Kotlin使用Viewpager2+Fragment+BottomNavigationView实现底部菜单栏的切换模块的样式。

效果图：

实现步骤：

1.首先在app的builde文件中引用ViewPager2

implementation 'androidx.viewpager2:viewpager2:1.0.0-alpha01'

2.添加首页的布局xml

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

    <androidx.viewpager2.widget.ViewPager2
        android:id="@ id/mainViewPager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@ id/navigationView" />

    <com.谷歌.android.material.bottomnavigation.BottomNavigationView
        android:id="@ id/navigationView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:background="#FFFFFF" />

</RelativeLayout>

3.创建home Activity页面

package com.example.tasklayoutdemo

import android.content.Context
import android.content.Intent
import androidx.fragment.app.Fragment
import androidx.viewpager2.widget.ViewPager2
import com.example.tasklayoutdemo.base.BaseAppActivity
import com.example.tasklayoutdemo.fragment.*
import com.谷歌.android.material.bottomnavigation.LabelVisibilityMode
import kotlinx.android.synthetic.main.home_activity.*

class Home : BaseAppActivity() {

    companion object {
        fun startHomeActivity(context: Context) {
            context.startActivity(Intent(context, Home::class.java))
        }

        //传递参数
        fun startHomeActivity2(
            context: Context,
            userid: String,
            token: String
        ) {
            context.startActivity(Intent(context, Home::class.java).apply {
                putExtra("UserId", userid)
                putExtra("Token", token)
            })
        }

    }

    override fun getLayout(): Int {
        return R.layout.home_activity
    }

    override fun initView() {


    }

    override fun initDate() {
        val fragmentArr = ArrayList<Fragment>()
        fragmentArr.add(ShoeYeFragment.instance)
        fragmentArr.add(InfoFragment.instance)
        fragmentArr.add(FindFragment.instance)
        fragmentArr.add(MineFragment.instance)

        navigationView.menu.add(0, 0, 1, "首页").setIcon(R.drawable.tab_1)
        navigationView.menu.add(0, 1, 1, "消息").setIcon(R.drawable.tab_2)
        navigationView.menu.add(0, 2, 1, "园地").setIcon(R.drawable.tab_4)
        navigationView.menu.add(0, 3, 1, "我的").setIcon(R.drawable.tab_3)
        /**
         * 否禁止用户滑动页面
         * */
        mainViewPager.isUserInputEnabled = false
        /**
         * 设置ViewPager2的滑动监听事件
         * isUserInputEnabled = true 时
         * */
        mainViewPager.registerOnPageChangeCallback(object : ViewPager2.OnPageChangeCallback() {
            override fun onPageSelected(position: Int) {
                //设置导航栏选中位置
                navigationView.menu.getItem(position).setChecked(true)
            }
        })

        mainViewPager.adapter = PagerAdapter(this, fragmentArr)
        navigationView.labelVisibilityMode = LabelVisibilityMode.LABEL_VISIBILITY_LABELED
        /**
         * 设置导航栏菜单项Item选中监听
         * */
        navigationView.setOnNavigationItemSelectedListener {
            mainViewPager.currentItem = it.itemId
            true
        }

    }
}

4.创建对应的Fragment页面（示例中需创建四个，我贴出来一个代码，其他的一样）

package com.example.tasklayoutdemo.fragment

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import com.example.tasklayoutdemo.R
import kotlinx.android.synthetic.main.shouye_fragment.*

class ShoeYeFragment : Fragment() {

    companion object {
        val instance: ShoeYeFragment by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) {
            ShoeYeFragment()
        }
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.shouye_fragment, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        tv_content.text = "我是Fragment，首页...."

    }

}

Fragment对应布局文件

........

5.创建 PagerAdapter 

package com.example.tasklayoutdemo.fragment

import androidx.fragment.app.Fragment
import androidx.fragment.app.FragmentActivity
import androidx.viewpager2.adapter.FragmentStateAdapter

class PagerAdapter : FragmentStateAdapter {
    private var fragments: List<Fragment>

    constructor(fragmentActivity: FragmentActivity, fragments: List<Fragment>) : super(
        fragmentActivity
    ) {
        this.fragments = fragments
    }

    constructor(fragment: Fragment, fragments: List<Fragment>) : super(
        fragment
    ) {
        this.fragments = fragments
    }

    override fun getItemCount(): Int {
        return fragments.size
    }

    override fun createFragment(position: Int): Fragment {
        return fragments[position]
    }

}

6.添加导航栏图标

完成，运行即可

7.进阶

其中app:itemTextColor="@drawable/main_bottom"为设置按钮选中与未选中时字体的颜色选择

app:itemIconTint="@drawable/main_bottom"为设置按钮选中与未选中时图标的颜色选择

main_bottom.xml文件为：

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:color="@android:color/holo_red_light" android:state_checked="true" />
    <item android:color="@color/black" android:state_checked="false" />
</selector>

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