如何使用子类中的变量集形成子类中的主类？

Question

我有一个Activity，其中有一个PlaceholderFragment 子类。我正在尝试将int 传递给PlaceholderFragment（这显然是有效的，我返回int 并将其打印到logcat，它确实是正确的），但是当onCreateView() 方法运行时，被传递的int突然消失并重置为0。我做错了什么？

这是我的Activity 中负责传递int 的代码：

public void setActivityTitle(){
    toolbar = findViewById(R.id.toolbar);

    resName = getIntent().getExtras().getString("Name");
    this.ResID = getIntent().getExtras().getInt("ResID");

    PlaceholderFragment pf = new PlaceholderFragment(ResID);
    //↑ I know that PlaceholderFragments should almost always have default constructors, but I was desperate. The default empty constructor is still there though
    pf.receiveResID(ResID);
    //↑ This method is basically a setter for the ResID int
    Log.e("SplitBill","ResID that we got from PlaceholderFragment is " + pf.getResID());
    //↑ This output is as expected, and matches the int that I pass down
    toolbar.setTitle(resName);
}

这里是PlaceholderFragment中int的相关声明字段、构造函数和getter/setter：

    private int ResID;

    public PlaceholderFragment() {

    }

    public PlaceholderFragment(int resID) {
        this.ResID = resID;
        Log.e("SplitBill","PlaceholderFragment: Received ResID as " + ResID);
        //↑ This returns the correct int to logcat
    }

    public int getResID() {
        return ResID;
        //Outside the subclass, this returns the correct int. Inside it, this returns 0
    }

    public void receiveResID(int ResID) {
        this.ResID = ResID;
        Log.e("SplitBill","PlaceholderFragment: Received ResID as " + ResID);
        //↑ This also works as expected
    }

这是我的onCreaeView() 中应该在数据库查询中使用int 的部分：

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        final View rootView = inflater.inflate(R.layout.fragment_menu, container, false);
        MenuViewModel viewModel = ViewModelProviders.of(this).get(MenuViewModel.class);
        //MenuActivity m = new MenuActivity();
        //int resID = m.getResID();
        //↑ This was my attempt at trying to get the int straight from a getter in the MenuActivity, it returns 0 
        Log.e("SplitBill","From onCreateView(): ResID is " + ResID);
        //↑ This also prints 0 to logcat
        viewModel.setResID(ResID);

编辑：SectionsPagerAdapter 是 MenuActivity 中的另一个子类。它负责获取PlaceholderFragment 的实例。这是它的全部内容：

public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        return PlaceholderFragment.newInstance(position + 1);
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }
}

Answer 1

您应该只对 Fragments 使用空构造函数。

public static MyFragment newInstance(int someInt) {
    MyFragment myFragment = new MyFragment();

    Bundle args = new Bundle();
    args.putInt("someInt", someInt);
    myFragment.setArguments(args);

    return myFragment;
}

MyFragment fragment = MyFragment.newInstance(5); 是获取片段实例并设置值的方式。

要在片段的onCreate() 方法中使用getArguments().getInt("someInt", 0); 来获取值