設定 OnPageChangeListener

如果你需要監聽所選頁面的更改，可以在 ViewPager 上實現 ViewPager.OnPageChangeListener 監聽器：

viewPager.addOnPageChangeListener(new OnPageChangeListener() {
    
    // This method will be invoked when a new page becomes selected. Animation is not necessarily complete.
    @Override
    public void onPageSelected(int position) {
         // Your code
    }
    
    // This method will be invoked when the current page is scrolled, either as part of 
    // a programmatically initiated smooth scroll or a user initiated touch scroll.
    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
         // Your code
    }
    
    // Called when the scroll state changes. Useful for discovering when the user begins
    // dragging, when the pager is automatically settling to the current page, 
    // or when it is fully stopped/idle.
    @Override
    public void onPageScrollStateChanged(int state) {
         // Your code
    }
});