Android BottomNavigationView結合ViewPager實現底部導航欄步驟詳解

<?xml version="1.0" encoding="utf-8"?>
<layout>
   <data>
   </data>
   <androidx.constraintlayout.widget.ConstraintLayout 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">
      <androidx.viewpager2.widget.ViewPager2
          android:id="@+id/vp2"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:orientation="horizontal"
          app:layout_constraintTop_toTopOf="parent"
          />
      <com.google.android.material.bottomnavigation.BottomNavigationView
          android:id="@+id/bnv"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          app:labelVisibilityMode="labeled"
          app:layout_constraintBottom_toBottomOf="parent"
          app:menu="@menu/bottom_view"
          />
   </androidx.constraintlayout.widget.ConstraintLayout>
</layout>```
# 第二步編寫 Menu
```kotlin
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/A"
        android:icon="@mipmap/ic_launcher"
        android:title="A" />
    <item
        android:id="@+id/B"
        android:icon="@drawable/ic_launcher_background"
        android:title="B" />
</menu>

class ViewPagerAdapter(fragmentActivity: FragmentActivity,private  val fragments:Map<Int,Fragment>) :FragmentStateAdapter(fragmentActivity){
    override fun getItemCount(): Int {
        return fragments.size
    }
    override fun createFragment(position: Int): Fragment {
       return fragments[position]!!
    }
}

class MainActivity : AppCompatActivity() {
    var binding:ActivityMainBinding?=null
    private val fragments= mapOf<Int,Fragment>(
        A to AFragment.newInstance("1","2"),
        B to BFragment.newInstance("3","4")
    )
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding=ActivityMainBinding.inflate(LayoutInflater.from(this))
        setContentView(binding?.root)
        binding?.apply {
            vp2.adapter=ViewPagerAdapter(this@MainActivity,fragments)
            BnvMediator(bnv, vp2) { bnv, vp2 ->
                vp2.isUserInputEnabled = true//為false ViewPager2 不能滑動
                bnv.itemIconTintList=null//顯示 BottomNavigationView 的圖標。
            }.attach()
        }
    }
    companion object{
        private val A:Int=0
        private val B:Int=1
    }
}

class BnvMediator(
    private val bnv: BottomNavigationView,
    private val vp2: ViewPager2,
    private val config: ((bnv: BottomNavigationView, vp2: ViewPager2) -> Unit)? = null
) {
    //存儲bottomNavigationView的menu的item和其自身position的對應關系
    private val map = mutableMapOf<MenuItem, Int>()
    init {
        //初始化bnv的item和index對應關系
        bnv.menu.forEachIndexed { index, item ->
            map[item] = index
        }
    }
    /**
     * 關聯BottomNavigationView和ViewPager2的選擇關系
     */
    fun attach() {
        config?.invoke(bnv, vp2)
        vp2.registerOnPageChangeCallback(object : ViewPager2.OnPageChangeCallback() {
            override fun onPageSelected(position: Int) {
                super.onPageSelected(position)
                bnv.selectedItemId = bnv.menu[position].itemId
            }
        })
        bnv.setOnNavigationItemSelectedListener { item ->
            vp2.currentItem = map[item] ?: error("沒有對應${item.title}的ViewPager2的元素")
            true
        }
    }
}