KotlinRadioGroup与ViewPager实现底层分页按钮方法

<RelativeLayout xmlns:Android="Http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <RadioGroup
        android:id = "@+id/rg_tab_bar"
        android:layout_width="match_parent"
        android:layout_alignParentBottom="true"
        android:layout_height="50dp"
        android:background="@color/white"
        android:orientation="horizontal">
        <RadioButton
            android:id="@+id/rb_home"
            android:text="主页"
            android:button="@null"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="match_parent"
            android:drawableTop="@drawable/tab_menu_home"
            android:gravity="center">
        </RadioButton>
        <RadioButton
            android:id="@+id/rb_query"
            android:text="查询"
            android:button="@null"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="match_parent"
            android:drawableTop="@drawable/tab_menu_query"
            android:gravity="center">
        </RadioButton>
        <RadioButton
            android:id="@+id/rb_predict"
            android:text="预测"
            android:button="@null"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="match_parent"
            android:drawableTop="@drawable/tab_menu_predict"
            android:gravity="center">
        </RadioButton>
        <RadioButton
            android:id = "@+id/rb_user"
            android:text="个人中心"
            android:button="@null"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="match_parent"
            android:drawableTop="@drawable/tab_menu_user"
            android:gravity="center">
        </RadioButton>
    </RadioGroup>
    <androidx.viewpager.widget.ViewPager
        android:id ="@+id/vpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@+id/rg_tab_bar"/>
</RelativeLayout>

class MyFragmentPagerAdapter(fm:FragmentManager) : FragmentPagerAdapter(fm) {
    private var homeF:HomeFragment = HomeFragment()
    private var queryF:QueryFragment = QueryFragment()
    private var preF:PredictFragment = PredictFragment()
    private var userF:UserFragment = UserFragment()
    override fun getCount(): Int {
        return 4
    }
    override fun getItem(position: Int): Fragment {
        var fragment =  when(position){
            0 -> {homeF}
            1 ->{queryF}
            2 ->{preF}
            3 ->{userF}
            else -> {homeF}
        }
        return fragment
    }
}

class MainActivity : AppCompatActivity(), RadioGroup.OnCheckedChangeListener,ViewPager.OnPageChangeListener
{
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        rg_tab_bar.setOnCheckedChangeListener(this)
        vpager.adapter = MyFragmentPagerAdapter(supportFragmentManager)
        vpager.setCurrentItem(0)
        rb_home.isChecked = true
        vpager.setOnPageChangeListener(this)
    }
    override fun onCheckedChanged(p0: RadioGroup?, p1: Int) {
        when(p1){
            R.id.rb_home->{vpager.setCurrentItem(0)}
            R.id.rb_query ->{vpager.setCurrentItem(1)}
            R.id.rb_predict ->{vpager.setCurrentItem(2)}
            R.id.rb_user ->{vpager.setCurrentItem(3)}
        }
    }
    override fun onPageScrolled(position: Int, positionOffset: Float, positionOffsetPixels: Int) {
    }
    override fun onPageSelected(position: Int) {
    }
    override fun onPageScrollStateChanged(state: Int) {
        if (state == 2){
            when(vpager.currentItem){
                0 -> rb_home.isChecked = true
                1 -> rb_query.isChecked = true
                2 -> rb_predict.isChecked = true
                3 -> rb_user.isChecked = true
            }
        }
    }
}