Question

是否可以使用Android导航组件中的navigate函数而不创建新的fragment实例，而是恢复前一个实例？

我试图恢复以前的片段，但是只有使用navigate函数才能在这些片段之间传输数据。

Answer 1

我相信您可以通过ViewModelProvider从活动中获取视图模型。

inline fun <reified VM : ViewModel> BaseFragment.vmProviderActivity() = lazy {

    val viewModel = activity?.let { ViewModelProviders.of(it).get(VM::class.java) }

    if (viewModel is BaseViewModel) {
        viewModel.kodeinInstance = kodein
    }

    return@lazy viewModel
}

Answer 2

还没有，但是我想他们正在努力。暂时始终会创建新实例。但是，如果您编写自己的导航器，则可能。

@Navigator.Name("keep_state_fragment") // `keep_state_fragment` is used in navigation xml
class KeepStateNavigator(
    private val context: Context,
    private val manager: FragmentManager, // Should pass childFragmentManager.
    private val containerId: Int
) : FragmentNavigator(context, manager, containerId) {

    override fun navigate(
        destination: Destination,
        args: Bundle?,
        navOptions: NavOptions?,
        navigatorExtras: Navigator.Extras?
    ): NavDestination? {
        val tag = destination.id.toString()
        val transaction = manager.beginTransaction()

        var initialNavigate = false
        val currentFragment = manager.primaryNavigationFragment
        if (currentFragment != null) {
            transaction.detach(currentFragment)
        } else {
            initialNavigate = true
        }

        var fragment = manager.findFragmentByTag(tag)
        if (fragment == null) {
            val className = destination.className
            fragment = manager.fragmentFactory.instantiate(context.classLoader, className)
            transaction.add(containerId, fragment, tag)
        } else {
            transaction.attach(fragment)
        }

        transaction.setPrimaryNavigationFragment(fragment)
        transaction.setReorderingAllowed(true)
        transaction.commitNow()

        return if (initialNavigate) {
            destination
        } else {
            null
        }
    }
}

然后您必须像下面这样呼叫

val navigator = KeepStateNavigator(this, navHostFragment.childFragmentManager, R.id.nav_host_fragment)
        navController.navigatorProvider.addProvider(navigator)

这是示例：Github

Answer 3

我改进了 Umut ADALI 的回答：

/**
 * Created by Nooi on 12,February,2021
 */
@Navigator.Name("fragment")
class PersistentFragmentNavigator(
    persistentClasses : Set<KClass<out Fragment>>,
    context : Context,
    manager : FragmentManager,
    containerId : Int
) : FragmentNavigator(context, manager, containerId) {

    private val navigatorHandler = PersistentNavigatorHandler(persistentClasses, manager, containerId)

    init {
        navigatorHandler.addTopFragmentIfRequired()
    }

    override fun instantiateFragment(context : Context, fragmentManager : FragmentManager, className : String, args : Bundle?) : Fragment {
        return navigatorHandler.instantiateFragment(className) {
            super.instantiateFragment(context, fragmentManager, className, args)
        }
    }
}

private class PersistentNavigatorHandler(
    private val persistentClasses : Set<KClass<out Fragment>>,
    private val manager : FragmentManager,
    private val containerId : Int
) {

    private val instances = mutableMapOf<String, Fragment>()

    fun addTopFragmentIfRequired() {
        manager.findFragmentById(containerId)
            ?.takeIf { fragment ->
                persistentClasses.any { it.qualifiedName == fragment.javaClass.name }
            }
            ?.let { fragment ->
                instances[fragment.javaClass.name] = fragment
            }
    }

    fun instantiateFragment(className : String, superCall : () -> Fragment) : Fragment {
        return instances[className] ?: superCall.invoke().also { fragment ->
            if (persistentClasses.any { it.qualifiedName == className }) {
                instances[className] = fragment
            }
        }
    }
}

在获取navController和navHostFragment之后，将此代码添加到您的活动的onCreate中：

    navController.navigatorProvider.addNavigator(PersistentFragmentNavigator(
        persistentClasses = setOf(SomePersistentFragment::class),
        context = this,
        manager = navHostFragment.childFragmentManager,
        containerId = R.id.nav_host_fragment
    ))

导航将与图形正常工作，但是当再次导航到标记为持久的片段时，将恢复先前的片段实例。在构建任何标签导航时非常有用。

导航时不创建Fragment-Navigation Component的新实例

3 个答案: