如何设置Bottom Navigation Bar默认选中Home图标

我正在使用Java开发一款Android应用，实现了Bottom Navigation Bar。进入包含该底部导航栏的Activity时，页面已显示Home Fragment，但导航栏图标仍默认选中Message，请问如何设置默认选中Home图标？

我的Activity代码 #

package com.example.a1221_test;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;

import android.os.Bundle;

import com.example.a1221_test.databinding.ActivitySecondBinding;
import com.google.android.material.bottomnavigation.BottomNavigationView;

public class SecondActivity extends AppCompatActivity {
    ActivitySecondBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = ActivitySecondBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
        replaceFragment(new c_HomeFragment());

        binding.bottomNavigationView2.setOnItemSelectedListener(item -> {
            switch (item.getItemId()){

                case R.id.home:
                    replaceFragment(new c_HomeFragment());
                    break;

                case R.id.message:
                    replaceFragment(new a_MessageFragment());
                    break;

                case R.id.food:
                    replaceFragment(new b_FoodFragment());
                    break;

                case R.id.closet:
                    replaceFragment(new d_ClosetFragment());
                    break;

                case R.id.question:
                    replaceFragment(new e_QuestionFragment());
                    break;
            }

            return true;
        });
    }

    private void replaceFragment(Fragment fragment){
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.frame_layout,fragment);
        fragmentTransaction.commit();
    }
}

我的布局文件 #

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".SecondActivity">

    <FrameLayout
        android:id="@+id/frame_layout"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintBottom_toTopOf="@+id/bottomNavigationView2"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

    </FrameLayout>

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottomNavigationView2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="#e8cfa6"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:itemIconTint="@drawable/bottom_navigation_colors"
        app:itemTextColor="@drawable/bottom_navigation_colors"
        app:itemTextAppearanceActive="@style/BottomNavigationViewTextStyle"
        app:itemTextAppearanceInactive="@style/BottomNavigationViewTextStyle"
        app:menu="@menu/bottom_nav_menu"
        />

</androidx.constraintlayout.widget.ConstraintLayout>

解决方法 #

方法一：通过代码主动设置默认选中项

在onCreate方法中，加载完Home Fragment后，调用BottomNavigationView的setSelectedItemId方法，直接指定选中Home对应的ID：

修改后的onCreate代码片段：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    binding = ActivitySecondBinding.inflate(getLayoutInflater());
    setContentView(binding.getRoot());
    replaceFragment(new c_HomeFragment());
    // 添加此行，设置默认选中Home
    binding.bottomNavigationView2.setSelectedItemId(R.id.home);

    binding.bottomNavigationView2.setOnItemSelectedListener(item -> {
        switch (item.getItemId()){
            case R.id.home:
                replaceFragment(new c_HomeFragment());
                break;
            case R.id.message:
                replaceFragment(new a_MessageFragment());
                break;
            case R.id.food:
                replaceFragment(new b_FoodFragment());
                break;
            case R.id.closet:
                replaceFragment(new d_ClosetFragment());
                break;
            case R.id.question:
                replaceFragment(new e_QuestionFragment());
                break;
        }
        return true;
    });
}

方法二：通过菜单文件调整默认选中行为

BottomNavigationView默认会选中菜单列表中的第一个菜单项，如果你的bottom_nav_menu.xml中Message项排在Home前面，就会默认选中Message。只需将Home项移到菜单的最顶部即可：

示例bottom_nav_menu.xml结构：

<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- 将Home放在第一个位置 -->
    <item
        android:id="@+id/home"
        android:icon="@drawable/ic_home"
        android:title="Home"/>
    <item
        android:id="@+id/message"
        android:icon="@drawable/ic_message"
        android:title="Message"/>
    <item
        android:id="@+id/food"
        android:icon="@drawable/ic_food"
        android:title="Food"/>
    <item
        android:id="@+id/closet"
        android:icon="@drawable/ic_closet"
        android:title="Closet"/>
    <item
        android:id="@+id/question"
        android:icon="@drawable/ic_question"
        android:title="Question"/>
</menu>

两种方法都能让导航栏默认选中Home图标，与已加载的Home Fragment状态保持一致。

内容的提问来源于stack exchange，提问作者Xiayan