Question

我有简化的示例程序，如下图所示。当我在第三个片段（ListFragment）中调用方法时出错。然而，即使从第三个片段中调用其他片段中的方法也没有问题。

当我将ListFragment更改为第一个或第二个片段时，我在ListFragment内调用方法时就可以了。我在第三个片段中放置的替代片段在我调用方法时会出错。

我不知道，我的错误在哪里。非常感谢您的回答。

注意：错误意味着程序关闭，并显示"Unfortunately, program has stooped."

这是我的计划：

MainActivity.java

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 java.util.Locale;

import id.my.zain.tupperware.fragment.ItemFragment;
import id.my.zain.tupperware.fragment.ListFragment;
import id.my.zain.tupperware.fragment.UserFragment;

public class MainActivity extends FragmentActivity {

    private UserFragment userFragment;
    private ItemFragment itemFragment;
    private ListFragment listFragment;

    SectionsPagerAdapter mSectionsPagerAdapter;
    ViewPager mViewPager;

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

        userFragment = new UserFragment();
        itemFragment = new ItemFragment();
        listFragment = new ListFragment();

        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);
    }

    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            Fragment fragment = new Fragment();

            switch (position) {
                case 0:
                    return fragment = userFragment;
                case 1:
                    return fragment = itemFragment;
                case 2:
                    return fragment = listFragment;
                default:
                    break;
            }
            return fragment;
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();
            switch (position) {
                case 0:
                    return getString(R.string.title_section1).toUpperCase(l);
                case 1:
                    return getString(R.string.title_section2).toUpperCase(l);
                case 2:
                    return getString(R.string.title_section3).toUpperCase(l);
            }
            return null;
        }
    }
}

UserFragment.java

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import id.my.zain.tupperware.R;
import id.my.zain.tupperware.form.UserForm;

public class UserFragment extends Fragment implements View.OnClickListener{

    private UserForm form;
    private ItemFragment itemFragment;
    private ListFragment listFragment;

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

        form = new UserForm(rootView);

        itemFragment = (ItemFragment) getActivity().getSupportFragmentManager()
                .findFragmentByTag("android:switcher:" + R.id.pager + ":1");
        listFragment = (ListFragment) getActivity().getSupportFragmentManager()
                .findFragmentByTag("android:switcher:" + R.id.pager + ":2");

        form.getUser_item().setOnClickListener(this);
        form.getUser_list().setOnClickListener(this);

        return rootView;
    }

    public void onClick(View v) {
        if (v == form.getUser_item()) {
            itemFragment.user();
        }

        else if (v == form.getUser_list()) {
            listFragment.user();
        }
    }

    public void item() {
        int a = 1;
    }

    public void list() {
        int a = 1;
    }
}

ItemFragment.java

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import id.my.zain.tupperware.R;
import id.my.zain.tupperware.form.ItemForm;

public class ItemFragment extends Fragment implements View.OnClickListener {

    private ItemForm form;
    private UserFragment userFragment;
    private ListFragment listFragment;

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

        form = new ItemForm(rootView);

        userFragment = (UserFragment) getActivity().getSupportFragmentManager()
                .findFragmentByTag("android:switcher:" + R.id.pager + ":0");
        listFragment = (ListFragment) getActivity().getSupportFragmentManager()
                .findFragmentByTag("android:switcher:" + R.id.pager + ":2");

        form.getItem_user().setOnClickListener(this);
        form.getItem_list().setOnClickListener(this);

        return rootView;
    }

    public void onClick(View v) {
        if (v == form.getItem_user()) {
            userFragment.item();
        }

        else if (v == form.getItem_list()) {
            listFragment.item();
        }
    }

    public void user() {
        int a = 1;
    }

    public void list() {
        int a = 1;
    }
}

ListFragment.java

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import id.my.zain.tupperware.R;
import id.my.zain.tupperware.form.ListForm;

public class ListFragment extends Fragment implements View.OnClickListener {

    private ListForm form;
    private UserFragment userFragment;
    private ItemFragment itemFragment;

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

        form = new ListForm(rootView);

        userFragment = (UserFragment) getActivity().getSupportFragmentManager()
                .findFragmentByTag("android:switcher:" + R.id.pager + ":0");
        itemFragment = (ItemFragment) getActivity().getSupportFragmentManager()
                .findFragmentByTag("android:switcher:" + R.id.pager + ":1");

        form.getList_user().setOnClickListener(this);
        form.getList_item().setOnClickListener(this);

        return rootView;
    }

    public void onClick(View v) {
        if (v == form.getList_user()) {
            userFragment.list();
        }

        else if (v == form.getList_item()) {
            itemFragment.list();
        }
    }

    public void user() {
        int a = 1;
    }

    public void item() {
        int a = 1;
    }
}

从UserActivity调用方法时出现错误Logcat

11-11 13:21:09.273    2806-2806/id.my.zain.tupperware E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at id.my.zain.tupperware.fragment.UserFragment.onClick(UserFragment.java:42)
            at android.view.View.performClick(View.java:4202)
            at android.view.View$PerformClick.run(View.java:17340)
            at android.os.Handler.handleCallback(Handler.java:725)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5039)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)

从ItemActivity调用方法时出现错误Logcat

11-11 15:02:03.104    3291-3291/id.my.zain.tupperware E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at id.my.zain.tupperware.fragment.ItemFragment.onClick(ItemFragment.java:42)
            at android.view.View.performClick(View.java:4202)
            at android.view.View$PerformClick.run(View.java:17340)
            at android.os.Handler.handleCallback(Handler.java:725)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5039)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

ViewPager存储当前，右边和左边的碎片。当您在UserFragment时，您只有UserFragment和ItemFragment。当你在ItemFragment时，你拥有所有的碎片。当您在ListFragment时，您只有ListFragment和ItemFragment。

您在UserFragment处有一个npe：

UserFragment.onClick(UserFragment.java:42)

UserFragment代码的这一部分导致你的npe：

public void onClick(View v) {
    if (v == form.getUser_item()) {
        itemFragment.user();
    }

    else if (v == form.getUser_list()) {
        listFragment.user();
       // debug here, you'll see that listFragment is null. thus listFragment.user() throws a npe.
    }
}

因为listFragment为空。处理另一个片段中片段的点击事件是个坏主意。尝试将您的点击侦听器放入相关片段。

修改
解决你的问题;考虑我的回答。
在UserFragment，您可以参考ItemFragment 在ItemFragment，您可以引用UserFragment和ListFragment 在ListFragment，您可以参考ItemFragment 其他参考文献将使你获得一个npe。

但正如我之前提到的，从片段到片段的直接引用是一个糟糕的设计。请参阅此文档的Communicating with the Activity部分并实现类似的内容。

编辑2：对于您的实现，ViewPagers setOffscreenPageLimit方法可能是您的解决方案。将其设置为2以上，然后重试。

Answer 2

这是一个糟糕的方法，你不应该从其他活动或片段调用其他的片段/活动方法，你可以使用Observer模式来实现这一点。

但这是更新的代码，它将适用于您的情况。

P

ublic void onClick(View v) {
        if (v == form.getItem_user()) {
            if(usreFragment == null)
              {
               userFragment = new UserFragment();
             }
            userFragment.item();
        }

        else if (v == form.getItem_list()) {
             if(listFragment == null)
             {
                listFragment = new ListFragment();
             }            
             listFragment.item();
        }
    }

在第三个片段调用方法时出错

2 个答案: