2 个片段事务在 onCreate() 中是不可能的

Question

我尝试在onCreate()中执行 2 个片段事务：

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    bottomNav_main.setOnNavigationItemSelectedListener(this)
    val firstFragment = OpeningFragment()
    firstFragment.arguments = intent.extras
    val transaction = supportFragmentManager.beginTransaction()
    transaction.add(R.id.fragment_layout, firstFragment, "add_activitymain")
    transaction.add(R.id.fragment_layout, OptionsFragment(), "add_options")
    transaction.add(R.id.fragment_layout, LeaderboardsFragment(), "add_leaderboards")
    transaction.commit()
    val transaction2 = supportFragmentManager.beginTransaction()
    val options: Fragment = supportFragmentManager.findFragmentByTag("add_options")!!
    val leaderboards: Fragment = supportFragmentManager.findFragmentByTag("add_leaderboards")!!
    transaction2.hide(options)
    transaction2.hide(leaderboards)
    transaction2.commit()
}

但是在变量上收到options NullPointerException。但是当我将第二个事务分离到onStart()方法中时，它工作正常：

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    bottomNav_main.setOnNavigationItemSelectedListener(this)
    val firstFragment = OpeningFragment()
    firstFragment.arguments = intent.extras
    val transaction = supportFragmentManager.beginTransaction()
    transaction.add(R.id.fragment_layout, firstFragment, "add_activitymain")
    transaction.add(R.id.fragment_layout, OptionsFragment(), "add_options")
    transaction.add(R.id.fragment_layout, LeaderboardsFragment(), "add_leaderboards")
    transaction.commit()
}
override fun onStart() {
    super.onStart()
    val transaction2 = supportFragmentManager.beginTransaction()
    val options: Fragment = supportFragmentManager.findFragmentByTag("add_options")!!
    val leaderboards: Fragment = supportFragmentManager.findFragmentByTag("add_leaderboards")!!
    transaction2.hide(options)
    transaction2.hide(leaderboards)
    transaction2.commit()
}

有人可以解释为什么会这样吗？

Answer 1

这主要是因为提交事务不会立即发生。它需要几秒钟并且异步工作。尝试 commitNow(( 使其即时和同步。

Answer 2

片段

转换在 onCreate(( 中是不可能的

尝试在 onViewCreated(( 中执行此操作

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    AppCompatDelegate.setCompatVectorFromResourcesEnabled(true)
    create()
}