1.ViewPager简单介绍

ViewPager就是一个简单的页面切换组件，我们可以往里面填充多个View，然后我们可以左右滑动，从而切换不同的View，我们可以通过setPageTransformer()方法为我们的ViewPager> 设置切换时的动画效果。和ListView，GridView一样，我们也需要一个Adapter> (适配器)将我们的View和ViewPager进行绑定，而ViewPager则有一个特定的Adapter——> PagerAdapter！另外，Google官方是建议我们使用Fragment来填充ViewPager的，这样> 可以更加方便的生成每个Page，以及管理每个Page的生命周期！Google给我们提供了两个Fragment> 专用的Adapter：FragmentPageAdapter和FragmentStatePagerAdapter。

2.实现效果图以及工程目录结构：

3.代码实现：
1)、相关资源文件
项目完整代码

2)、主界面布局文件：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    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_height="50dp"
        android:layout_alignParentBottom="true"
        android:background="@color/bg_white"
        android:orientation="horizontal">

        <RadioButton
            android:id="@+id/rb_main"
            style="@style/tab_menu_item"
            android:drawableTop="@drawable/tab_menu_channel"
            android:text="主页"
            android:textSize="14sp"/>

        <RadioButton
            android:id="@+id/rb_query"
            style="@style/tab_menu_item"
            android:drawableTop="@drawable/tab_menu_message"
            android:text="查询"
            android:textSize="14sp"/>

        <RadioButton
            android:id="@+id/rb_predict"
            style="@style/tab_menu_item"
            android:drawableTop="@drawable/tab_menu_better"
            android:text="预测"
            android:textSize="14sp"/>

        <RadioButton
            android:id="@+id/rb_user"
            style="@style/tab_menu_item"
            android:drawableTop="@drawable/tab_menu_setting"
            android:text="个人中心"
            android:textSize="14sp"/>

    </RadioGroup>

    <View
        android:id="@+id/div_tab_bar"
        android:layout_width="match_parent"
        android:layout_height="2px"
        android:layout_above="@id/rg_tab_bar"
        android:background="@color/div_white" />

    <androidx.viewpager.widget.ViewPager
        android:id="@+id/vpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@id/div_tab_bar"/>

</RelativeLayout>

3）、四个fragment及其布局文件
HomeFragment:

package com.example.buttommenubar;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class HomeFragment extends Fragment {

    private TextView textView;
    @Nullable
    @Override

    //类似于Activity里面的setContentView();
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view=inflater.inflate(R.layout.fragment_home,container,false);
        return view;
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        textView=view.findViewById(R.id.home);
    }
}

布局文件

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

    <TextView
        android:id="@+id/home"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="这里是主界面"
        android:textAlignment="center"
        android:textSize="25sp">
    </TextView>

</LinearLayout>

剩余三个改下名字就好了，这里就不列上代码了。

4)、极为关键的一步，自定义FragmentPagerAdapter
代码如下：

package com.example.buttommenubar;

import android.view.ViewGroup;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentPagerAdapter;


public class MyFragmentPagerAdapter extends FragmentPagerAdapter {
    private final int PAGER_COUNT = 4;

    private HomeFragment homeFragment;
    private PredictFragment predictFragment;
    private QueryFragment queryFragment;
    private UserFragment userFragment;

    public MyFragmentPagerAdapter(FragmentManager fm) {
        super(fm);
        homeFragment=new HomeFragment();
        predictFragment=new PredictFragment();
        queryFragment=new QueryFragment();
        userFragment=new UserFragment();
    }

    @Override
    public Fragment getItem(int position) {
        Fragment fragment = null;
        switch (position) {
            case MainActivity.PAGE_ONE:
                fragment = homeFragment;
                break;
            case MainActivity.PAGE_TWO:
                fragment = queryFragment;
                break;
            case MainActivity.PAGE_THREE:
                fragment = predictFragment;
                break;
            case MainActivity.PAGE_FOUR:
                fragment = userFragment;
                break;
        }
        return fragment;
    }

    @Override
    public int getCount() {
        return PAGER_COUNT;
    }

    @Override
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        return super.instantiateItem(container, position);
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        super.destroyItem(container, position, object);
    }
}

5）、MainActivity.java

package com.example.buttommenubar;

import androidx.appcompat.app.AppCompatActivity;
import androidx.viewpager.widget.ViewPager;

import android.os.Bundle;
import android.widget.RadioButton;
import android.widget.RadioGroup;


public class MainActivity extends AppCompatActivity implements RadioGroup.OnCheckedChangeListener,
        ViewPager.OnPageChangeListener {

    private RadioGroup rg_tab_bar;
    private RadioButton rb_main;
    private RadioButton rb_query;
    private RadioButton rb_predict;
    private RadioButton rb_user;
    private ViewPager vpager;

    private MyFragmentPagerAdapter mAdapter;

    //代表页面的常量
    public static final int PAGE_ONE = 0;
    public static final int PAGE_TWO = 1;
    public static final int PAGE_THREE = 2;
    public static final int PAGE_FOUR = 3;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getSupportActionBar().hide();
        setContentView(R.layout.activity_main);

        mAdapter = new MyFragmentPagerAdapter(getSupportFragmentManager());
        initViews();
        rb_main.setChecked(true);
    }

    private void initViews() {
        rg_tab_bar =  findViewById(R.id.rg_tab_bar);
        rb_main =  findViewById(R.id.rb_main);
        rb_query =  findViewById(R.id.rb_query);
        rb_predict =  findViewById(R.id.rb_predict);
        rb_user =  findViewById(R.id.rb_user);
        rg_tab_bar.setOnCheckedChangeListener(this);

        vpager = findViewById(R.id.vpager);
        vpager.setAdapter(mAdapter);
        vpager.setCurrentItem(0);
        vpager.addOnPageChangeListener(this);
    }

    @Override
    public void onCheckedChanged(RadioGroup group, int checkedId) {
        switch (checkedId) {
            case R.id.rb_main:
                vpager.setCurrentItem(PAGE_ONE);
                break;
            case R.id.rb_query:
                vpager.setCurrentItem(PAGE_TWO);
                break;
            case R.id.rb_predict:
                vpager.setCurrentItem(PAGE_THREE);
                break;
            case R.id.rb_user:
                vpager.setCurrentItem(PAGE_FOUR);
                break;
        }
    }

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

    }

    @Override
    public void onPageSelected(int position) {

    }

    @Override
    public void onPageScrollStateChanged(int state) {
        if (state == 2) {
            switch (vpager.getCurrentItem()) {
                case PAGE_ONE:
                    rb_main.setChecked(true);
                    break;
                case PAGE_TWO:
                    rb_query.setChecked(true);
                    break;
                case PAGE_THREE:
                    rb_predict.setChecked(true);
                    break;
                case PAGE_FOUR:
                    rb_user.setChecked(true);
                    break;
            }
        }
    }

    @Override
    public void onPointerCaptureChanged(boolean hasCapture) {

    }
}