如何检测ViewPager子项在MainActivity中单击？

Question

我有一张卡片清单，每张卡片都有两面（正面和背面）。我想在ViewPager中将此卡片（正面）显示为CardView，当用户单击卡片时，卡片将翻转并出现在背面，然后我也需要显示一个按钮以在MainAcitvity中转到下一张卡片（正面）， 如何检测ViewPager在MainActiviy中单击以在ViewPager中显示下一张卡片？

public Object instantiateItem(ViewGroup container, int position) {
    TextView txtFront, txtBack, txtFrontHeader, txtBackHeader;
    final CardView fr,bk;
    layoutInflater=LayoutInflater.from(context);
    View view=layoutInflater.inflate(R.layout.cards_review_layout,container,false);
    String s = context.getString(R.string.box);

    txtFrontHeader = (TextView) view.findViewById(R.id.front_header_txt);
    txtBackHeader = (TextView) view.findViewById(R.id.back_header_txt);
    txtFront = (TextView) view.findViewById(R.id.display_card_front_text);
    txtBack = (TextView) view.findViewById(R.id.display_card_back_text);
    fr = (CardView) view.findViewById(R.id.card_front);
    bk = (CardView) view.findViewById(R.id.card_back);

    txtFrontHeader.setText(cardList.get(position).getGroupName() + " " + cardList.get(position).getId());
    txtBackHeader.setText(s + " " + boxString(cardList.get(position).getBoxNo()));
    txtFront.setText(cardList.get(position).getFront());
    txtBack.setText(cardList.get(position).getRear());

    fr.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            fr.animate().rotationX(-90).setDuration(300).setListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    super.onAnimationEnd(animation);
                    fr.setVisibility(View.GONE);
                    bk.setRotationX(90);
                    bk.setVisibility(View.VISIBLE);
                    bk.animate().rotationX(0).setDuration(300).setListener(null);
                }
            });
        }
    });
    bk.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            bk.animate().rotationX(-90).setDuration(300).setListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    super.onAnimationEnd(animation);
                    bk.setVisibility(View.GONE);
                    fr.setRotationX(90);
                    fr.setVisibility(View.VISIBLE);
                    fr.animate().rotationX(0).setDuration(300).setListener(null);
                }
            });
        }
    });
    container.addView(view,0);
    return view;
}

我在MainActivity中使用此代码，但是onClickListener不起作用

adapter=new Adapter(cards,this);
    viewPager=(ViewPager) findViewById(R.id.pager_review);
    viewPager.setAdapter(adapter);
    viewPager.setPadding(0,0,0,0);
    btn=(Botton) findViewById(R.id.next_card);
    viewPager.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            btn.setVisibility(View.VISIBLE);
        }
    });