java - 片段 - 标签、条件和展示

Question

我将非常感谢分析这些代码行，因为我不明白它们之间的关系。特别是如果我我会将标签更改为 2 个不同的标签，应用程序中没有任何变化。另外，如果条件if去掉，也没有变化。评论里我有简短的翻译，这三行我还是看不懂。

Fragment fr = manager.findFragmentByTag("AddCity");

这个条件满足什么作用？

if (fr != null) {// if ???
                manager.beginTransaction().remove(fr).commit();
}

和:

fr2.show(manager, "AddCity");  

这是完整的代码片段

/*
        I create a variable manager, and then I get a FragmentManager and assign it to the variable manager,
         thanks to the variable manager, I can run actions on fragments
         */
        FragmentManager manager = getSupportFragmentManager();

        Fragment fr = manager.findFragmentByTag("AddCity");//I created fr variable and ???
        Fragment1 fr1 = (Fragment1)
                manager.findFragmentById(R.id.list); // I created fr1 variable and assigned fragment list to the variable fr1
        if (view.equals(b1)) {
            if (fr != null) {// if ???
                manager.beginTransaction().remove(fr).commit();//Creates a new transaction to change fragments at runtime, removes an existing fragment, plans to commit the transaction.
            }
            Fragment2 fr2 = new Fragment2();//Created object Fragment2
            fr2.show(manager, "AddCity");//Display the dialog box by adding the fragment to the given FragmentManager.
        }

编辑:

这是我的主要 Activity :

public class MainActivity extends AppCompatActivity implements View.OnClickListener {


    Button b1, b2;
    static int itemSelected;
    static boolean isSelected;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        b1 = findViewById(R.id.button1);
        b2 = findViewById(R.id.button2);
        b1.setOnClickListener(this);
        b2.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {

        FragmentManager manager = getSupportFragmentManager();//
        Fragment fr = manager.findFragmentByTag("addCity");
        Fragment1 fr1 = (Fragment1)
                getSupportFragmentManager().findFragmentById(R.id.list);
        if (view.equals(b1)) {
            if (fr != null) {
                manager.beginTransaction().remove(fr).commit();
            }
            Fragment2 fr2 = new Fragment2();
            fr2.show(manager, "");
        }
        if (view.equals(b2)) {
            if (!fr1.cities.isEmpty()) {
                if (isSelected) {
                    fr1.cities.remove(itemSelected);
                    fr1.adapter.notifyDataSetChanged();
                    itemSelected = 0;
                    isSelected = false;
                    fr1.getListView()
                            .setSelector(android.R.color.transparent);
                } else {
                    Toast.makeText(this, "Select city to be deleted",
                            Toast.LENGTH_LONG).show();
                }
            }
        }

    }
}

片段 1:

public class Fragment1 extends ListFragment {

    ArrayList<String> cities = new ArrayList<>();
    ArrayAdapter<String> adapter;
    String name;

    public Fragment1() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup viewGroup,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_fragment1,
                viewGroup, false);
        adapter = new ArrayAdapter<>(getActivity(),
                android.R.layout.simple_list_item_1, cities);
        setListAdapter(adapter);

        return view;
    }

    public void onListItemClick(ListView listView, View view,
                                int position, long id) {
        MainActivity.itemSelected = position;
        MainActivity.isSelected = true;
        getListView().setSelector(android.R.color.holo_blue_dark);
    }

}

片段2:

public class Fragment2 extends DialogFragment {

    EditText et;
    public Fragment2() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup viewGroup,
                             Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_fragment2, viewGroup, false);
    }

    public void onViewCreated(View view, Bundle savedInstanceState) {

        et = view.findViewById(R.id.editText);
        Button b = view.findViewById(R.id.button);
        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String country = et.getText().toString();
                Fragment1 fr1 = (Fragment1)
                        getFragmentManager().findFragmentById(R.id.list);
                if (!country.equals("")) {
                    fr1.cities.add(country);
                    fr1.adapter.notifyDataSetChanged();
                }
                getActivity().getSupportFragmentManager().beginTransaction()
                        .remove(Fragment2.this).commit();
            }
        });
    }

}

Answer 1

您正在添加另一个同名片段。您可以尝试将名称更改为其他名称并尝试吗？

喜欢

fr2.show(manager, "Add District");

这样你就可以找到不同之处。

添加片段

Fragment fr2 = new Fragment();

FragmentManager manger = getSupportFragmentManager();

        FragmentTransaction ft = manger.beginTransaction();

        ft.add(R.id.placeHolder, fr2,tag);

        ft.commitAllowingStateLoss();

        if(fr2.isAdded()){

           fr2.show(manager, "");

        }

在调用 show 之前，您必须添加片段。