有沒有辦法使用相同的布局動態創建片段並向它們顯示數據？

Question

我編寫了從遠程端點獲取 JSON 對象的代碼。 對於這個問題，我傳入了一個模擬對象，它是一個對象數組。 我的目標是解構對象，並為數組的每個元素在 ViewPager 中創建一個片段。 每個片段都應該顯示它所代表的對象的 id。 該對象如下所示：

{'data':[{'id':1},{'id':2}]}

如果我取消對 FragmentClass 中的代碼的注釋，每個片段將顯示“測試”。 但是，如果我嘗試根據通過參數傳遞的值設置文本，則會遇到以下錯誤：

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.os.Bundle.get(java.lang.String)' on a null object reference

FragmentClass 的代碼是：

public class FragmentClass extends Fragment {
    private TextView txtId;

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

        txtId = (TextView) rootView.findViewById(R.id.txtId);
        txtId.setText(getArguments().get("id").toString());

//        txtId.setText("test");
        return rootView;
    }
}

ViewPager 類 ScreenSliderPagerActivity 的代碼是：

public class ScreenSliderPagerActivity extends FragmentActivity {
    private ViewPager mPager;
    private PagerAdapter pagerAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Intent intent = getIntent();
        String data = intent.getStringExtra("data");
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_screen_slide);

        // Instantiate a ViewPager and a PagerAdapter.
        mPager = (ViewPager) findViewById(R.id.pager);
        pagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager(), data);
        mPager.setAdapter(pagerAdapter);
    }

    @Override
    public void onBackPressed() {
        if (mPager.getCurrentItem() == 0) {
            // If the user is currently looking at the first step, allow the system to handle the
            // Back button. This calls finish() on this activity and pops the back stack.
            super.onBackPressed();
        } else {
            // Otherwise, select the previous step.
            mPager.setCurrentItem(mPager.getCurrentItem() - 1);
        }
    }

    private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
        public int NUM_PAGES; //making this a public variable within the inner class allows me to dynamically change the number of fragments created. In this instance, 2 will be created, one for each element of the array.

        public ScreenSlidePagerAdapter(FragmentManager fm, String data) {
            super(fm);
            try {
                JSONObject json = new JSONObject(data);
                JSONArray jsonArray = json.getJSONArray("data");

                NUM_PAGES = jsonArray.length();

                Log.d("NUM_PAGES", String.valueOf(NUM_PAGES)); //2
                FragmentTransaction ft = fm.beginTransaction();
                Bundle args = new Bundle();

// I think the problem is something in the loop, but I cannot seem to figure another way to do this.
                for (int i = 0; i < NUM_PAGES; i++){
                    JSONObject obj = (JSONObject) jsonArray.get(i);

                    FragmentClass fragment = new FragmentClass();
                    ft.add(R.id.pager, fragment);

                    args.putString("id", obj.get("id").toString());
                    fragment.setArguments(args);

                }
                ft.commit();

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

        @Override
        public Fragment getItem(int position) {
            return new FragmentClass();
        }

        @Override
        public int getCount() {
            return NUM_PAGES;
        }
    }
}

最后，我的包含模擬對象的 MainActivity 是：

public class MainActivity extends AppCompatActivity {
    private Button button;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button = findViewById(R.id.btn);
    }
    public void handleClick(View view){
        String mockData = "{'data':[{'id':1},{'id':2}]}";
        Intent intent = new Intent(MainActivity.this, ScreenSliderPagerActivity.class);
        intent.putExtra("data",mockData);
        MainActivity.this.startActivity(intent);

    }
}

Answer 1

FragmentStatePagerAdapter不能那樣工作。 您無法將適配器構造函數中的片段添加到片段管理器，並期望將其添加到正確的位置。 您必須使用getItem(int)方法中的參數返回構造的片段：

private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
    public int NUM_PAGES; //making this a public variable within the inner class allows me to dynamically change the number of fragments created. In this instance, 2 will be created, one for each element of the array.
    private ArrayList<FragmentClass> mFragments = new ArrayList<>();

    public ScreenSlidePagerAdapter(FragmentManager fm, String data) {
        super(fm);
        try {
            JSONObject json = new JSONObject(data);
            JSONArray jsonArray = json.getJSONArray("data");

            NUM_PAGES = jsonArray.length();

            Log.d("NUM_PAGES", String.valueOf(NUM_PAGES)); //2
            Bundle args = new Bundle();

            for (int i = 0; i < NUM_PAGES; i++){
                JSONObject obj = (JSONObject) jsonArray.get(i);

                FragmentClass fragment = new FragmentClass();
                args.putString("id", obj.get("id").toString());
                fragment.setArguments(args);
                mFragments.add(fragment);
            }

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

    @Override
    public Fragment getItem(int position) {
        return mFragments.get(position);
    }

    @Override
    public int getCount() {
        return NUM_PAGES;
    }
}

Answer 2

為了解決這個問題，我們真的需要了解適配器是如何工作的

適配器中的函數 getItem() 將根據項目位置獲取顯示的片段。 看你沒有在這個片段中傳入這里返回的參數。

        @Override
        public Fragment getItem(int position) {
            return new FragmentClass();  // <- here you just created a branch new Fragment
        }

您不應該在適配器的構造函數中自己進行片段事務。 這就是適配器所做的。

執行此操作的正常方法是初始化參數列表，然后將其傳遞到 getItem() 中返回的片段中

   
   private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
        public int NUM_PAGES;
        public JSONArray jsonArray;

        public ScreenSlidePagerAdapter(FragmentManager fm, String data) {
            super(fm);

            JSONObject json = new JSONObject(data);
            JSONArray jsonArray = json.getJSONArray("data");

        }

        @Override
        public Fragment getItem(int position) {
            
            JSONObject obj = jsonArray.get(i);
            
            Fragment f = FragmentClass();
            Bundle args = new Bundle();
            args.putString("id", obj.get("id").toString());
            f.setArguments(args);
            return f;
        }

        @Override
        public int getCount() {
            return NUM_PAGES;
        }
    }