fragment的切換問題一直都比較麻煩绽诚，好在官方最新的jetpack提供了簡(jiǎn)單的解決方案监署，但也是不能拿來直接用，在此記錄下一些解決方法寄啼。

1.創(chuàng)建Android studio默認(rèn)ButtomNavigationView

MainActivity的布局很簡(jiǎn)單逮光，一個(gè)fragment一個(gè)BottomNavigationView，注意到ConstraintLayout有個(gè)paddingTop="?attr/actionBarSize"這就比較奇怪墩划，貌似如果父布局有toolbar時(shí)才有用涕刚。fragment的類是NavHostFragment，這就是jetpack的組件了走诞。注意app:menu指定了NavigationView的menu副女，fragment的app:navGraph指定了一個(gè)navigation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"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="?attr/actionBarSize">

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/nav_view"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="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" />

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@id/nav_view"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/mobile_navigation" />

</androidx.constraintlayout.widget.ConstraintLayout>

我們打開res/navigation/mobile_navigation蚣旱，有三個(gè)fragment標(biāo)簽碑幅，其父標(biāo)簽navigation通過app:startDestination指定了起始fragment。

<?xml version="1.0" encoding="utf-8"?>
<navigation 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/mobile_navigation"
    app:startDestination="@+id/navigation_home">

    <fragment
        android:id="@+id/navigation_home"
        android:name="com.latlaj.buttomnavigationtest.ui.home.HomeFragment"
        android:label="@string/title_home"
        tools:layout="@layout/fragment_home" />

    <fragment
        android:id="@+id/navigation_dashboard"
        android:name="com.latlaj.buttomnavigationtest.ui.dashboard.DashboardFragment"
        android:label="@string/title_dashboard"
        tools:layout="@layout/fragment_dashboard" />

    <fragment
        android:id="@+id/navigation_notifications"
        android:name="com.latlaj.buttomnavigationtest.ui.notifications.NotificationsFragment"
        android:label="@string/title_notifications"
        tools:layout="@layout/fragment_notifications" />
</navigation>

打開src/menu/bottom_nav_menu塞绿。

<?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>

每個(gè)item有和navigation中fragment同樣的id沟涨，通過下面MainActivity中onCreate用NavigationUI的方法setupWithNavController的綁定實(shí)現(xiàn)跳轉(zhuǎn)。

BottomNavigationView navView = findViewById(R.id.nav_view);
NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
NavigationUI.setupWithNavController(navView, navController);

2.用ViewModel保存View數(shù)據(jù)

Bottom Navigation Activity項(xiàng)目每個(gè)fragment都有ViewModel.png

最后用Activity的ViewModel解決問題射沟。新建MainViewModel.java：

package com.latlaj.buttomnavigationtest;
import android.view.View;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;
public class MainViewModel extends ViewModel {
    private MutableLiveData<View> mDashboardView;
    private MutableLiveData<View> mHomeView;
    private MutableLiveData<View> mNotificationsView;
    public MainViewModel() {
        mDashboardView = new MutableLiveData<>();
        mHomeView = new MutableLiveData<>();
        mNotificationsView = new MutableLiveData<>();
    }
    public void setDashboardView(View v) {
        mDashboardView.setValue(v);
    }
    public void setHomeView(View v) {
        mHomeView.setValue(v);
    }
    public void setNotificationsView(View v) {
        mNotificationsView.setValue(v);
    }
    public LiveData<View> getDashboardView() {
        return mDashboardView;
    }
    public LiveData<View> getHomeView() {
        return mHomeView;
    }
    public LiveData<View> getNotificationsView() {
        return mNotificationsView;
    }
}

使用方法，其實(shí)拿到的是同一個(gè)mainViewModel實(shí)例：

//在MainActivity中使用
MainViewModel mainViewModel = new ViewModelProvider(this).get(MainViewModel.class);
//在HomeFragment中使用,在onViewCreated()中或之后調(diào)用
MainViewModel mainViewModel = new ViewModelProvider(getActivity()).get(MainViewModel.class);

以HomeFragment為例与境，我們不使用ViewModel的觀察者方法验夯，直接在onDestroyView調(diào)用時(shí)保存View。

@Override
public void onDestroyView() {
    super.onDestroyView();
    mainViewModel.setView(root);
}

private View root;
private MainViewModel mainViewModel;
@Override
public View onCreateView(@NonNull LayoutInflater inflater,
                         ViewGroup container, Bundle savedInstanceState) {
    mainViewModel=new ViewModelProvider(getActivity())
            .get(MainViewModel.class);
    root=mainViewModel.getHomeView().getValue();
    if(root==null){
        //正常初始化View
        root = inflater.inflate(R.layout.fragment_home, container, false);
        //...
    }
    //...
    return root;
}

然而這有一個(gè)問題摔刁，root在這種情況下可能會(huì)指定多個(gè)parent導(dǎo)致報(bào)錯(cuò)挥转，加入去除parent的代碼：

@Override
public void onDestroyView() {
    super.onDestroyView();
    //其實(shí)以下代碼不一定寫在onDestroyView()，可以獲取指定后root的地方都可以
    ViewGroup parent = (ViewGroup) root.getParent();
    if(parent!=null){
        parent.removeView(root);
    }
    mainViewModel.setView(root);
}

3.取消煩人的fragment重建

NavigationUI.setupWithNavController.png

發(fā)現(xiàn)只給navView設(shè)置了一個(gè)監(jiān)聽暇务，于是我們可以重新設(shè)置監(jiān)聽：

BottomNavigationView navView = findViewById(R.id.nav_view);
NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
NavigationUI.setupWithNavController(navView, navController);
navView.setOnNavigationItemSelectedListener(
        new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                if (item.isChecked()){
                    //...其他動(dòng)作
                    return true;//單擊事件被消耗
                }
                return onNavDestinationSelected(item, navController);
            }
        });

這樣的結(jié)果是單擊當(dāng)前Tag不再有動(dòng)作。
希望大家能提供更好的辦法怔软。