Android-Jetpack笔记-Navigation之Fragment支持复用
上篇文章Android-Jetpack笔记-Navigation之Fragment使用提到，每次切换目的地，fragment是反复销毁重建的，按照谷歌推荐的1个APP只需1个activity的思路开发，这样是没问题的，但是这里的fragment是作为首页的3个常驻页面，我们是希望能够保存起来的，毕竟，销毁重建需要重新请求网络数据，重新初始化view，严重影响用户体验，那么接下来继续上篇文章的分析，支持Fragment的复用。
Jetpack笔记代码
本文源码基于SDK 29，IDE是 Android Studio Arctic Fox | 2020.3.1 Patch 2
解决
给上篇文章的项目加些日志，从面板页切到通知页，再从通知页切回面板页，查看日志，
可见面板页发生了销毁重建，接着创建一个ReuseFragmentNavigatorKotlin继承FragmentNavigator并重写navigate方法，直接把父类的实现copy过来，第1步，在调用fragment初始化的地方，改成复用：
//ReuseFragmentNavigatorKotlin.kt
override fun navigate(
destination: Destination,
args: Bundle?,
navOptions: NavOptions?,
navigatorExtras: Navigator.Extras?,
): NavDestination?{
//......
//fix 1: 把类名作为tag，寻找已存在的Fragment
//（如果想只针对个别fragment进行保活复用，可以在tag上做些标记比如加个前缀，这里不再展开）
var frag = mFragmentManager.findFragmentByTag(className)
frag = frag ?: instantiateFragment(mContext, mFragmentManager, className, args)
//......
}
第2步，将ft.replace换成show和hide，
//ReuseFragmentNavigatorKotlin.kt
override fun navigate(
destination: Destination,
args: Bundle?,
navOptions: NavOptions?,
navigatorExtras: Navigator.Extras?,
): NavDestination?{
//......
val fragments = mFragmentManager.fragments
for (fragment in fragments) {
ft.hide(fragment!!)
}
if (!frag.isAdded) {
ft.add(mContainerId, frag, className)
}
//ft.replace(mContainerId, frag);
//fix 2: replace换成show和hide
ft.show(frag)
ft.setPrimaryNavigationFragment(frag)
//......
}
第3步，反射获取父类的mBackStack，
//ReuseFragmentNavigatorKotlin.kt
override fun navigate(
destination: Destination,
args: Bundle?,
navOptions: NavOptions?,
navigatorExtras: Navigator.Extras?,
): NavDestination?{
//......
//fix 3: mBackStack是私有的，而且没有暴露出来，需要反射获取
var mBackStack: ArrayDeque<Int>? = null
try {
val aClass = FragmentNavigator::class.java
val mBackStackField = aClass.getDeclaredField("mBackStack")
mBackStackField.isAccessible = true
//mBackStack = mBackStackField.get(this) as ArrayDeque //也行
mBackStack = mBackStackField[this] as ArrayDeque<Int>
} catch (e: Exception) {
e.printStackTrace()
return null
}
//......
}
第4步，把父类的私有方法generateBackStackName也copy过来，
//ReuseFragmentNavigatorKotlin.kt
//fix 4: 从父类那边copy过来即可
private fun generateBackStackName(backStackIndex: Int, destId: Int): String {
return "$backStackIndex-$destId"
}
第5步，给ReuseFragmentNavigatorKotlin加上注解给他取个名字，原因放后面说，
//fix 5: 需要指定1个名字，源码里自带的名字有navigation、activity、fragment、dialog
@Navigator.Name("reuseFragment")
class ReuseFragmentNavigatorKotlin(
private val mContext: Context,
private val mFragmentManager: FragmentManager,
private val mContainerId: Int,
) : FragmentNavigator(mContext, mFragmentManager, mContainerId){
}
至此ReuseFragmentNavigatorKotlin就写好了
完整代码
@Navigator.Name("reuseFragment")
class ReuseFragmentNavigatorKotlin(
private val mContext: Context,
private val mFragmentManager: FragmentManager,
private val mContainerId: Int,
) : FragmentNavigator(mContext, mFragmentManager, mContainerId) {
override fun navigate(
destination: Destination,
args: Bundle?,
navOptions: NavOptions?,
navigatorExtras: Navigator.Extras?,
): NavDestination? {
if (mFragmentManager.isStateSaved) {
Log.i(TAG, "Ignoring navigate() call: FragmentManager has already"
+ " saved its state")
return null
}
var className = destination.className
if (className[0] == '.') {
className = mContext.packageName + className
}
var frag = mFragmentManager.findFragmentByTag(className)
frag = frag ?: instantiateFragment(mContext, mFragmentManager, className, args)
frag.arguments = args
val ft = mFragmentManager.beginTransaction()
var enterAnim = navOptions?.enterAnim ?: -1
var exitAnim = navOptions?.exitAnim ?: -1
var popEnterAnim = navOptions?.popEnterAnim ?: -1
var popExitAnim = navOptions?.popExitAnim ?: -1
if (enterAnim != -1 || exitAnim != -1 || popEnterAnim != -1 || popExitAnim != -1) {
enterAnim = if (enterAnim != -1) enterAnim else 0
exitAnim = if (exitAnim != -1) exitAnim else 0
popEnterAnim = if (popEnterAnim != -1) popEnterAnim else 0
popExitAnim = if (popExitAnim != -1) popExitAnim else 0
ft.setCustomAnimations(enterAnim, exitAnim, popEnterAnim, popExitAnim)
}
val fragments = mFragmentManager.fragments
for (fragment in fragments) {
ft.hide(fragment!!)
}
if (!frag.isAdded) {
ft.add(mContainerId, frag, className)
}
//ft.replace(mContainerId, frag);
ft.show(frag)
ft.setPrimaryNavigationFragment(frag)
var mBackStack: ArrayDeque<Int>? = null
try {
val aClass = FragmentNavigator::class.java
val mBackStackField = aClass.getDeclaredField("mBackStack")
mBackStackField.isAccessible = true
//mBackStack = mBackStackField.get(this) as ArrayDeque //也行
mBackStack = mBackStackField[this] as ArrayDeque<Int>
} catch (e: Exception) {
e.printStackTrace()
return null
}
@IdRes val destId = destination.id
val initialNavigation = mBackStack.isEmpty()
// TODO Build first class singleTop behavior for fragments
val isSingleTopReplacement = (navOptions != null && !initialNavigation
&& navOptions.shouldLaunchSingleTop()
&& mBackStack.peekLast() == destId)
val isAdded: Boolean
isAdded = if (initialNavigation) {
true
} else if (isSingleTopReplacement) {
if (mBackStack.size > 1) {
mFragmentManager.popBackStack(
generateBackStackName(mBackStack.size, mBackStack.peekLast()),
FragmentManager.POP_BACK_STACK_INCLUSIVE)
ft.addToBackStack(generateBackStackName(mBackStack.size, destId))
}
false
} else {
ft.addToBackStack(generateBackStackName(mBackStack.size + 1, destId))
true
}
if (navigatorExtras is Extras) {
for ((key, value) in navigatorExtras.sharedElements) {
ft.addSharedElement(key!!, value!!)
}
}
ft.setReorderingAllowed(true)
ft.commit()
// The commit succeeded, update our view of the world
return if (isAdded) {
mBackStack.add(destId)
destination
} else {
null
}
}
private fun generateBackStackName(backStackIndex: Int, destId: Int): String {
return "$backStackIndex-$destId"
}
companion object {
private const val TAG = "ReuseFragmentNavigator"
}
}
接下来要如何把它使用进去呢？
先在布局文件中去掉app:navGraph="@navigation/publicwelfare_navigation"，然后来到activity，编写如下代码，
//PublicWelfareActivity.kt
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
//获取页面容器NavHostFragment
val fragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment) as Fragment
//获取导航控制器
NavHostFragment.findNavController(fragment)
.apply {
//自定义Fragment导航器
val fragmentNavigator =
ReuseFragmentNavigatorKotlin(this@PublicWelfareActivity,
fragment.childFragmentManager,
fragment.id)
//添加自定义的Fragment导航器 到 导航器提供者 中
navigatorProvider.addNavigator(fragmentNavigator)
//手动创建导航图
graph = initNavGraph(navigatorProvider, fragmentNavigator)
}
}
//手动创建导航图，把目的地添加进来
private fun initNavGraph(
provider: NavigatorProvider,
fragmentNavigator: FragmentNavigator,
): NavGraph {
return NavGraph(NavGraphNavigator(provider)).apply {
//用自定义的导航器来创建目的地
val prosFragDest: FragmentNavigator.Destination =
fragmentNavigator.createDestination().apply {
//这些是之前去掉的导航文件 publicwelfare_navigation 中的配置信息，对照配置上去就行
id = R.id.projectsFragment
className = ProjectsFragment::class.qualifiedName!!
label = "公益项目列表"
}
//添加目的地到导航图中
addDestination(prosFragDest)
val proDetailDest: FragmentNavigator.Destination =
fragmentNavigator.createDestination().apply {
id = R.id.donateWollarFragment
className = DonateWollarFragment::class.qualifiedName!!
label = "公益项目详情"
}
addDestination(proDetailDest)
//创建并添加更多目的地到导航图...
//...
//设置最初目的地
startDestination = R.id.projectsFragment
}
}
具体流程都写在代码注释里了，至此就实现了对fragment的复用。
前边提到的自定义导航器需要指定名字@Navigator.Name("reuseFragment")，是因为不同类型的目的地（页面）需要使用不同的导航器，在NavigatorProvider里有个map存储了多个导航器，
//NavigatorProvider.java
private final HashMap<String, Navigator<? extends NavDestination>> mNavigators =
new HashMap<>();
// "navigation" NavGraphNavigator
// "activity" ActivityNavigator
// "fragment" FragmentNavigator
// "dialog" DialogFragmentNavigator
// "reuseFragment" ReuseFragmentNavigatorKotlin 这个就是我们自定义的导航器
然后，使用自定义导航器ReuseFragmentNavigatorKotlin来createDestination创建目的地，这样就把导航器和目的地绑定在一起了。可以看出，Navigation的思想是，把各种类型的页面都抽象成目的地Destination，进行统一跳转，不同的导航器则封装了不同类型页面跳转的实现，由NavController统一调度，而许许多多的目的地则编织成了一个导航图NavGraph。