啟動畫面使用AsyncTask

Question

我有一個使用AsyncTask的啟動畫面，它將從數據庫下載一些數據並將數據存儲在ArrayList 。 此ArrayList將在MainActivity.class片段中用於RecyclerView 。

問題是，當我從Android Studio運行應用程序到手機時，一切運行正常。 但是，當我銷毀該應用程序並通過手機手動運行它時，它將顯示空白屏幕，然后崩潰。 如果崩潰后我再次運行，該應用程序將運行。 因此，僅當我從Android Studio運行它或當它崩潰后，此應用程序始終可以運行。

該錯誤表明它是由空列表引起的。 如果我沒記錯的話，我認為活動被銷毀后， AsyncTask似乎無法正常工作。 但是我不知道如何解決。 請幫我解決這個問題。

SplashScreen.java

public class SplashScreenActivity extends AppCompatActivity {
    public static Event event;
    private static List<Feed> feedList;
    private static List<Event> eventList;
    private static List<Event> todayList;
    private static List<Event> upcomingList;
    private static List<Partner> partnerList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_splashscreen);
        Time today = new Time(Time.getCurrentTimezone());
        today.setToNow();
        Config.TODAY_DATE = String.valueOf(today.monthDay) + "-" + String.valueOf(today.month) + "-" + String.valueOf(today.year);
        new DownloadData().execute("");
    }

    class DownloadData extends AsyncTask<String, Integer, String>
    {
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            startActivity(new Intent(getBaseContext(), WelcomeActivity.class));
            finish();
        }

        @Override
        protected String doInBackground(String... params) {
            RequestHandler rh = new RequestHandler();
            String JSON_STRING = rh.sendGetRequest(Config.URL_GET_ALL_DATA);
            JSONObject jsonObject;
            eventList       = new ArrayList<>();
            todayList       = new ArrayList<>();
            upcomingList    = new ArrayList<>();
            partnerList     = new ArrayList<>();
            feedList        = new ArrayList<>();
            try {
                jsonObject = new JSONObject(JSON_STRING);

                JSONArray getEvent = jsonObject.getJSONArray(Config.TAG_JSON_EVENT);
                for (int i = 0; i < getEvent.length(); i++) {
                    int id              = getEvent.getJSONObject(i).getInt(Config.TAG_ID);
                    int eoId        = getEvent.getJSONObject(i).getInt(Config.TAG_EO_ID);
                    String eoName       = getEvent.getJSONObject(i).getString(Config.TAG_EO_NAME);
                    String title        = getEvent.getJSONObject(i).getString(Config.TAG_TITLE);
                    String day      = getEvent.getJSONObject(i).getString(Config.TAG_DAY);
                    String date         = getEvent.getJSONObject(i).getString(Config.TAG_DATE);
                    int price       = getEvent.getJSONObject(i).getInt(Config.TAG_PRICE);
                    event = new Event(id, eoId, eoName, title, day, date, price);
                    eventList.add(event);
                    if(Config.TODAY_DATE.equals(event.getDate())){
                        todayList.add(event);
                    } else {
                        upcomingList.add(event);
                    }

                }

                JSONArray getPartner = jsonObject.getJSONArray(Config.TAG_JSON_PARTNER);
                for (int i = 0; i < getPartner.length(); i++) {
                    int pId         = getPartner.getJSONObject(i).getInt(Config.TAG_ID);
                    String pName    = getPartner.getJSONObject(i).getString(Config.TAG_NAME);
                    String pEmail   = getPartner.getJSONObject(i).getString(Config.TAG_EMAIL);
                    String pPhone   = getPartner.getJSONObject(i).getString(Config.TAG_PHONE);
                    String pPhoto   = getPartner.getJSONObject(i).getString(Config.TAG_PHOTO_URL);
                    Partner partner = new Partner(pId, pName, pEmail, pPhone, pPhoto);
                    partnerList.add(partner);
                }

                JSONArray getArticle = jsonObject.getJSONArray(Config.TAG_JSON_ARTICLE);
                for (int i = 0; i < getArticle.length(); i++) {
                    int feedId              = getArticle.getJSONObject(i).getInt(Config.TAG_ID);
                    String feedAuthor       = getArticle.getJSONObject(i).getString(Config.TAG_FEED_AUTHOR);
                    String feedTitle        = getArticle.getJSONObject(i).getString(Config.TAG_FEED_TITLE);
                    String feedContent      = getArticle.getJSONObject(i).getString(Config.TAG_FEED_CONTENT);
                    String feedDate         = getArticle.getJSONObject(i).getString(Config.TAG_FEED_DATE);
                    String feedThumbnail    = getArticle.getJSONObject(i).getString(Config.TAG_FEED_THUMBNAIL);
                    Feed feed = new Feed(feedId, feedAuthor, feedTitle, feedContent, feedDate, feedThumbnail);
                    feedList.add(feed);
                }

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

    }

    public static List<Feed> getFeedList(){ return feedList;}
    public static List<Event> getEventList() {return eventList;}
    public static List<Event> getTodayList() { return todayList;}
    public static List<Event> getUpcomingList() { return upcomingList;}
    public static List<Partner> getPartnerList() {return partnerList;}

}

DiscoverFragment.java

public class DiscoverFragment extends Fragment implements ViewPager.OnPageChangeListener, View.OnClickListener {

    protected View view;
    private LinearLayout pager_indicator;
    private int dotsCount;
    private ImageView[] dots;

    private List<Feed> feedList;
    private List<Event> eventList;
    private List<Partner> partnerList;

    public DiscoverFragment() {}

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

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_discover, container, false);

        RecyclerView recyclerViewEvent      = (RecyclerView) view.findViewById(R.id.discover_event_recycler_view);
        RecyclerView recyclerViewPartner    = (RecyclerView) view.findViewById(R.id.discover_partner_recycler_view);

        ClickableViewPager intro_images = (ClickableViewPager) view.findViewById(R.id.pager_introduction);
        pager_indicator         = (LinearLayout) view.findViewById(R.id.viewPagerCountDots);

        eventList       = SplashScreenActivity.getEventList();
        partnerList     = SplashScreenActivity.getPartnerList();
        feedList        = SplashScreenActivity.getFeedList();

        EventAdapter eventAdapter = new EventAdapter(getContext(), eventList);
        DiscoverPartnerAdapter discoverPartnerAdapter = new DiscoverPartnerAdapter(getContext(), partnerList);
    DiscoverFeedAdapter mAdapter = new DiscoverFeedAdapter(getContext(), feedList);

        final LinearLayoutManager layoutManagerEvent = new LinearLayoutManager(getContext());
        final LinearLayoutManager layoutManagerPartner = new LinearLayoutManager(getContext());

        layoutManagerEvent.setOrientation(LinearLayoutManager.HORIZONTAL);
        layoutManagerPartner.setOrientation(LinearLayoutManager.HORIZONTAL);

        addBottomDots(0);
        intro_images.setAdapter(mAdapter);
        intro_images.setCurrentItem(0);
        intro_images.addOnPageChangeListener(this);
        intro_images.setOnItemClickListener(new ClickableViewPager.OnItemClickListener() {
            @Override
            public void onItemClick(int position) {
                Config.FEED_ID = position;
                startActivity(new Intent(getContext(), ArticleActivity.class));
            }
        });

        return view;
    }


    private void addBottomDots(int currentPage) {
        dots = new ImageView[feedList.size()]; //the problem
        pager_indicator.removeAllViews();
        for (int i = 0; i < dots.length; i++) {
            dots[i] = new ImageView(getContext());
            dots[i].setImageDrawable(getResources().getDrawable(R.drawable.nonselecteditem_dot));
            LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
                    LinearLayout.LayoutParams.WRAP_CONTENT,
                    LinearLayout.LayoutParams.WRAP_CONTENT
            );
            params.setMargins(4, 0, 4, 0);
            pager_indicator.addView(dots[i], params);
        }
        if (dots.length > 0)
            dots[currentPage].setImageDrawable(getResources().getDrawable(R.drawable.selecteditem_dot));
    }


    @Override
    public void onClick(View v) {
        switch (v.getId()) {
        }
    }

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
    }

    @Override
    public void onPageSelected(int position) {
        addBottomDots(position);
        for (int i = 0; i < dotsCount; i++) {
            dots[i].setImageDrawable(getResources().getDrawable(R.drawable.nonselecteditem_dot));
        }
        dots[position].setImageDrawable(getResources().getDrawable(R.drawable.selecteditem_dot));
    }
}

LogCat

01-29 00:40:57.565 32535-32535/com.irmaelita.esodiaapp E/AndroidRuntime: FATAL EXCEPTION: main
                                                                         Process: com.irmaelita.esodiaapp, PID: 32535
                                                                         java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
                                                                             at com.irmaelita.esodiaapp.fragment.DiscoverFragment.addBottomDots(DiscoverFragment.java:181)
                                                                             at com.irmaelita.esodiaapp.fragment.DiscoverFragment.onCreateView(DiscoverFragment.java:158)

Answer 1

feedList為null。 您在執行DownloadData任務時創建feedList實例。 但是，應在創建片段視圖時在addBottomDots調用feedList.size() 。 因此，最有可能在執行DownloadData任務之前調用addBottomDots 。 您需要修復它。

Answer 2

發現片段中的供稿列表在初始化時將為空。 請在執行此操作之前先設置一個null檢查。這與從android studio中運行無關。如果我對它的理解正確，則您嘗試在完成后從splasScreenActivity訪問列表。 即在后期執行中，您完成了當前活動，而該片段在主活動中，因此列表為空。因此，如果是這種情況（請更正我的意思），然后將數據集中下載或以最佳方式發送有意圖地進行主要活動並在那里使用它。 另外，當從android studio運行時，請手動殺死該應用程序並再次運行，同時手機已連接並查看它在當前情況下是否崩潰。

Answer 3

使用sendBroadcast將數據從doInBackground發送到MainActivity

在DownloadData類中添加廣播方法

private void broadcast(SplashParcel parcel) {
    Intent i = new Intent("splash_parcel");
    i.putExtra("values", parcel);
    sendBroadcast(i);
}

@Override
protected String doInBackground(String... params) {
    // your code
    // ..
    try {
        // your code
        // ..


        // send splashParcel to MainActivity
        SplashParcel splashParcel = new SplashParcel(feedList, eventList, todayList, upcomingList, partnerList);
        broadcast (splashParcel);   

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

添加新的類SplashParcel.java

 public class SplashParcel implements Parcelable { public static final Creator<SplashParcel> CREATOR = new Creator<SplashParcel>() { @Override public SplashParcel createFromParcel(Parcel in) { return new SplashParcel(in); } @Override public SplashParcel[] newArray(int size) { return new SplashParcel[size]; } }; private static List<Feed> _feedList; private static List<Event> _eventList; private static List<Event> _todayList; private static List<Event> _upcomingList; private static List<Partner> _partnerList; protected SplashParcel(Parcel in) { _feedList = new ArrayList<Feed>(); in.readList(_feedList, null); _eventList = new ArrayList<Event>(); in.readList(_eventList, null); _todayList = new ArrayList<Event>(); in.readList(_todayList, null); _upcomingList = new ArrayList<Event>(); in.readList(_upcomingList, null); _partnerList = new ArrayList<Partner>(); in.readList(_partnerList, null); } public SplashParcel(List<Feed> feedList, List<Event> eventList, List<Event> todayList, List<Event> upcomingList, List<Partner> partnerList) { _feedList = feedList; _eventList = eventList; _todayList = todayList; _upcomingList = upcomingList; _partnerList = partnerList; } public SplashParcel() { } public List<Feed> getFeedList() { return _feedList; } public void setFeedList(List<Feed> feedList) { _feedList = feedList; } public List<Event> getEventList() { return _eventList; } public void setEventList(List<Event> eventList) { _eventList = eventList; } public List<Event> getTodayList() { return _todayList; } public void setTodayList(List<Event> todayList) { _todayList = todayList; } public List<Event> getUpcomingList() { return _upcomingList; } public void setUpcomingList(List<Event> upcomingList) { _upcomingList = upcomingList; } public List<Partner> getPartnerList() { return _partnerList; } public void setPartnerList(List<Partner> partnerList) { _partnerList = partnerList; } @Override public int describeContents() { return 0; } @Override public void writeToParcel(Parcel parcel, int i) { parcel.writeList(_feedList); parcel.writeList(_eventList); parcel.writeList(_todayList); parcel.writeList(_upcomingList); parcel.writeList(_partnerList); } } 

MainActivity.java

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    SplashParcel splashParcel = getArguments().getParcelable("splash_parcel");  
    if(splashParcel != null) {  
        // your splashParcel ready in here
        List<Feed> feedList = splashParcel.getFeedList()
        List<Event> eventList = splashParcel.getEventList()
        List<Event> todayList = splashParcel.getTodayList();
        List<Event> upcommingList = splashParcel.getUpcomingList();
        List<Partner> partnerList = splashParcel.getPartnerList();
    }
}

在DiscoverFragment的 onCreateView中：

 @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { SplashParcel splashParcel = getArguments().getParcelable("splash_parcel"); if(splashParcel != null) { // your splashParcel ready in here List<Feed> feedList = splashParcel.getFeedList() List<Event> eventList = splashParcel.getEventList() List<Event> todayList = splashParcel.getTodayList(); List<Event> upcommingList = splashParcel.getUpcomingList(); List<Partner> partnerList = splashParcel.getPartnerList(); } }