Main Activity 不会与 Fragment 2 通信

Question

我已经设法弄清楚如何使用接口 java 类将信息从两个片段（片段 1a 和片段 1b）发送到主活动，但是我在将信息从主活动发送到另一个片段时遇到了真正的困难（片段 2)。我已经尝试了十几件事，但在以下几点上，我在 Main Activity 和 Fragment 2 中不断收到 Null Pointer 异常：

主要活动：

        //Send data to Even Split Fragment 2
        evenSplit_CalculationFragment2.tellMeWhatEachPersonOwesES(eachPersonOwesESString);

片段 2：

public void tellMeWhatEachPersonOwesES (String eachPersonOwesThisESString) {

    amountEachPersonOwesES.setText(eachPersonOwesThisESString);

};

如果有人可以提供帮助，将不胜感激。 Main Activity、Fragment 2 和 Communicator 界面的完整代码如下：

主要活动：

package apps.created.jay.splititapp;

import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;


public class MainActivity extends ActionBarActivity implements View.OnClickListener, Communicator  {

    Button nextButton, calculateButton, backButton;
    partySizeAndMethodFragment partySize_MethodFragment;
    evenSplitCalculationFragment1 evenSplit_CalculationFragment1;
    evenSplitCalculationFragment2 evenSplit_CalculationFragment2;
    byWhoHadWhatCalculationFragment1 byWhoHadWhat_CalculationFragment1;
    byWhoHadWhatCalculationFragment2 byWhoHadWhat_CalculationFragment2;
    String numberInPartyString;
    String splitMethodString;
    String billGrandTotalString;
    String eachPersonOwesESString;
    double numberInPartyDouble, eachPersonOwesESDouble, billGrandTotalDouble;
    TextView codeTesterView;

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

        //Implement Initial Fragment (i.e. Set Party Size and Method of Split)
        FragmentManager splitItFragmentManager = getFragmentManager();
        FragmentTransaction splitItFragmentTransaction = splitItFragmentManager.beginTransaction();
        partySize_MethodFragment = new partySizeAndMethodFragment();
        splitItFragmentTransaction.add(R.id.fragment_container, partySize_MethodFragment);
        splitItFragmentTransaction.commit();

        //Initialise Next Button
        nextButton = (Button) findViewById(R.id.next1button);
        nextButton.setOnClickListener(this);

        //Initialise Calculate Button
        calculateButton = (Button) findViewById(R.id.calculateButton);
        calculateButton.setOnClickListener(this);

        //Initialise Back Button
        backButton = (Button) findViewById(R.id.backButton);
        backButton.setOnClickListener(this);

    }

    //COMMUNICATIONS

    //Method of Split (from Party Size & Method Fragment)

    @Override
    public void respond(String splitMethodData) {

        splitMethodString = splitMethodData;

    }

    // Party Size (from Party Size & Method Fragment)

    @Override
    public void partyOfWhat(String numberInPartyData) {

        numberInPartyString = numberInPartyData;

    }

    // Total Bill (from Even Split Calculation Fragment 1)

    @Override
    public void billGrandTotal(String billGrandTotalData) {

        billGrandTotalString = billGrandTotalData;

    }


    //On Button Click

    @Override
    public void onClick(View v) {

        FragmentManager splitItFragmentManager = getFragmentManager();
        FragmentTransaction splitItFragmentTransaction = splitItFragmentManager.beginTransaction();

        if(v.getId() == R.id.next1button) {

            if (splitMethodString.equals("byWhoHadWhat")) {

                byWhoHadWhat_CalculationFragment1 = new byWhoHadWhatCalculationFragment1();
                splitItFragmentTransaction.replace(R.id.fragment_container, byWhoHadWhat_CalculationFragment1);
                splitItFragmentTransaction.addToBackStack(null);
                splitItFragmentTransaction.commit();

                //Replace Next Button with Calculate & Back Button
                nextButton.setVisibility(View.GONE);
                calculateButton.setVisibility(View.VISIBLE);
                backButton.setVisibility(View.VISIBLE);


            } else if (splitMethodString.equals("evenly")) {

                //Replace Fragment

                evenSplit_CalculationFragment1 = new evenSplitCalculationFragment1();
                splitItFragmentTransaction.replace(R.id.fragment_container, evenSplit_CalculationFragment1);
                splitItFragmentTransaction.addToBackStack(null);
                splitItFragmentTransaction.commit();

                //Replace Next Button with Calculate & Back Button
                nextButton.setVisibility(View.GONE);
                calculateButton.setVisibility(View.VISIBLE);
                backButton.setVisibility(View.VISIBLE);

            }
        }

        else if (v.getId() == R.id.backButton){

                splitItFragmentTransaction.replace(R.id.fragment_container, partySize_MethodFragment);
                splitItFragmentTransaction.addToBackStack(null);
                splitItFragmentTransaction.commit();

                //Replace Next Button with Calculate & Back Button
                nextButton.setVisibility(View.VISIBLE);
                calculateButton.setVisibility(View.GONE);
                backButton.setVisibility(View.GONE);

        }

        else if (v.getId() == R.id.calculateButton){

            //Convert Strings pulled from Fragments into Doubles
            numberInPartyDouble = Double.parseDouble(numberInPartyString);

            billGrandTotalDouble = Double.parseDouble(billGrandTotalString.replace(",",""));

            eachPersonOwesESString = String.valueOf(billGrandTotalDouble/numberInPartyDouble);


            //Replace Even Split Fragment 1 with Even Split Fragment 2
            evenSplit_CalculationFragment2 = new evenSplitCalculationFragment2();
            splitItFragmentTransaction.replace(R.id.fragment_container, evenSplit_CalculationFragment2);
            splitItFragmentTransaction.addToBackStack(null);
            splitItFragmentTransaction.commit();

            //Send data to Even Split Fragment 2
            evenSplit_CalculationFragment2.tellMeWhatEachPersonOwesES(eachPersonOwesESString);

            //Hide Calculate Button
            calculateButton.setVisibility(View.GONE);

        }
    }

}

片段 2：

package apps.created.jay.splititapp;

import android.app.Fragment;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

/**
 * Created by Jay on 02/06/2015.
 */
public class evenSplitCalculationFragment2 extends Fragment {

    String eachPersonOwesThisESString;
    TextView amountEachPersonOwesES;
    View evenSplitView2;
    Communicator comm;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        //Define the View and inflate Fragment 2
        evenSplitView2 = inflater.inflate(R.layout.even_split_calculation_fragment_layout_2, container, false);
        return evenSplitView2;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        amountEachPersonOwesES = (TextView) evenSplitView2.findViewById(R.id.even_split_amount_due);
        comm = (Communicator) getActivity();
    }

    //COMMUNICATIONS

    //What Each Person Owes (from Main Activity)

    public void tellMeWhatEachPersonOwesES (String eachPersonOwesThisESString) {

        amountEachPersonOwesES.setText(eachPersonOwesThisESString);

    }




}

界面：

public interface Communicator {

    public void respond (String data);
    public void partyOfWhat (String data);
    public void billGrandTotal (String data);

}

Answer 1

看看这个教程 https://developer.android.com/training/basics/fragments/communicating.html

您需要回调才能在片段和活动之间进行通信

这是来自谷歌页面的快速示例：

public class HeadlinesFragment extends ListFragment {
    OnHeadlineSelectedListener mCallback;

    // Container Activity must implement this interface
    public interface OnHeadlineSelectedListener {
        public void onArticleSelected(int position);
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        // This makes sure that the container activity has implemented
        // the callback interface. If not, it throws an exception
        try {
            mCallback = (OnHeadlineSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement OnHeadlineSelectedListener");
        }
    }

    ...
}

public static class MainActivity extends Activity
        implements HeadlinesFragment.OnHeadlineSelectedListener{
    ...

    public void onArticleSelected(int position) {
        // The user selected the headline of an article from the HeadlinesFragment
        // Do something here to display that article
    }
}

我建议您从上面的链接下载示例并试一试。非常好。

希望对你有帮助