因为碎片并不是一直都在（重新）创建。它们被创建然后被重复使用。为了让你的textview适当地改变文本，你需要使用OnPageChangeCallback
就像这样：

viewPager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            super.onPageScrolled(position, positionOffset, positionOffsetPixels);
        }

        @Override
        public void onPageSelected(int position) {
            super.onPageSelected(position);

            //your code
            switch (position)
            {
                case 0:
                {
                    myTextView.setText("0");
                    //return discover_page;
                }
                case 1:
                {
                    myTextView.setText("1");
                    //return top_page;
                }
                case 2:
                {
                    myTextView.setText("2");
                    //return friends_page;
                }
                //default: return null;
            }
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            super.onPageScrollStateChanged(state);
        }
    });