底部导航栏

1.添加相应的文件

2.分别加入4个Fragment以及布局文件

3.MianActivity的引用

4.MainActivity的布局文件

添加相应的文件：

 a.先引入导航栏的依赖

implementation 'com.android.support:design:+'

  b.准备导航栏的图片素材（我这里的是4个主页，我准备了8个，4个为true的，4个为false的）

  c.创建一个menu文件，然后再里面创建一个Menu resource file文件

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/menu_home"
        android:title="首页"
        app:showAsAction="ifRoom"
        android:enabled="true"
        android:icon="@drawable/home_t"/>
    <item
        android:id="@+id/menu_classification"
        android:title="分类"
        app:showAsAction="ifRoom"
        android:enabled="true"
        android:icon="@drawable/classification_f"/>
    <item
        android:id="@+id/menu_shoppingcart"
        android:title="购物车"
        app:showAsAction="ifRoom"
        android:enabled="true"
        android:icon="@drawable/shoppingcart_f"/>
    <item
        android:id="@+id/menu_personal"
        android:title="我的"
        app:showAsAction="ifRoom"
        android:enabled="true"
        android:icon="@drawable/personal_f"/>
</menu>

 c.在drawable中创建一个bottom_navigtion_item_selector.xml文件

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

    <item android:color="#157AE2" android:state_checked="true" />
    <item android:color="#CCCCCC" android:state_checked="false" />
</selector>

  d.创建一个绑定显示未选中导航栏描述字的类

public class BottomNavigationViewHelper {
    @SuppressLint("RestrictedApi")
    public static void disableShiftMode(BottomNavigationView view) {
        BottomNavigationMenuView menuView = (BottomNavigationMenuView) view.getChildAt(0);
        try {
            Field shiftingMode = menuView.getClass().getDeclaredField("labelVisibilityMode");
            shiftingMode.setAccessible(true);
            shiftingMode.setInt(menuView, 1);//labelVisibilityMode
//            shiftingMode.setBoolean(menuView, false);//mShiftingMode
            shiftingMode.setAccessible(false);
            for (int i = 0; i < menuView.getChildCount(); i++) {
                BottomNavigationItemView item = (BottomNavigationItemView) menuView.getChildAt(i);
                item.setShifting(false);
                item.setChecked(item.getItemData().isChecked());
            }
        } catch (NoSuchFieldException e) {
            Log.e("BNVHelper", "Unable to get shift mode field", e);
        } catch (IllegalAccessException e) {
            Log.e("BNVHelper", "Unable to change value of shift mode", e);
        }
    }
}

分别加入4个Fragment以及布局文件（这里我发其中的一个碎片的代码）

public class ClassificationFragment extends Fragment {
    private View view;
    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        view = inflater.inflate(R.layout.fragment_classification,container,false);

        return view;
    }
}

MianActivity的引用（直接贴出MainActivity的代码）

public class MainActivity extends AppCompatActivity {
    private BottomNavigationView bottomNavigationView;
    private int lastIndex;
    List<Fragment> mFragment;
    private Toolbar toolbar;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initDate();
        initBottomNavigation();
    }

    private void initDate(){
        mFragment = new ArrayList<>();
        mFragment.add(new HomeFragment());
        mFragment.add(new ClassificationFragment());
        mFragment.add(new ShoppingCartFragment());
        mFragment.add(new PersonalFragment());
        setFragmentPosition(0);
    }

    private void initBottomNavigation(){
        bottomNavigationView = findViewById(R.id.bv_bottomNavigation);
        bottomNavigationView.setItemIconTintList(null); //取消图片默认颜色
        BottomNavigationViewHelper.disableShiftMode(bottomNavigationView); //显示未选中导航栏的描述字
        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
                resetToDefaultIcon();//重置到默认不选中图片
                switch (menuItem.getItemId()){
                    case R.id.menu_home:
                        menuItem.setIcon(R.drawable.home_t); //设置选中的图片
                        setFragmentPosition(0); //选中的碎片
                        break;
                    case R.id.menu_classification:
                        menuItem.setIcon(R.drawable.classification_t); //设置选中的图片
                        setFragmentPosition(1);
                        break;
                    case R.id.menu_shoppingcart:
                        menuItem.setIcon(R.drawable.shoppingcart_t); //设置选中的图片
                        setFragmentPosition(2);
                        break;
                    case R.id.menu_personal:
                        menuItem.setIcon(R.drawable.personal_t); //设置选中的图片
                        setFragmentPosition(3);
                        break;
                        default:break;
                }
                return true;
            }
        });
    }

    //重置图标
    private void resetToDefaultIcon() {
        MenuItem home =  bottomNavigationView.getMenu().findItem(R.id.menu_home);
        home.setIcon(R.drawable.home_f);
        MenuItem classification =  bottomNavigationView.getMenu().findItem(R.id.menu_classification);
        classification.setIcon(R.drawable.classification_f);
        MenuItem shoppingcart =  bottomNavigationView.getMenu().findItem(R.id.menu_shoppingcart);
        shoppingcart.setIcon(R.drawable.shoppingcart_f);
        MenuItem personal =  bottomNavigationView.getMenu().findItem(R.id.menu_personal);
        personal.setIcon(R.drawable.personal_f);

    }

    //设置选中的Fragment
    private void setFragmentPosition(int position){
        FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
        Fragment currentFragment = mFragment.get(position);
        Fragment lastFragment = mFragment.get(lastIndex);
        lastIndex = position;
        fragmentTransaction.hide(lastFragment);
        if(!currentFragment.isAdded()){
            getSupportFragmentManager().beginTransaction().remove(currentFragment).commit();
            fragmentTransaction.add(R.id.ll_frameLayout,currentFragment);
        }
        fragmentTransaction.show(currentFragment);
        fragmentTransaction.commitAllowingStateLoss();
    }
}

MainActivity布局文件的代码

<?xml version="1.0" encoding="utf-8"?>
<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">

    <FrameLayout
        android:id="@+id/ll_frameLayout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@id/bv_bottomNavigation" />

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bv_bottomNavigation"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:background="@color/colorWhite"
        app:itemIconTint="@drawable/bottom_navigation_item_selector"
        app:itemTextColor="@drawable/bottom_navigation_item_title"
        app:menu="@menu/main_bottom_navigation">

    </com.google.android.material.bottomnavigation.BottomNavigationView>
</RelativeLayout>