java - ArrayList 每次返回 fragment 时将他的大小加倍

Question

我是一名新的 Android 开发人员，我尝试创建一些旅游应用程序"

在我的应用程序中，我有 HomeFragment，它有 3 个按钮: “景点” “事件” “酒店”

public class HomeFragment extends Fragment implements View.OnClickListener {

private ArrayList<Category> mAttractions= new ArrayList<>();
private ArrayList<Category> mEvents= new ArrayList<>();
private ArrayList<Category> mHotels= new ArrayList<>();


private int mAttractionsBtnId = R.id.attractions_btn;
private int mEventsBtnId = R.id.events_btn;
private int mHotelsBtnId = R.id.hotels_btn;

public HomeFragment() {
    // Required empty public constructor
}

public void initCategoriesData() {
    // init attraction array list
    mAttractions.add(new Category("Jaffa Port",5,R.drawable.attractions_jaffa_port));
    mAttractions.add(new Category("Beit Hatfusot",5,R.drawable.attractions_beit_hatfutsot));

    mAttractions.add(new Category("Independence Hall",5,R.drawable.attractions_independence_hall));

    mAttractions.add(new Category("Jaffa Bazar",5,R.drawable.attractions_jaffa_bazar));

    mAttractions.add(new Category("Neve Tzedek St",5,R.drawable.attractions_neve_tzedek));

    mAttractions.add(new Category("Rothschild Boulevard St",5,R.drawable.attractions_rothschild_boulevard));


    // init Events array list
    mEvents.add(new Category("cacdas",3));
    mEvents.add(new Category("cacdas",3));
    mEvents.add(new Category("cacdas",3));
    mEvents.add(new Category("cacdas",3));
    mEvents.add(new Category("cacdas",3));
    mEvents.add(new Category("cacdas",3));
    mEvents.add(new Category("cacdas",3));
    mEvents.add(new Category("cacdas",3));

    // init Hotels array list
    mHotels.add(new Category("hotels",3));
    mHotels.add(new Category("hotels",3));
    mHotels.add(new Category("hotels",3));
    mHotels.add(new Category("hotels",3));
    mHotels.add(new Category("hotels",3));
    mHotels.add(new Category("hotels",3));




}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_home, container, false);

    // Initiate data for each category
    initCategoriesData();


    // Find Buttons
    Button attractionsBtn = (Button)view.findViewById(R.id.attractions_btn);
    Button hotelsBtn = (Button)view.findViewById(R.id.hotels_btn);
    Button eventsBtn = (Button)view.findViewById(R.id.events_btn);

    // Set buttons OnClickListener
    attractionsBtn.setOnClickListener(this);
    hotelsBtn.setOnClickListener(this);
    eventsBtn.setOnClickListener(this);


    return view;
}


@Override
public void onClick(View v) {
    // Start new Fragment when Button Clicked
    FragmentManager fragmentManager = getFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction().addToBackStack(null);

    Bundle bundle = new Bundle();


    // Pass the right data in the bundle
    switch (v.getId()){
        case R.id.attractions_btn:
            bundle.putSerializable("Data",mAttractions);
            break;

        case R.id.hotels_btn:
            bundle.putSerializable("Data",mHotels);
            break;

        case R.id.events_btn:
            bundle.putSerializable("Data",mEvents);
            break;

    }

    ArrayList<Category> cat = (ArrayList<Category>)bundle.getSerializable("Data");
    int size = cat.size();

    MasterFragment masterFragment = new MasterFragment();
    masterFragment.setArguments(bundle);

    fragmentTransaction.replace(R.id.fragment_container, masterFragment);
    fragmentTransaction.commit();
}

因为所有的内容都是一样的(只是类别名称不同) 我决定使用 MasterFragment 以避免重复每个类别的代码。

所有“数据”都在 HomeFragment 中创建，每次用户单击其中一个按钮时，应用程序都会创建新的 MasterFragment 并将 Bundle 中的数据发送给他，以便让他知道他需要哪些数据以显示。

这是“MasterFragment”的样子:

public class MasterFragment extends Fragment {

private ArrayList<Category> mData;

public MasterFragment() {
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle bundle) {
    // Inflate the layout for this fragment
    View rootView = inflater.inflate(R.layout.item_list, container, false);
    mData = (ArrayList<Category>)getArguments().getSerializable("Data");

    // Create an {@link WordAdapter}, whose data source is a list of {@link Category}s. The
    // adapter knows how to create list items for each item in the list.
    CategoryAdapter adapter = new CategoryAdapter(getActivity(), mData, R.color.colorAccent);

    // Find the {@link ListView} object in the view hierarchy of the {@link Activity}.
    // There should be a {@link ListView} with the view ID called list, which is declared in the
    // word_list.xml layout file.
    ListView listView = (ListView) rootView.findViewById(R.id.list);

    // Make the {@link ListView} use the {@link WordAdapter} we created above, so that the
    // {@link ListView} will display list items for each {@link Word} in the list.
    listView.setAdapter(adapter);


    return rootView;
}

一切正常，当用户点击例如 AttractionButton 时，应用程序显示的内容很棒(只有设计不好，哈哈) 但!! 当我单击“后退按钮”并返回到 HomeFragment 时 然后再次单击 AttractionButton 它会复制 Arraylist (从 6 项到 12 项等等……)

我通过在“initCategoriesData”方法中初始化 ArrayList 变量解决了这个问题

但我还是不明白为什么这个应用会像她那样做 我很欣赏对此的一个很好的解释，它真的会帮助我更好地理解事情的运作方式

提前致谢，Niv :)

Answer 1

因为你的 fragment 没有被破坏，当你回来的时候，你的数组列表已经有 6 个项目(因为 fragment 没有被重新创建)。所以你也添加了 6 个项目，arraylist 大小变为 12。你可以做这样的事情来解决这个问题。

private ArrayList<Category> mAttractions;
private ArrayList<Category> mEvents;
private ArrayList<Category> mHotels;

public void initCategoriesData() {

//initialize array list here

mAttractions= new ArrayList<>()
mEvents= new ArrayList<>();
mHotels= new ArrayList<>();

// init attraction array list
mAttractions.add(new Category("Jaffa Port",5,R.drawable.attractions_jaffa_port));
mAttractions.add(new Category("Beit Hatfusot",5,R.drawable.attractions_beit_hatfutsot));

mAttractions.add(new Category("Independence Hall",5,R.drawable.attractions_independence_hall));

mAttractions.add(new Category("Jaffa Bazar",5,R.drawable.attractions_jaffa_bazar));

mAttractions.add(new Category("Neve Tzedek St",5,R.drawable.attractions_neve_tzedek));

mAttractions.add(new Category("Rothschild Boulevard St",5,R.drawable.attractions_rothschild_boulevard));


// init Events array list
mEvents.add(new Category("cacdas",3));
mEvents.add(new Category("cacdas",3));
mEvents.add(new Category("cacdas",3));
mEvents.add(new Category("cacdas",3));
mEvents.add(new Category("cacdas",3));
mEvents.add(new Category("cacdas",3));
mEvents.add(new Category("cacdas",3));
mEvents.add(new Category("cacdas",3));

// init Hotels array list
mHotels.add(new Category("hotels",3));
mHotels.add(new Category("hotels",3));
mHotels.add(new Category("hotels",3));
mHotels.add(new Category("hotels",3));
mHotels.add(new Category("hotels",3));
mHotels.add(new Category("hotels",3));

希望对你有帮助。