片段简历：显示和隐藏交易不起作用

Question

我想恢复后台堆栈中的 android 片段。在底部导航的不同选项卡之间切换时，我不希望重新创建片段的视图。

我阅读了this、this、this 以及有关 stackoverflow 的其他一些与此相关的问题。常见的建议是使用片段交易的显示和隐藏方法，但它不起作用。这是我的 kotlin 代码：

    bottomnavigation.setOnNavigationItemSelectedListener {
        item ->
        when(item.itemId){
            R.id.first_fragment_item -> {
                var fragment:Fragment = FirstFragment.newInstance()
                replaceFragment(fragment)
                return@setOnNavigationItemSelectedListener true
            }
            R.id.second_fragment_item -> {
                var fragment:Fragment = SecondFragment.newInstance()
                replaceFragment(fragment)
                return@setOnNavigationItemSelectedListener true
            }

        }
        return@setOnNavigationItemSelectedListener false

    }

}

  fun replaceFragment(fragment:Fragment) {

    var fragmentName:String = fragment::class.simpleName!!

    if(supportFragmentManager.findFragmentByTag(fragmentName)==null) {
        val fragmentTransaction = supportFragmentManager.beginTransaction()
        fragmentTransaction.replace(R.id.fragment_container, fragment, fragmentName)
        fragmentTransaction.addToBackStack(fragmentName)
        fragmentTransaction.commit()
    }
    else{
        if(fragmentName == "FirstFragment") {
            supportFragmentManager.beginTransaction().
                     hide(supportFragmentManager.findFragmentByTag("SecondFragment"))
                    .show(supportFragmentManager.findFragmentByTag("FirstFragment"))
                    .commit()
        }
        else{
            supportFragmentManager.beginTransaction()
                    .hide(supportFragmentManager.findFragmentByTag("FirstFragment"))
                    .show(supportFragmentManager.findFragmentByTag("SecondFragment"))
                    .commit()
        }
    }

}

总是显示 backstack 的最后一个片段。当我想隐藏它并显示时 另一个片段，那个片段屏幕变成白色，上面什么都没有。

我的底部导航有四个项目，但出于测试目的，我只使用了两个。其中两个片段是 FirstFragment 和 SecondFragment。我正在使用 v4.app.fragments。

Answer 1

在Kishan Viramgama 评论的帮助下，我解决了我的问题。我使用“添加”方法而不是“替换”方法。这是我恢复片段的代码：`

fun replaceFragment(fragment: Fragment) {
    var nameOfFragment: String? = fragment::class.simpleName
    var fragmentTransaction = supportFragmentManager.beginTransaction()
    var fm: FragmentManager = supportFragmentManager
    if (supportFragmentManager.findFragmentByTag(nameOfFragment) == null) {
        fragmentTransaction.add(R.id.root_fragments, fragment, nameOfFragment)      //if fragment is not added to the backstack,add it/ don't use replace because it creates new fragment emptying fragment container
        fragmentTransaction.addToBackStack(nameOfFragment)
        fragmentTransaction.commit()
    } else {
        var fragmentToShow:Fragment = supportFragmentManager.findFragmentByTag(nameOfFragment) //if fragment is already in backstack,show it and hide all other fragments.
        for (i in 0 until fm.backStackEntryCount) {
            var fragmentToHide: Fragment = fm.findFragmentByTag(fm.getBackStackEntryAt(i).name)
            if (fragmentToHide::class.simpleName != fragment::class.simpleName)
                fragmentTransaction.hide(fragmentToHide)
        }
        fragmentTransaction.show(fragmentToShow)
        fragmentTransaction.commit()
    }

}

`