android - getSupportFragmentManager()。findFragmentById给我一个nullPointer

我有带viewpager的应用程序。当我单击第一个片段上的按钮时，我需要调用其他片段methon。

这是我的邮件活动

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Note: Text content in the code blocks is automatically word-wrapped
package src.com.programmingmobile.pageviewer;

package src.com.programmingmobile.pageviewer;

import java.util.ArrayList;
import java.util.List;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.widget.Toast;

public class PageViewActivity extends FragmentActivity implements Communicator{
    MyPageAdapter pageAdapter;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        try{
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_page_view);
        List<Fragment> fragments = getFragments();
        pageAdapter = new MyPageAdapter(getSupportFragmentManager(),  fragments);
        ViewPager pager = (ViewPager)findViewById(R.id.viewpager);
        pager.setAdapter(pageAdapter);

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

    }

    private List<Fragment> getFragments(){
        List<Fragment> fList = new ArrayList<Fragment>();

        fList.add(MyFragment.newInstance("Fragment 1"));
        fList.add(MyFragment2.newInstance("Fragment 2"));
        //fList.add(MyFragment.newInstance("Fragment 3"));

        return fList;
    }

    private class MyPageAdapter extends FragmentPagerAdapter {

        private List<Fragment> fragments;

        public MyPageAdapter(FragmentManager fm, List<Fragment> fragments) {
            super(fm);
            this.fragments = fragments;
        }
        @Override
        public Fragment getItem(int position) {
            return this.fragments.get(position);
        }

        @Override
        public int getCount() {
            return this.fragments.size();
        }
    }

    @Override
    public void respond(String data) {
        // TODO Auto-generated method stub
        MyFragment2 fragment = (MyFragment2) getSupportFragmentManager().findFragmentById(R.id.Fragment2);
        fragment.print();

    }
}

这是我的片段

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package src.com.programmingmobile.pageviewer;

import java.io.InputStream;

import javax.net.ssl.ManagerFactoryParameters;

import com.epapyrus.plugpdf.SimpleDocumentReader;
import com.epapyrus.plugpdf.SimpleDocumentReaderListener;
import com.epapyrus.plugpdf.SimpleReaderFactory;
import com.epapyrus.plugpdf.core.viewer.DocumentState;

import android.app.Activity;
import android.content.res.AssetManager;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;

public class MyFragment extends Fragment{
    public static final String EXTRA_MESSAGE = "EXTRA_MESSAGE";

    Communicator com;

    public void onAttach(Activity a) {
        super.onAttach(a);
        com = (Communicator) a;
    }

    public static final MyFragment newInstance(String message) {
        MyFragment f = new MyFragment();
        Bundle bdl = new Bundle(1);
        bdl.putString(EXTRA_MESSAGE, message);
        f.setArguments(bdl);
        return f;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        String message = getArguments().getString(EXTRA_MESSAGE);
        View v = inflater.inflate(R.layout.myfragment_layout, container, false);
        TextView messageTextView = (TextView) v.findViewById(R.id.textView);
        messageTextView.setText(message);
        // OpenPdf();

        LinearLayout mLinearLayout = (LinearLayout) inflater.inflate(
                R.layout.myfragment_layout, container, false);

        // note that we're looking for a button with id="@+id/myButton" in your
        // inflated layout
        // Naturally, this can be any View; it doesn't have to be a button
        Button mButton = (Button) mLinearLayout.findViewById(R.id.button1);
        mButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                com.respond("Fragment ");
            }
        });

        return mLinearLayout;
    }



}

这是fragment2

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Note: Text content in the code blocks is automatically word-wrapped
package src.com.programmingmobile.pageviewer;

import java.io.InputStream;

import com.epapyrus.plugpdf.SimpleDocumentReader;
import com.epapyrus.plugpdf.SimpleDocumentReaderListener;
import com.epapyrus.plugpdf.SimpleReaderFactory;
import com.epapyrus.plugpdf.core.viewer.DocumentState;

import android.content.res.AssetManager;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;

public class MyFragment2 extends Fragment {
    public static final String EXTRA_MESSAGE = "EXTRA_MESSAGE";

    // create a listener for receiving provide pdf loading results
    SimpleDocumentReaderListener m_listener = new SimpleDocumentReaderListener() {

        @Override
        public void onLoadFinish(DocumentState.OPEN state) {
        }
    };

    public static final MyFragment2 newInstance(String message) {
        MyFragment2 f = new MyFragment2();
        Bundle bdl = new Bundle(1);
        bdl.putString(EXTRA_MESSAGE, message);
        f.setArguments(bdl);
        return f;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        String message = getArguments().getString(EXTRA_MESSAGE);
        View v = inflater.inflate(R.layout.myfragment_layout2, container, false);
        TextView messageTextView = (TextView) v.findViewById(R.id.textView);
        messageTextView.setText(message);
        //OpenPdf();
        return v;
    }

    public void print(){
        Toast.makeText(getActivity(), " inside Respond ", 100);
    }

}

这是myfragment_layout xml文件

view plainprint?
Note: Text content in the code blocks is automatically word-wrapped
package src.com.programmingmobile.pageviewer;

import java.io.InputStream;

import javax.net.ssl.ManagerFactoryParameters;

import com.epapyrus.plugpdf.SimpleDocumentReader;
import com.epapyrus.plugpdf.SimpleDocumentReaderListener;
import com.epapyrus.plugpdf.SimpleReaderFactory;
import com.epapyrus.plugpdf.core.viewer.DocumentState;

import android.app.Activity;
import android.content.res.AssetManager;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class MyFragment extends Fragment {
    public static final String EXTRA_MESSAGE = "EXTRA_MESSAGE";

    Communicator com;

    public void onAttach(Activity a) {
        super.onAttach(a);
        com = (Communicator) com;
    }

    public static final MyFragment newInstance(String message) {
        MyFragment f = new MyFragment();
        Bundle bdl = new Bundle(1);
        bdl.putString(EXTRA_MESSAGE, message);
        f.setArguments(bdl);
        return f;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        String message = getArguments().getString(EXTRA_MESSAGE);
        View v = inflater.inflate(R.layout.myfragment_layout, container, false);
        TextView messageTextView = (TextView) v.findViewById(R.id.textView);
        messageTextView.setText(message);
        // OpenPdf();
        return v;
    }

    public void onclick(View v) {

        com.respond("Fragment ");
    }

}

这是my_fragment2 xml文件

view plainprint?
Note: Text content in the code blocks is automatically word-wrapped
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/Fragment2"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:textAppearance="?android:attr/textAppearanceLarge"/>

</RelativeLayout>

当我运行应用程序时，它会给我带button的第一个片段。当我单击按钮时，它会给我以下错误

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10-07 09:37:52.610: E/AndroidRuntime(10907): FATAL EXCEPTION: main
10-07 09:37:52.610: E/AndroidRuntime(10907): java.lang.NullPointerException
10-07 09:37:52.610: E/AndroidRuntime(10907):    at src.com.programmingmobile.pageviewer.PageViewActivity.respond(PageViewActivity.java:66)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at src.com.programmingmobile.pageviewer.MyFragment$1.onClick(MyFragment.java:63)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.view.View.performClick(View.java:3620)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.view.View$PerformClick.run(View.java:14322)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.os.Handler.handleCallback(Handler.java:605)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.os.Handler.dispatchMessage(Handler.java:92)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.os.Looper.loop(Looper.java:137)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.app.ActivityThread.main(ActivityThread.java:4507)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at java.lang.reflect.Method.invokeNative(Native Method)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at java.lang.reflect.Method.invoke(Method.java:511)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:978)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)
10-07 09:37:52.610: E/AndroidRuntime(10907):    at dalvik.system.NativeStart.main(Native Method)

给出空指针的行在下面

view plainprint?
Note: Text content in the code blocks is automatically word-wrapped
MyFragment2 fragment = (MyFragment2) getSupportFragmentManager().findFragmentById(R.id.Fragment2);

最佳答案

观察source for FragmentPagerAdapter，他们使用FragmentManager.findFragmentByTag(name)，其中name的形式为

"android:switcher:" + viewId + ":" + id;

其中viewId是android:id的ViewPager，而id是getItemId(position)的结果（在您的情况下，可能只是位置）。

注意：这并不安全，并且支持库的版本可能会有所不同。在所有以前的发行版中，它一直保持不变，但是您应该在两个组件之间（在技术上，在第一个片段，活动然后是第二个片段之间）考虑setting up an event callback interface。