如何在ViewPager中访问项目（对象）？

Question

I've created an acitvity that must have two pages. Those two pages shouldn't work like two different and unrelated activities, but like, let's say, one activity with two work area. Switching between them just with sliding right or left. So I've impleneted it and switching works, but I can't get access to the object inside ViewPager . Description: Parent activity activity_setdetail.xml has two area: list view_setdetail_1.xml - of items and view_setdetail_2.xml - some settings. So, those two areas relate to different *.xml .

XMLs

activity_setdetail.xml

    <android.support.v4.view.ViewPager
        android:id="@+id/vpager"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginBottom="2dp"
        android:layout_marginEnd="2dp"
        android:layout_marginLeft="2dp"
        android:layout_marginRight="2dp"
        android:layout_marginStart="2dp"
        android:layout_marginTop="2dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>

view_setdetail_1.xml it goes to ViewPager vpager as right side area

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ListView
        android:id="@+id/listViewOfWords"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="2dp"
        android:layout_marginLeft="2dp"
        android:layout_marginRight="2dp"
        android:layout_marginStart="2dp"
        android:layout_marginTop="8dp"
        android:clickable="true"
        android:focusable="true"
        android:paddingBottom="20dp"
        android:textAlignment="center"
        app:layout_constraintBottom_toTopOf="@+id/txtToLBL"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


    <TextView
        android:id="@+id/txtToLBL"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:text="@string/view_goToSettings"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent" />

</android.support.constraint.ConstraintLayout>

view_setdetail_2.xml it goes to ViewPager vpager as left side area

   <?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/txtCardsQTYinLessonlbl"
        android:layout_width="0dp"
        android:layout_height="38dp"
        android:layout_marginLeft="2dp"
        android:layout_marginStart="2dp"
        android:layout_marginTop="2dp"
        android:gravity="center_vertical"
        android:text="@string/txtCardsQTYinLessonLBLString"
        android:textAlignment="center"
        android:textColor="?attr/editTextColor"
        android:textSize="16sp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/etxtCardsQTYinLesson"
        android:layout_width="34dp"
        android:layout_height="wrap_content"
        android:layout_marginEnd="2dp"
        android:layout_marginLeft="2dp"
        android:layout_marginRight="2dp"
        android:layout_marginStart="2dp"
        android:layout_marginTop="2dp"
        android:ems="10"
        android:imeOptions="actionDone"
        android:inputType="number"
        android:text="99"
        android:textAlignment="center"
        android:textSize="14sp"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toRightOf="@+id/txtCardsQTYinLessonlbl"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/txtTolistLBL"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:text="@string/view_goToList"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent" />

</android.support.constraint.ConstraintLayout>

Resource file that enumerate screens for activity_setdetail.xml

public enum ResourcesModel {


    FIRST_SCREEN(R.string.view_setdetail_1, R.layout.view_setdetail_1), SECOND_SCREEN(R.string.view_setdetail_2, R.layout.view_setdetail_2);

    private int mTitleResourceId;
    private int mLayoutResourceId;

    ResourcesModel(int titleResId, int layoutResId) {
        mTitleResourceId = titleResId;
        mLayoutResourceId = layoutResId;
    }

    public int getTitleResourceId() {
        return mTitleResourceId;
    }

    public int getLayoutResourceId() {
        return mLayoutResourceId;
    }
}

Adapter:

import android.content.Context;
import android.support.v4.view.PagerAdapter;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import am.slotvin.hoodiesoft.by.enums.ResourcesModel;

public class SimplePagerAdapter extends PagerAdapter {

    private Context mContext;

    public SimplePagerAdapter(Context context) {
        mContext = context;
    }

    @Override
    public Object instantiateItem(ViewGroup viewGroup, int position) {
        ResourcesModel resources = ResourcesModel.values()[position];
        LayoutInflater inflater = LayoutInflater.from(mContext);
        ViewGroup layout = (ViewGroup) inflater.inflate(
                resources.getLayoutResourceId(), viewGroup, false);
        viewGroup.addView(layout);
        return layout;
    }

    @Override
    public void destroyItem(ViewGroup viewGroup, int position, Object view) {
        viewGroup.removeView((View) view);
    }

    @Override
    public int getCount() {
        return ResourcesModel.values().length;
    }

    @Override
    public boolean isViewFromObject(View view, Object object) {
        return view == object;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        ResourcesModel customPagerEnum = ResourcesModel.values()[position];
        System.out.println("SimplePagerAdapter CharSequence getPageTitle" + position);
        return mContext.getString(customPagerEnum.getTitleResourceId());
    }

}

Java class for activity_setdetail.xml . I've reduced code just to only textview . But in reality I have ListView there and other stuff. Nevertheless, I can't influence even to textview .

public class SetDetail extends AppCompatActivity {

    private String setName;
    private int setID;
    TextView txtToLBL;

    SimplePagerAdapter spa;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setName = getIntent().getStringExtra("setName");
        setID = getIntent().getIntExtra("setID", 0);
        setTitle(this.getString(R.string.activity_setdetailLabel) + setName);
        setContentView(R.layout.activity_setdetail);

        spa = new SimplePagerAdapter(this);

        ViewPager viewPager = (ViewPager) findViewById(R.id.vpager);
        if (viewPager != null) {
            viewPager.setAdapter(spa);

        }

        txtToLBL = (TextView) findViewById(R.id.txtToLBL); //doesn't work
        //txtToLBL.setText("bla-bla-bla"); //error - android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference

        LayoutInflater inflater = LayoutInflater.from(this);
        View v = inflater.inflate(R.layout.view_setdetail_1, null, false);
        txtToLBL = (TextView) v.findViewById(R.id.txtToLBL);
        txtToLBL.setText("bla-bla-bla");//also doesn't work. No errors, just no changes.

    }

}

So the question is - How to gegt access to elements on views that loaded in a ViewPager ? I found similar questions, but they didn't work for me.

UPDATE 1: So, I've completely moved all code (also a bunch of listeners) to my SimplePagerAdapter . Now it should be renamed, because it is not universal anylonger and closely-coupled with the java class of parent activity. Looks not very good, but the only way I found at the moment. Though works OK, without freezing. Still be good to hear more elegant solution.

Answer 1

A ViewPager inflates 3 fragments at a time, the current fragment in view and a fragment on both left and right side of it. Since, your ViewPager has only two fragments both are inflated. So you can use something like viewPager.findViewById(resId) where resId is the Id of the view you want to access

Answer 2

I have the same kind of issue, the viewPager and the PagerAdapter should have been designed with a callback for when it's finished loading it's views. If I put code I want to run inside onPageSelected it all works, its just not available onCreate or onResume (presumably because the view is not instantiated by the pager yet.. If I had hair I'd be pulling it out trying to set default text on TextViews when my activity is Created...