如何从 ViewPager2 的 NavHostFragment 内的片段导航回来? [英] How to navigate back from fragments inside NavHostFragment of ViewPager2?

问题描述

从 ViewPager2 的嵌套片段返回的正确方法是什么?

What is the proper way of navigating back from nested fragments of ViewPager2?

尽管使用 app:defaultNavHost=true" 和 FragmentContainerView 在页面的嵌套片段中按后退按钮调用 Activity 的后按而不是导航回上一个片段.

Despite using app:defaultNavHost="true"with FragmentContainerView pressing back button while in a nested fragment of a page calls Activity's back press instead of navigating back to previous fragment.

推荐答案

根据 创建NavHostFragment文档，app:defaultNavHost=true"调用setPrimaryNavigationFragment() 首次添加 Fragment 时 - 它是 setPrimaryNavigationFragment() 自动将返回按钮按下事件路由到该片段.

As per the Create a NavHostFragment documentation, app:defaultNavHost="true" calls setPrimaryNavigationFragment() when the Fragment is first added - it is setPrimaryNavigationFragment() that routes back button press events to that fragment automatically.

在 ViewPager2 中，负责创建和添加 Fragment 的是 ViewPager2.由于 Fragment 层次结构的每一层都需要为主导航片段，通过 XML 添加子片段仍然不能解决缺失的链接:ViewPager2 中的 Fragment 需要为主导航片段.

In a ViewPager2 though, it is the ViewPager2 that is responsible for creating and adding the Fragment. Since every level of the Fragment hierarchy needs to be the primary navigation fragment, adding a child fragment via XML still doesn't solve the missing link: that the Fragment in the ViewPager2 needs to be the primary navigation fragment.

因此，当 Fragment 成为活动 Fragment 时，您需要挂钩回调并调用 setPrimaryNavigationFragment().ViewPager2 1.1.0-alpha01 在FragmentTransactionCallback，特别是onFragmentMaxLifecyclePreUpdated()，每当 Fragment 的 Lifecycle 状态发生更改时都会调用它:当它更改为 RESUMED 时，该 Fragment 现在是活动片段，应该成为主要导航片段，作为 onPost 回调的一部分.

Therefore, you need to hook into the callbacks for when a Fragment is made the active Fragment and call setPrimaryNavigationFragment(). ViewPager2 1.1.0-alpha01 adds exactly this API in the FragmentTransactionCallback, specifically, the onFragmentMaxLifecyclePreUpdated(), which is called whenever the Lifecycle state of a Fragment is changed: when it is changed to RESUMED, that Fragment is now the active fragment and should become the primary navigation Fragment as part of the onPost callback.

private class Adapter(parentFragment: Fragment) : FragmentStateAdapter(parentFragment) {
    init {
        // Add a FragmentTransactionCallback to handle changing
        // the primary navigation fragment
        registerFragmentTransactionCallback(object : FragmentTransactionCallback() {
            override fun onFragmentMaxLifecyclePreUpdated(
                    fragment: Fragment,
                    maxLifecycleState: Lifecycle.State
            ) = if (maxLifecycleState == Lifecycle.State.RESUMED) {
                // This fragment is becoming the active Fragment - set it to
                // the primary navigation fragment in the OnPostEventListener
                OnPostEventListener {
                    fragment.parentFragmentManager.commitNow {
                        setPrimaryNavigationFragment(fragment)
                    }
                }
            } else {
                super.onFragmentMaxLifecyclePreUpdated(fragment, maxLifecycleState)
            }
        })
    }

    // The rest of your FragmentStateAdapter...
}

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