_POSTERPATH = "poster_path";

        JSONObject movieJson = new JSONObject(movieJSONstr);
        JSONArray movieArray = movieJson.getJSONArray(OWM_RESULTS);

        Log.v("SIZE MOVIE ARRAY: ", Integer.toString(movieArray.length()));

        for(int i = 0; i < movieArray.length(); i++) {
            JSONObject movieObj = movieArray.getJSONObject(i);
            mArrayImages[i] = movieObj.getString(OWM_POSTERPATH);
            Log.v("String "+i+":", mArrayImages[i]);
        }

        return null;
    }

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

        HttpURLConnection urlConnection = null;
        BufferedReader reader = null;

        String movieJSONstr = null;

        try{
            URL url = new URL(params[0]);
            urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.setRequestMethod("GET");
            urlConnection.connect();

            InputStream inputStream = urlConnection.getInputStream();
            StringBuffer buffer = new StringBuffer();
            if(inputStream == null){
                return null;
            }
            reader = new BufferedReader(new InputStreamReader(inputStream));

            String line;
            while((line = reader.readLine()) != null){
                buffer.append(line + "\n");
            }

            if(buffer.length() == 0){
                movieJSONstr = null;
            }
            movieJSONstr = buffer.toString();
            Log.v("URLoutput: ", movieJSONstr);
        } catch (IOException e) {
            Log.e(LOG_TAG, "Error ", e);

            return null;
        } finally{
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
            if (reader != null) {
                try {
                    reader.close();
                } catch (final IOException e) {
                    Log.e("MovieFragment", "Error closing stream", e);
                }
            }
        }

        try{
            return getMovieImagesFromJson(movieJSONstr);
        }catch (JSONException e) {
            Log.e(LOG_TAG, e.getMessage(), e);
            e.printStackTrace();
        }

        return null;
    }
}
}

Answer 1

你应该在MovieFragment的onCreateView方法中获得gridview引用

Answer 2

问题在于片段添加。

来自https://developer.android.com/reference/android/app/FragmentTransaction.html#commit()

提交不会立即执行，稍后会完成

在您查找网格时，片段尚未附加，因此此时网格不存在

在检索视图之前，您必须等待该片段真正附加

Answer 3

对于findViewById来电，您需要对片段xml进行充气，然后使用返回的视图“查找”您的观点。

 @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        GridView grid = rootView.findViewById(R.id.gridview);
        new FetchMoviesTask().execute("http://api.themoviedb.org/3/movie/popular?api_key=37068e0a72b2cc1751b4246899923ba7");

    }

您还可以通过让onPostExecute返回字符串数组

，轻松地在AsyncTasks doInBackground中设置网格适配器

public class FetchMoviesTask extends AsyncTask<String, Void, String[]> {

 @Override
 protected String[] doInBackground(String... params) {
    //get your string array.
     //mArrayImages = assign the value 

      return mArrayImages;
     }
@Override
protected onPostExecute(String[] strings) {
    gridView.setAdapter(new ImageAdapter(getActivity(), strings);
  }

MainActivity找不到gridView

3 个答案: