Android解决fragment来回切换重新实例化问题

1. 关于Android studio新建的Bottom Navigation Activity

2. 解决思路

3. 具体操作

4. 关于androidstudio自生成的xml文件的代码修改

1. 关于Android studio新建的Bottom Navigation Activity

AndroidStudio自带的生成的Navigation控制fragment的转换是直接使用的replace()方法，这会使每次点击底部的按钮都会重新实例化一个fragment，无论是跳转到其它fragment还是点击当前fragment对应的按钮。

而要使fragment不被回收并重新实例化，需要使用hide()和show()方法，具体实现下文。

2. 解决思路

首先在MainActivity实例化fragment，将所有fragment包含在list中。方便使用FragmentTransaction来实现对fragment的操作

        private val home = HomeFragment()private val dash = DashboardFragment()private val notification = NotificationsFragment()val list = arrayListOf(home, dash, notification)

构建一个FragmentTransaction来实现操作，以及两个Int变量获取当前fragment和要跳转的目的fragment

        private lateinit var transaction: FragmentTransactionvar mFragment = 0var toFragment = 0

这样就能实现对fragment的精准操控，这些定义都是全局变量

3. 具体操作

1. 首先是第一次打开的初始化，显示homefragment，即打开的默认的fragment

    private fun init(){transaction = supportFragmentManager.beginTransaction()transaction.add(R.id.nav_host_fragment_activity_main, home).show(home).commit()}

R.id.nav_host_fragment_activity_main是activity_main中fragment控件的id，这是androidstudio生成时默认的。

2. 其次是转换fragment的实现函数通过两个简单的if语句实现不同的hide和show函数

    private fun switchFragment(transaction: FragmentTransaction){val from = list[mFragment]val to = list[toFragment]if (mFragment == toFragment) returnif (from.isAdded) {if (to.isAdded) {transaction.hide(from).show(to).commit()} else {transaction.add(R.id.nav_host_fragment_activity_main, to).hide(from).show(to).commit()}} else {if (to.isAdded) {transaction.add(R.id.nav_host_fragment_activity_main, from).hide(from).show(to).commit()} else {transaction.add(R.id.nav_host_fragment_activity_main, from).add(R.id.nav_host_fragment_activity_main, to).hide(from).show(to).commit()}}}

具体逻辑自己理解

3. 最后是菜单按钮的点击事件

    private fun click(){binding.navView.setOnItemSelectedListener{transaction = supportFragmentManager.beginTransaction()when(it.itemId) {R.id.navigation_home -> {toFragment = 0switchFragment(transaction)mFragment = 0}R.id.navigation_dashboard -> {toFragment = 1switchFragment(transaction)mFragment = 1}R.id.navigation_notifications -> {toFragment = 2switchFragment(transaction)mFragment = 2}}true}}

每次跳转前给toFragment赋值，跳转后给mFragment赋值，这样switch函数就能知道当前的fragment和目的fragment。

4. 关于androidstudio自生成的xml文件的代码修改

activity_main.xml文件中fragment标签，取消使用navigation绑定几个fragment，去掉最后一个属性即可

<fragmentandroid:id="@+id/nav_host_fragment_activity_main"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" 这一行删掉
/>

这样的话，res文件夹中的navigation文件夹就可以直接删除

贴一个MainActivity的全部代码吧

package com.example.testfragmentimport android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.fragment.app.FragmentTransaction
import com.example.testfragment.databinding.ActivityMainBinding
import com.example.testfragment.ui.DashboardFragment
import com.example.testfragment.ui.HomeFragment
import com.example.testfragment.ui.NotificationsFragmentclass MainActivity : AppCompatActivity() {companion object{private val home = HomeFragment()private val dash = DashboardFragment()private val notification = NotificationsFragment()val list = arrayListOf(home, dash, notification)private lateinit var transaction: FragmentTransactionvar mFragment = 0var toFragment = 0}private lateinit var binding: ActivityMainBindingoverride fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)binding = ActivityMainBinding.inflate(layoutInflater)setContentView(binding.root)init()click()}private fun init(){transaction = supportFragmentManager.beginTransaction()transaction.add(R.id.nav_host_fragment_activity_main, home).show(home).commit()}private fun click(){binding.navView.setOnItemSelectedListener{transaction = supportFragmentManager.beginTransaction()when(it.itemId) {R.id.navigation_home -> {toFragment = 0switchFragment(transaction)mFragment = 0}R.id.navigation_dashboard -> {toFragment = 1switchFragment(transaction)mFragment = 1}R.id.navigation_notifications -> {toFragment = 2switchFragment(transaction)mFragment = 2}}true}}private fun switchFragment(transaction: FragmentTransaction){val from = list[mFragment]val to = list[toFragment]if (mFragment == toFragment) returnif (from.isAdded) {if (to.isAdded) {transaction.hide(from).show(to).commit()} else {transaction.add(R.id.nav_host_fragment_activity_main, to).hide(from).show(to).commit()}} else {if (to.isAdded) {transaction.add(R.id.nav_host_fragment_activity_main, from).hide(from).show(to).commit()} else {transaction.add(R.id.nav_host_fragment_activity_main, from).add(R.id.nav_host_fragment_activity_main, to).hide(from).show(to).commit()}}}
}

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