当我将数据传递给第三个片段时出现异常，但它可以将数据传递给第二个片段

Question

当我在三个片段之间传递数据时，我的三个片段应用程序出现问题。

我使用一个接口来传递值，片段的视图页面，我将片段保存在 mainActivity.java 的列表中。问题似乎是我猜的片段实例。

在我的 mainFragment (fragment1.java) 上，我有两个按钮，一个用于向第二个片段发送数据，一个用于向第三个片段发送数据。

应用程序正常工作，当我按下按钮时，数据显示在第二个片段上。

如果我按下第二个按钮将数据发送到第三个片段，则会出现异常。

数据按应有的方式传输到第三个片段，但如果在我转到第二个片段并再次返回第一个片段之前按下第二个按钮，则会出现异常。

我不知道我是否说清楚了，但似乎这是一个实例错误，尽管 frag1 和 frag2 具有相同的代码。

这是我的代码：

ma​​inactivity.java

public class MainActivity extends FragmentActivity 
implements ICommunicator{

private String Importeddata;
private PagerAdapter mPagerAdapter;
private List<Fragment> fragments;
private ViewPager mpager;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.viewpager_layout);
    initialisePaging();
}


private void initialisePaging() {
    // TODO Auto-generated method stub
    fragments = new Vector<Fragment>();
    fragments.add(Fragment.instantiate(this, Fragment1.class.getName()));
    fragments.add(Fragment.instantiate(this, Fragment2.class.getName()));
    fragments.add(Fragment.instantiate(this, Fragment3.class.getName()));

    mPagerAdapter = new PagerAdapter(this.getSupportFragmentManager(), fragments);

    mpager = (ViewPager)findViewById(R.id.viewpager);
    mpager.setAdapter(mPagerAdapter);       
}


public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}


@Override
//Interface method implementation
public void respond(String data,int frItem) {
    // TODO Auto-generated method stub
    switch(frItem)
    {
    case 1:
        /*
         * THIS WAY WORKS 1
         * Fragment2 fr2 =  (Fragment2)fragments.get(1);
        Toast.makeText(this, "you passed values"+data, Toast.LENGTH_SHORT).show();
        System.out.println("you passed values"+data);
        fr2.changeData(data);*/


        /* THIS WAY WORKS 2 
         * Fragment2 fr2=(Fragment2)getSupportFragmentManager().findFragmentByTag("android:switcher:"+R.id.viewpager+":1");  
           fr2.changeData(data);
        */

        /* THIS WAY WORKS 3
         * 
         */
        Fragment2 fr2=(Fragment2) mPagerAdapter.instantiateItem(mpager, 1);     
        fr2.changeData(data);
        break;
    case 2:
        /*
         * 1st way works, only if i first go to fragment 2 and return to firstfragment*/
        /*otherwise i get a nullpointer EXCEPTION */

        /*Fragment3 fr3 =  (Fragment3)fragments.get(2);
        Toast.makeText(this, "you passed values"+data, Toast.LENGTH_SHORT).show();
        System.out.println("you passed values"+data);
        fr3.changeData(data);*/

        /*2nd way  works, only if i first go to fragment 2 and return to firstfragment*/
        /*otherwise i get a nullpointer EXCEPTION */
        Fragment3 fr3=(Fragment3)getSupportFragmentManager().findFragmentByTag("android:switcher:"+R.id.viewpager+":2");  
        fr3.changeData(data);
    }

}
}

下面是 FRAGMENT1.JAVA，它有两个按钮

public class Fragment1 extends Fragment{

    Button btn1;
    Button btn2;
    ICommunicator icommunicator;

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        Log.e(getTag(), "onActivityCreated 1");

        icommunicator = (ICommunicator) getActivity();  

        btn1 = (Button) getActivity().findViewById(R.id.Btn_Send);
        btn1.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                Toast.makeText(getActivity(), "you pressed the button", Toast.LENGTH_LONG).show();
                icommunicator.respond("hello from Fragment 1",1);               
            }           
        });

        btn2 = (Button)getActivity().findViewById(R.id.Btn_Send2);
        btn2.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                Toast.makeText(getActivity(), "you pressed button 2", Toast.LENGTH_SHORT).show();
                icommunicator.respond("hello from fragment1 fragment 2",2);

            }
        });
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        Log.e(getTag(), "onAttach 1");
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.e(getTag(), "onDestroy 1");
    }

    @Override
    public void onPause() {
        super.onPause();
        Log.e(getTag(), "onPause 1");
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.e(getTag(), "onResume 1");
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        Log.e(getTag(), "onCreateView 1");
        if (container == null){
            return null;
        }
        return inflater.inflate(R.layout.fragment1_layout,
                container,false);
    }
    }

**below is the fragment2.java, the fragment that i send the btn1 message**

    public class Fragment2 extends Fragment{

    private String data;
    private TextView txt;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        Log.e(getTag(), "onCreateView 2");
        txt = (TextView) getActivity().findViewById(R.id.txt_importData);
        if (container == null){
            return null;
        }
        return inflater.inflate(R.layout.fragment2_layout,
                container,false);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        Log.e(getTag(), "onActivityCreated 2");
        txt = (TextView) getActivity().findViewById(R.id.txt_importData);
        if (this.data == null)
            txt.setText("i wait value....");
        else
            txt.setText(this.data);
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        Log.e(getTag(), "onAttach 2");
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.e(getTag(), "onDestroy 2");
    }

    @Override
    public void onPause() {
        super.onPause();
        Log.e(getTag(), "onPause 2");
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.e(getTag(), "onResume 2");
    }

    public void changeData(String importdata){
        this.data = importdata;
        txt.setText(importdata);
    }
    }

下面是 fragment3.java，我发送 btn2 消息的片段 数据被发送到 fragment3.java ，到目前为止一切顺利，但我得到一个空指针异常，除非我先到第二个片段并返回到第一个片段，然后我按下第二个按钮，这似乎是什么片段的视图实例错误。

public class Fragment3 extends Fragment{

private String data;
private TextView txt;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    Log.e(getTag(), "onCreateView 3");
    txt = (TextView) getActivity().findViewById(R.id.txt_fragment3);
    if (container == null){
        return null;
    }
    return inflater.inflate(R.layout.fragment3_layout,
            container,false);

}


@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    txt = (TextView) getActivity().findViewById(R.id.txt_fragment3);
    Log.e(getTag(), "onActivityCreated 3");
    if (this.data == null)
        txt.setText("i wait value....");
    else
        txt.setText(this.data);
}


@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    Log.e(getTag(), "onAttach 3");
}


@Override
public void onDestroy() {
    super.onDestroy();
    Log.e(getTag(), "onDestroy 3");
}


@Override
public void onPause() {
    super.onPause();
    Log.e(getTag(), "onPause 3");
}


@Override
public void onResume() {
    super.onResume();
    Log.e(getTag(), "onResume 3");
}

public void changeData(String importdata){
    this.data = importdata;
    txt.setText(importdata);
}
}

如果有人可以帮助我或说出他的意见，我将不胜感激。

Answer 1

我需要通过片段进行通信的接口，即片段相互交互的方式，在您的回答中，如果我没记错的话，您建议从 clickListener 内部传递数据。

至于 setoffscreenPageLimit ，你能告诉我我在哪里告诉代码缓存三页还是 viewpager 默认只缓存三页的方式？

我补充说：

mpager.setOffscreenPageLimit(3);

它有效！ 谢谢。

Answer 2

它只缓存 3 个页面。本身，页面在右侧和左侧。添加这个：

mpager.setOffscreenPageLimit(3); // 你有 3 个片段

你可以只做这个，而不是做所有这些：

btn1 = (Button) getActivity().findViewById(R.id.Btn_Send);
btn1.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            TextView txt = (TextView) getActivity().findViewById(R.id.txt_importData);
            txt.setText("whatever you want");             
        }           
    });

btn2 = (Button)getActivity().findViewById(R.id.Btn_Send2);
btn2.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            TextView txt = (TextView) getActivity().findViewById(R.id.txt_fragment3);
            txt.setText("whatever you want");
        }
    })