Android - 无法在 View Pager 适配器中更新文本视图

Question

我有一个计数器视图，它应该随着用户触摸minus和plus按钮而更新。

我正在尝试更新ViewPager的适配器instantiateItem方法中的计数器，但它没有任何效果。

我只能在instantiateItem中更新它的值。除了instantiateItem，还有什么办法到达setText()吗？对不起，我是安卓系统的新手。

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

对于初学者来说，这是一个很好的问题，Ian，请检查我对所做的每一个更改的评论：

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);
}
}