java - 如何修复空 fragment ？

Question

我是 Android 开发的新手，正在尝试创建自己的应用。它应该使用 YouTube 数据 API 显示特定的 YouTube channel 。我从 Android Studio 中的标准底部导航模板开始，并使用 Github 上的以下项目来获得一些启动帮助。 https://github.com/stressGC/Remake-YouTube-Android

我必须更改一些内容，例如代码中已弃用的 http 调用，以使其与新的 Android APK 一起运行。从我的角度来看，一切似乎都很好:我可以看到 API 内容看起来不错，并且每个标题/描述/发布日期都放在相应的变量中。日志中也没有错误消息。当我启动模拟器时，应用程序运行良好。但是一旦我切换到“仪表板” fragment (放置代码的地方)，它就是空的。

DashboardFragment.java

public class DashboardFragment extends Fragment {
    private static String API_KEY = "hidden"; //normaler API key ohne limits, kein oauth
    private static String CHANNEL_ID = "hidden";
    private static String CHANNEL_GET_URL = "https://www.googleapis.com/youtube/v3/search?part=snippet&order=date&channelId="+CHANNEL_ID+"&maxResults=20&key="+API_KEY+"";

    private RecyclerView mList_videos = null;
    private VideoPostAdapter adapter = null;
    private ArrayList<YouTubeDataModel> mListData = new ArrayList<>();

    public DashboardFragment () {

    }

    @Override
    public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.fragment_dashboard, container, false);
        mList_videos = (RecyclerView) view.findViewById(R.id.mList_videos);
        initList(mListData);
        new RequestYouTubeAPI().execute();
        return view;
    }


    private void initList(ArrayList<YouTubeDataModel> mListData) {
        mList_videos.setLayoutManager(new LinearLayoutManager(getActivity()));
        adapter = new VideoPostAdapter(getActivity(), mListData);
        mList_videos.setAdapter(adapter);
    }

    // create asynctask to get data from youtube
    private class RequestYouTubeAPI extends AsyncTask<Void, String, String>{

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected String doInBackground(Void... params) {
            URL url = null;
            String json = null;
            StringBuffer sb = new StringBuffer();

            try {
                url = new URL(CHANNEL_GET_URL);
            } catch (MalformedURLException e) {
                e.printStackTrace();
            }
            try {
                //HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
                HttpURLConnection urlConnection = NetCipher.getHttpsURLConnection(url);
                InputStream in = new BufferedInputStream(urlConnection.getInputStream());
                BufferedReader br = new BufferedReader(new InputStreamReader(in));
                String inputLine = "";
                while ((inputLine = br.readLine()) != null) {
                    sb.append(inputLine);
                }
                json = sb.toString();
                return json;

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


        @Override
        protected void onPostExecute(String response) {
            super.onPostExecute(response);
            if(response != null){
                try {
                    JSONObject jsonObject = new JSONObject(response);
                    Log.e("response", jsonObject.toString());
                    mListData = parseVideoListFromResponse(jsonObject);
                    initList(mListData);
                    //adapter.notifyDataSetChanged();
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public ArrayList<YouTubeDataModel> parseVideoListFromResponse(JSONObject jsonObject) {
        ArrayList<YouTubeDataModel> mList = new ArrayList<>();

        if (jsonObject.has("items")) {
            try {
                JSONArray jsonArray = jsonObject.getJSONArray("items");
                for (int i = 0; i < jsonArray.length(); i++) {
                    JSONObject json = jsonArray.getJSONObject(i);
                    if (json.has("id")) {
                        JSONObject jsonID = json.getJSONObject("id");
                        String video_id = "";
                        if (jsonID.has("videoId")) {
                            video_id = jsonID.getString("videoId");
                        }
                        if (jsonID.has("kind")) {
                            if (jsonID.getString("kind").equals("youtube#video")) {
                                YouTubeDataModel youtubeObject = new YouTubeDataModel();
                                JSONObject jsonSnippet = json.getJSONObject("snippet");
                                String title = jsonSnippet.getString("title");
                                String description = jsonSnippet.getString("description");
                                String publishedAt = jsonSnippet.getString("publishedAt");
                                String thumbnail = jsonSnippet.getJSONObject("thumbnails").getJSONObject("high").getString("url");

                                youtubeObject.setTitle(title);
                                youtubeObject.setDescription(description);
                                youtubeObject.setPublishedAt(publishedAt);
                                youtubeObject.setThumbnail(thumbnail);
                                youtubeObject.setVideo_id(video_id);
                                mList.add(youtubeObject);

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

        return mList;
    }
}

VideoPostAdapter.java

public class VideoPostAdapter extends RecyclerView.Adapter<VideoPostAdapter.YouTubePostHolder> {

    private ArrayList<YouTubeDataModel> dataSet;
    private Context mContext = null;

    public VideoPostAdapter(Context mContext, ArrayList<YouTubeDataModel> dataSet) {
        this.dataSet = dataSet;
        this.mContext = mContext;
    }

    @NonNull
    @Override
    public YouTubePostHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.youtube_post_layout,parent,false);
        YouTubePostHolder postHolder = new YouTubePostHolder(view);
        return postHolder;
    }

    @Override
    public void onBindViewHolder(@NonNull YouTubePostHolder holder, int position) {

        // set the views here
        TextView textViewTitle = holder.textViewTitle;
        TextView textViewDes = holder.textViewDes;
        TextView textViewDate = holder.textViewDate;
        ImageView ImageThumb = holder.ImageThumb;

        YouTubeDataModel object = dataSet.get(position);

        textViewTitle.setText(object.getTitle());
        textViewDes.setText(object.getDescription());
        textViewDate.setText(object.getPublishedAt());
        // image will be downloaded from url
    }

    @Override
    public int getItemCount() {
        return dataSet.size();
    }

    public static class YouTubePostHolder extends RecyclerView.ViewHolder{
        TextView textViewTitle;
        TextView textViewDes;
        TextView textViewDate;
        ImageView ImageThumb;

        public YouTubePostHolder(@NonNull View itemView) {
            super(itemView);
            this.textViewTitle = (TextView) itemView.findViewById(R.id.textViewTitle);
            this.textViewDes = (TextView) itemView.findViewById(R.id.textViewDes);
            this.textViewDate = (TextView) itemView.findViewById(R.id.textViewDate);
            this.ImageThumb = (ImageView) itemView.findViewById(R.id.ImageThumb);
        }
    }
}

YouTubeDataModel.java

public class YouTubeDataModel {
    private String title = "";
    private String description = "";
    private String publishedAt = "";
    private String thumbnail = "";

    public String getVideo_id() {
        return video_id;
    }

    public void setVideo_id(String video_id) {
        this.video_id = video_id;
    }

    private String video_id = "";

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getPublishedAt() {
        return publishedAt;
    }

    public void setPublishedAt(String publishedAt) {
        this.publishedAt = publishedAt;
    }

    public String getThumbnail() {
        return thumbnail;
    }

    public void setThumbnail(String thumbnail) {
        this.thumbnail = thumbnail;
    }
}

youtube_post_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="200dp">

        <ImageView
            android:id="@+id/ImageThumb"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="@color/colorPrimary"/>

        <TextView
            android:id="@+id/textViewDate"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="published at"
            android:singleLine="true"
            android:layout_alignParentRight="true"
            android:layout_margin="5dp"
            android:textColor="@android:color/white"
            android:textSize="12dp"/>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:layout_margin="10dp"
            android:orientation="vertical">

            <TextView
                android:id="@+id/textViewTitle"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="video Title"
                android:singleLine="true"
                android:textColor="@android:color/white"
                android:textSize="22dp"/>

            <TextView
                android:id="@+id/textViewDes"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="video description"
                android:singleLine="true"
                android:textColor="@android:color/white"
                android:textSize="12dp"/>

        </LinearLayout>

    </RelativeLayout>

</LinearLayout>

fragment_dashboard.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/mList_videos"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

不幸的是，我不知道为什么 fragment 仍然是空的。 Android Studio 日志中没有任何错误，我真的希望你能帮助我:/

Answer 1

在您的 RequestYouTubeAPI ASyncTask 中，您有以下错误代码:

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

然后在 onPostExecute 中你有以下内容:

    @Override
    protected void onPostExecute(String response) {
        super.onPostExecute(response);
        if(response != null){
            try {
                JSONObject jsonObject = new JSONObject(response);
                Log.e("response", jsonObject.toString());
                mListData = parseVideoListFromResponse(jsonObject);
                initList(mListData);
                //adapter.notifyDataSetChanged();
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    }

因此，如果您收到错误，您将返回 null，如果 onPostExecute 收到一个 null 响应，它什么也不做。

所以这个地方你可能有错误，因此是一个空白 fragment 。

---

在你解决这个问题之前，你可以证明这是这样发生的:

    @Override
    protected void onPostExecute(String response) {
        super.onPostExecute(response);
        if(response == null){
            Log.e("TUT", "We did not get a response, not updating the UI.");
        } else {
            try {
                JSONObject jsonObject = new JSONObject(response);
                Log.e("response", jsonObject.toString());
                mListData = parseVideoListFromResponse(jsonObject);
                initList(mListData);
                //adapter.notifyDataSetChanged();
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    }

---

您可以通过两种方式解决此问题:

在 doInBackground 中将 catch 更改为:

        } catch (IOException e) {
            Log.e("TUT", "error", e);
            // Change this JSON to match what the parse expects, so you can show an error on the UI
            return "{\"yourJson\":\"error!\"}";
        }

或onPostExecute:

        if(response == null){
            List errorList = new ArrayList();
            // Change this data model to show an error case to the UI
            errorList.add(new YouTubeDataModel("Error");
            mListData = errorList;
            initList(mListData);
        } else {
            try {
                JSONObject jsonObject = new JSONObject(response);
                Log.e("response", jsonObject.toString());
                mListData = parseVideoListFromResponse(jsonObject);
                initList(mListData);
                //adapter.notifyDataSetChanged();
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }

希望对您有所帮助，代码中可能还有其他错误，但如果 API、Json、授权、互联网等出现问题，则可能会发生这种情况。