Android - 无法在 View Pager Adapter 中更新 textview

Question

I have a counter view which should update as user touch minus and plus buttons.

I'm trying to update counter within ViewPager's adapter instantiateItem method, but it doesn't make any effect.

I can update it's value only in instantiateItem . Is there any way to setText() outside of instantiateItem ? Sorry for dumb question, i'm a newbie in Android.

public class OrderVPAdapter extends PagerAdapter implements View.OnClickListener {

    private ArrayList<SomeItem> list;
    private Context context;

    public TextView counterView;
    View view;

    // I skipped some code for better understanding

    int counter = 0;

    @NonNull
    @Override
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        LayoutInflater layoutInflater = LayoutInflater.from(context);

        view = layoutInflater.inflate(R.layout.vp_item, container, false);

        counterView = view.findViewById(R.id.counter);

        Button plus, minus;
        plus = view.findViewById(R.id.btn_plus);
        plus.setOnClickListener(this);

        minus = view.findViewById(R.id.btn_minus);
        minus.setOnClickListener(this);

        container.addView(view, 0);

        return view;
    }

    @Override
    public void onClick(View view) {
        if(view.getId() == R.id.btn_plus) {
            counter++;
        }
        else {
            if(counter > 0)
                counter--;
        }
        counterView.setText(counter);
    }

Answer 1

It's a good question for beginners Ian, check my comments for each change I have made:

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.viewpager.widget.PagerAdapter;

public class OrderVPPageAdapter extends PagerAdapter {

    // 10 counters, one for each page
    int[] counter = new int[10];

    @Override
    public int getCount() {
        return 10;
    }

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

    @NonNull
    @Override
    public Object instantiateItem(final ViewGroup container, int position) {
        LayoutInflater layoutInflater = LayoutInflater.from(container.getContext());
        View view = layoutInflater.inflate(R.layout.vp_item, container, false);

        // counterView needs to be local variable so it can serve as          
        // reference for each page when updated inside buttons' OnClickListener
        TextView counterView = view.findViewById(R.id.counter);
        Button plus = view.findViewById(R.id.btn_plus);
        Button minus = view.findViewById(R.id.btn_minus);

        // Set up counter value every time as this method will be 
        // called and update UI whenever you swipe between pages
        counterView.setText(String.valueOf(counter[position]));

        plus.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // Update specific page counter in array position
                counter[position]++;
                counterView.setText(String.valueOf(counter[position]));
            }
        });

        minus.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (counter[position] > 0)
                    counter[position]--;
                counterView.setText(String.valueOf(counter[position]));
            }
        });

        container.addView(view);

        return view;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, @NonNull Object object) {
        container.removeView((View) object);
    }
}