android BottomNavigationView 底部显示3个以上的item

首先需要在(app)build.gradle 的dependencies里面引入

implementation 'com.google.android.material:material:1.0.0'

新建MainActivity找到对应的xml 加入viewpager 和 BottomNavigationView

<?xml version="1.0" encoding="utf-8"?>
<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:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

    <androidx.viewpager.widget.ViewPager
            android:id="@+id/viewpager"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintBottom_toTopOf="@id/nav_view"/>

    <com.google.android.material.bottomnavigation.BottomNavigationView
            android:id="@+id/nav_view"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginEnd="0dp"
            android:layout_marginStart="0dp"
            android:background="?android:attr/windowBackground"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:menu="@menu/bottom_nav_menu"/>

</androidx.constraintlayout.widget.ConstraintLayout>

在res-menu 文件夹下新建 bottom_nav_menu.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <item
            android:id="@+id/navigation_home"
            android:icon="@drawable/ic_home_black_24dp"
            android:title="@string/title_home"/>

    <item
            android:id="@+id/navigation_dashboard"
            android:icon="@drawable/ic_dashboard_black_24dp"
            android:title="@string/title_dashboard"/>

    <item
            android:id="@+id/navigation_notifications"
            android:icon="@drawable/ic_notifications_black_24dp"
            android:title="@string/title_notifications"/>

</menu>

重写 MyPagerAdapter

import androidx.fragment.app.Fragment
import androidx.fragment.app.FragmentManager
import androidx.fragment.app.FragmentPagerAdapter

class MyPagerAdapter(fm: FragmentManager,val fts:List<Fragment>) : FragmentPagerAdapter(fm) {

    override fun getItem(position: Int): Fragment {
        return fts[position]
    }

    override fun getCount(): Int {
        return fts.size
    }

}

新建fragment  页面xml 就一个TextView就不贴了（这里就用一个fragment模拟一下，实际应该建立多个fragment类）

class HomeFragment : Fragment() {
    private var param: String? = null
    private lateinit var baseView:View

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        arguments?.let {
            param = it.getString("param")
        }
    }

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
        baseView = inflater.inflate(R.layout.fragment_home, container, false)
        baseView.text.text = param
        return baseView
    }

}

现在开始在 MainActivity 写代码了

class MainActivity : AppCompatActivity() {

    private val onNavigationItemSelectedListener = BottomNavigationView.OnNavigationItemSelectedListener {
        when(it.itemId){
            R.id.navigation_home -> {
                viewpager.currentItem = 0
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_dashboard -> {
                viewpager.currentItem = 1
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_notifications -> {
                viewpager.currentItem = 2
                return@OnNavigationItemSelectedListener true
            }
        }
        false
    }

    private val onPageChangeListener = object : ViewPager.OnPageChangeListener {
        override fun onPageScrollStateChanged(state: Int) {

        }

        override fun onPageScrolled(position: Int, positionOffset: Float, positionOffsetPixels: Int) {

        }

        override fun onPageSelected(position: Int) {
            when(position){
                0 -> {nav_view.selectedItemId = R.id.navigation_home}
                1 -> {nav_view.selectedItemId = R.id.navigation_dashboard}
                2 -> {nav_view.selectedItemId = R.id.navigation_notifications}
            }
        }
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        nav_view.setOnNavigationItemSelectedListener(onNavigationItemSelectedListener)
        val f1 = HomeFragment()
        var args = Bundle()
        args.putString("param",getString(R.string.title_home))
        f1.arguments = args
        val f2 = HomeFragment()
        args = Bundle()
        args?.putString("param",getString(R.string.title_dashboard))
        f2.arguments = args
        val f3 = HomeFragment()
        args = Bundle()
        args?.putString("param",getString(R.string.title_notifications))
        f3.arguments = args
        val fts = listOf(f1, f2, f3)
        val fm = supportFragmentManager
        viewpager.adapter = MyPagerAdapter(fm,fts)

        viewpager.addOnPageChangeListener(onPageChangeListener)
    }
}

你现在可以用app:labelVisibilityMode="[labeled, unlabeled, selected, auto]

不设置默认就是 auto

labeled     所有的标签都是可见的。
unlabeled  只显示图标。
selected    将只显示选定项和移位项的标签。
auto         将根据您拥有的项目数选择“标记”或“选定”。标记为1-3项，选择为3+项。