android - 为什么 fragment 的父级在暂停后为空？

Question

当我运行应用程序并使用它时没有崩溃。 BottomSheetBehavior 工作正常。但是，如果我在后台长时间暂停后返回应用程序，当尝试将 view.parent as View 转换为 View 以查找具有 BottomSheetBehavior 的 View 时，它会因空点异常而崩溃.

为什么 fragment 暂停后没有父级？如何解决？

我尝试做 binding.root.parent as View 和其他 requireActivity().findViewById(R.id.fragment_sheet_container)。但同一案例的崩溃连续发生。

@AndroidEntryPoint
class ToolboxSheetFragment : Fragment(R.layout.fragment_toolbox_sheet) {
    private lateinit var binding: FragmentToolboxSheetBinding
    private lateinit var behavior: BottomSheetBehavior<View>

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        binding = FragmentToolboxSheetBinding.bind(view)
        behavior = BottomSheetBehavior.from(
            view.parent as View // null cannot be cast to non-null type android.view.View
        )
    }

fragment 的父级是下一个布局的主要 Activity :

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".view.MainActivity">

    // ...

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragment_sheet_container"
        android:name=".view.fragment.ToolboxSheetFragment"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:behavior_hideable="false"
        app:behavior_peekHeight="?actionBarSize"
        app:layout_behavior="com.google.android.material.bottomsheet.BottomSheetBehavior"
        tools:layout="@layout/fragment_toolbox_sheet" />

</androidx.coordinatorlayout.widget.CoordinatorLayout>

java.lang.RuntimeException: Unable to start activity ComponentInfo{
.view.MainActivity}: java.lang.NullPointerException: null cannot be cast to non-null type android.view.View
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2792)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2870)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1601)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:172)
        at android.app.ActivityThread.main(ActivityThread.java:6590)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: null cannot be cast to non-null type android.view.View
        at .view.fragment.ToolboxSheetFragment.onViewCreated(ToolboxSheetFragment.kt:46)
        at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:3128)
        at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:552)
        at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:261)
        at androidx.fragment.app.FragmentStore.moveToExpectedState(FragmentStore.java:113)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1424)
        at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:2968)
        at androidx.fragment.app.FragmentManager.dispatchActivityCreated(FragmentManager.java:2886)
        at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:263)
        at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:351)
        at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:246)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1335)
        at android.app.Activity.performStart(Activity.java:7043)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2755)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2870) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1601) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:172) 
        at android.app.ActivityThread.main(ActivityThread.java:6590) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

Answer 1

如果能拿到完整的project就更好了。但我在这里建议几种方法:

方式一:

如何将行为 保留在 Activity 类中。实例化它 onCreate()。并像这样从 ToolboxSheetFragment 使用它。

class MainActivity : BaseActivity() {
    var behavior: BottomSheetBehavior? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        behavior = BottomSheetBehavior.from(R.id.sheet_id)
    }
}

然后从 ToolboxSheetFragment

class ToolboxSheetFragment : Fragment(R.layout.fragment_toolbox_sheet) {
    private lateinit var binding: FragmentToolboxSheetBinding
    var behavior: BottomSheetBehavior? = null

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        binding = FragmentToolboxSheetBinding.bind(view)
        behavior = (requireActivity() as MainActivity).behavior
    }

方式二:

在 ToolboxSheetFragment 上使用 addOnAttachStateChangeListener:

@AndroidEntryPoint
class ToolboxSheetFragment : Fragment(R.layout.fragment_toolbox_sheet) {
    private lateinit var binding: FragmentToolboxSheetBinding
    private lateinit var behavior: BottomSheetBehavior<View>

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        view.addOnAttachStateChangeListener(object : View.OnAttachStateChangeListener {
            override fun onViewAttachedToWindow(v: View?) {
                if(v!=null){
                    binding = FragmentToolboxSheetBinding.bind(v)
                    behavior = BottomSheetBehavior.from(
                        v.parent as View 
                    )
                }
            }

            override fun onViewDetachedFromWindow(v: View?) {
                view.removeOnAttachStateChangeListener(this)
            }

        })
    }