android - 无法使用 ViewPager 和 FragmentPagerAdapter 访问 fragment View

Question

试图通过摸索学习Android开发，如果问题太简单，请原谅。

我有以下代码由于 NullPointerException 而不断崩溃 - 我已经尝试了几种方法来访问 fragment 的 View ，但我总是得到 null。谁能帮我解决我做错的事情？

我想做的是在滚动页面时将 EditText 的内容保存到 SharedPreferences。对于我的生活，我似乎无法访问显示选项卡时加载的 fragment 的 View 。

public class ViewPagerAdapter extends FragmentPagerAdapter {

    final int PAGE_COUNT = 3;
    // Tab Titles
    private String tabtitles[] = new String[] { "Profile", "Address", 
    "Security"};
    Context context;

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

    @Override
    public int getCount() {
        return PAGE_COUNT;
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            // Open FragmentTab1.java
            case 0:
                CreateUserFragmentProfile fragmenttab1 = new 
                CreateUserFragmentProfile();
                return fragmenttab1;
            // Open FragmentTab2.java
            case 1:
                CreateUserFragmentLocation fragmenttab2 = new 
                CreateUserFragmentLocation();
                return fragmenttab2;
            // Open FragmentTab3.java
            case 2:
                CreateUserFragmentSecurity fragmenttab3 = new 
                CreateUserFragmentSecurity();
                return fragmenttab3;

        }
        return null;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return tabtitles[position];
    }

}

Fragment 代码扩展了 Fragment，几乎什么都不做。

public class CreateUserFragmentSecurity extends Fragment {

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup 
    container, @Nullable Bundle savedInstanceState) {
        //return super.onCreateView(inflater, container, 
    savedInstanceState);
        View view = inflater.inflate(R.layout.create_fragment_security, 
    container, false);


        return view;

    }

    public void  storeData(){

    }
}

Activity代码如下:

public class CreateUser extends AppCompatActivity {


    public SharedPreferences preferences;
    public SharedPreferences.Editor editor;
    public View currView;
    public String fname, lname, dob, gender, email, addr1, addr2, addr3, 
    addrstate, city, passwd, passwdPlain;
    public Long mobile, pin;
    //public FragmentPagerAdapter adapter;

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

        preferences = getSharedPreferences("Reviv", MODE_PRIVATE);
        editor = preferences.edit();

        // Locate the viewpager in activity_create_user.xml
        final ViewPager viewPager = (ViewPager) findViewById(R.id.pager);

        // Set the ViewPagerAdapter into ViewPager

        final FragmentPagerAdapter adapter = new 
        ViewPagerAdapter(getSupportFragmentManager());
        viewPager.setAdapter(adapter);
        viewPager.setOffscreenPageLimit(3);

        viewPager.addOnPageChangeListener(new 
            ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, 
            int positionOffsetPixels) {
                boolean hasChanged = false;
                switch (position){ // store data on tab switch
                    case 0: // Profile tab
                        //currView = 
               viewPager.getChildAt(viewPager.getCurrentItem());  // returns 
               null
                        currView = 
               adapter.getItem(viewPager.getCurrentItem()).getView();  // 
               crashes, returns null
                        EditText etFname = 
               currView.findViewById(R.id.etFname);


                        fname = etFname.getText().toString();

                        hasChanged = false;

                        if(!StringUtils.isEmpty(fname)) {
                            editor.putString("fname", fname);
                            hasChanged = true;
                        }

                        if(hasChanged == true) editor.commit();

                        break;
                    case 1: // Address tab
                        // currView = 
             viewPager.getChildAt(viewPager.getCurrentItem()); // returns 
             null
                        currView = 
             adapter.getItem(viewPager.getCurrentItem()).getView();
                        EditText etAddr1 = 
             currView.findViewById(R.id.etAddr1);



                        addr1 = etAddr1.getText().toString();


                        hasChanged = false;

                        if(!StringUtils.isEmpty(addr1)) {
                            editor.putString("addr1", addr1);
                            hasChanged = true;
                        }


                        if(hasChanged == true) editor.commit();


                        break;
                    case 2: // Security tab
                        //currView = 
          viewPager.getChildAt(viewPager.getCurrentItem()); // returns null
                        currView = 
          adapter.getItem(viewPager.getCurrentItem()).getView();

                        EditText etPasswd = 
          currView.findViewById(R.id.etPasswd);


                        passwd = etPasswd.getText().toString();

                        hasChanged = false;

                        if(!StringUtils.isEmpty(passwd)) {
                            editor.putString("passwd", passwd);
                            hasChanged = true;
                        }



                        if(hasChanged == true) editor.commit();

                        break;
                }

            }

            @Override
            public void onPageSelected(int position) {
               // do nothing
            }

            @Override
            public void onPageScrollStateChanged(int state) {
                // do nothing
            }
        });
    }
}

我的堆栈轨迹如下:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference at com.portmanteau.reviv.CreateUser$1.onPageScrolled(CreateUser.java:58) at android.support.v4.view.ViewPager.dispatchOnPageScrolled(ViewPager.java:1921) at android.support.v4.view.ViewPager.onPageScrolled(ViewPager.java:1895) at android.support.v4.view.ViewPager.pageScrolled(ViewPager.java:1833) at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:690) at android.support.v4.view.ViewPager.onLayout(ViewPager.java:1777) at android.view.View.layout(View.java:19678) at android.view.ViewGroup.layout(ViewGroup.java:6057) at android.support.constraint.ConstraintLayout.onLayout(ConstraintLayout.java:1197) at android.view.View.layout(View.java:19678) at android.view.ViewGroup.layout(ViewGroup.java:6057) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) at android.widget.FrameLayout.onLayout(FrameLayout.java:261) at android.view.View.layout(View.java:19678) at android.view.ViewGroup.layout(ViewGroup.java:6057) at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:434) at android.view.View.layout(View.java:19678) at android.view.ViewGroup.layout(ViewGroup.java:6057) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) at android.widget.FrameLayout.onLayout(FrameLayout.java:261) at android.view.View.layout(View.java:19678) at android.view.ViewGroup.layout(ViewGroup.java:6057) at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791) at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635) at android.widget.LinearLayout.onLayout(LinearLayout.java:1544) at android.view.View.layout(View.java:19678) at android.view.ViewGroup.layout(ViewGroup.java:6057) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) at android.widget.FrameLayout.onLayout(FrameLayout.java:261) at com.android.internal.policy.DecorView.onLayout(DecorView.java:758) at android.view.View.layout(View.java:19678) at android.view.ViewGroup.layout(ViewGroup.java:6057) at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2510) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2219) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1405) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6829) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:966) at android.view.Choreographer.doCallbacks(Choreographer.java:778) at android.view.Choreographer.doFrame(Choreographer.java:713) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:952) at android.os.Handler.handleCallback(Handler.java:789) at android.os.Handler.dispatchMessage(Handler.java:98) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6803) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

Answer 1

尝试这样的事情:

@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
    boolean hasChanged = false;
    if (positionOffset > 0) {
        currView = getSupportFragmentManager().findFragmentByTag("android:switcher:" + R.id.pager + ":" + position).getView();
        if (currView != null) {
            EditText editText = null;
            if (position == 0) editText = currView.findViewById(R.id.etPasswdPlain);
            else if (position == 1) editText = currView.findViewById(R.id.etAddr1);
            else if (position ==2) editText = currView.findViewById(R.id.etFname);
            if (editText != null) {
                String value = editText.getText().toString();
                Log.d("TAG", "Value is " + value);
            }
        }
    }
}

请注意，这仅在您从左向右滑动(从第 1 页到第 2 页以及从第 2 页到第 3 页)时才有效。

即当您从第 1 页滑动到第 2 页时，第 1 页 中的数据将被保存并

当您从第2页滑动到第3页时，第2页中的数据将被保存。

对于从右向左滑动，假设从第3页滑动到第2页时必须保存第3页数据并保存>page 2 数据从page 2 滑动到page 1，你必须做一些逻辑，但你可以从这里得到想法。