第三个片段的调用方法时出错

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问题描述

我有下面简化的示例程序.当我在第三个片段( ListFragment )中调用方法时出现错误.而即使我从第三个片段中调用另一个片段中的方法也没关系.

I have sample program like at bellow that I have simplified. There is an error when I call a method in third fragment (ListFragment). Whereas there is alright when I call method in other fragment even though from third fragment.

当我将 ListFragment 更改为第一个或第二个片段时，然后当我在 ListFragment 中调用一个方法时，就可以了.当我在第三个片段中调用一个方法时，放置在第三个片段中的替换片段会出错.

When I change ListFragment to first or second fragment, then there is alright when I call a method inside ListFragment. And the substitutes fragment that I place in third fragment get error when I call a method on that.

我不知道，我的错误在哪里.非常感谢您的回答.

I have no idea, where is my mistakes. Thank you very much for any answer.

注意:错误表示程序已关闭，并出现不幸的是，程序已弯曲."

Note: Error meaning that program close, and appear "Unfortunately, program has stooped."

这是我的程序:

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;
    }
}

当我从UserActivity调用方法时出现错误Logcat

Error Logcat when I call method from UserActivity

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

Error Logcat when I call method from ItemActivity

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)

推荐答案

ViewPager 存储当前，右边和任何左边的片段.当您使用 UserFragment 时，您只有 UserFragment 和 ItemFragment .当您位于 ItemFragment 中时，便拥有了所有片段.当您位于 ListFragment 中时，您只有 ListFragment 和 ItemFragment .

ViewPager stores current,right and if any left fragments. When you are in UserFragment you only have UserFragment and ItemFragment. when you are in ItemFragment you have all your fragments. And when you are in ListFragment you only have ListFragment and ItemFragment.

您在 UserFragment 处有一个npe:

UserFragment.onClick(UserFragment.java:42)

这部分代码位于 UserFragment ，导致您的npe:

This part of your code at UserFragment, causes your 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.处理另一个片段中的一个片段的单击事件是一个坏主意.尝试将点击侦听器放到相关片段中.

Because listFragment is null. It is a bad idea to handle click events of a fragment in another fragment. Try to put your click listeners in to the relevant fragments.

解决你的问题考虑我的答案.
在 UserFragment 上，您可以引用 ItemFragment .
在 ItemFragment 上，您可以同时引用 UserFragment 和 ListFragment .
在 ListFragment 上，您可以引用 ItemFragment .
其他参考文献会使您感到不安.

To solve your npe; consider my answer.
On UserFragment, you can have reference to ItemFragment.
On ItemFragment, you can have references to both UserFragment and ListFragment.
On ListFragment, you can have reference to ItemFragment.
Other references are going to make you get a npe.

但是正如我之前提到的，要直接从片段引用到片段是一个不好的打算.请参见本文档的与Activity通讯部分并实现类似的功能.

But as I mentioned before, it is a bad desing to have direct references from fragments to fragments. See this document's Communicating with the Activity part and implement something like that.

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.

EDIT 2: For your implementation ViewPagers setOffscreenPageLimit method maybe a solution for you. Set it more than 2 and try again.

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