快速点击Android概览按钮两次时出现内部异常/崩溃

Question

我有一个名为CustomArFragment（Sceneform 1.15的ArFragment子类）的片段，它包含在MainActivity中。在某个时刻，我通过使用新实例替换来重置片段。我通过在MainActivity中调用以下方法来执行此操作：

fun reset() {
        val oldFragment = (supportFragmentManager.findFragmentById(R.id.custom_ar_fragment)) as? CustomArFragment ?: return
        val newArFragment = CustomArFragment()

        val transaction = supportFragmentManager.beginTransaction()
        transaction.remove(oldFragment)
        transaction.add(R.id.custom_ar_fragment, newArFragment)
        transaction.commit()
}

现在，正如我所说的，替换一开始似乎可以正常工作，并且新片段在替换后像往常一样运行。然而，如果我在调用reset()之后的任何时候快速连续按两次android概览按钮，我会收到以下错误提示，这似乎与我无法跟踪的一些内部调用有关：

java.lang.NullPointerException: throw with null exception
        at com.google.ar.sceneform.utilities.Preconditions.checkNotNull(SourceFile:3)
        at com.google.ar.sceneform.SceneView.onLayout(SourceFile:41)
        at com.google.ar.sceneform.ArSceneView.onLayout(SourceFile:79)
        at android.view.View.layout(View.java:20729)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20729)
        at android.view.ViewGroup.layout(ViewGroup.java:6198)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20729)
        at android.view.ViewGroup.layout(ViewGroup.java:6198)
        at androidx.constraintlayout.widget.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
        at android.view.View.layout(View.java:20729)
        at android.view.ViewGroup.layout(ViewGroup.java:6198)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20729)
        at android.view.ViewGroup.layout(ViewGroup.java:6198)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1812)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1656)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1565)
        at android.view.View.layout(View.java:20729)
        at android.view.ViewGroup.layout(ViewGroup.java:6198)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20729)
        at android.view.ViewGroup.layout(ViewGroup.java:6198)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1812)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1656)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1565)
        at android.view.View.layout(View.java:20729)
        at android.view.ViewGroup.layout(ViewGroup.java:6198)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at com.android.internal.policy.DecorView.onLayout(DecorView.java:757)
        at android.view.View.layout(View.java:20729)
        at android.view.ViewGroup.layout(ViewGroup.java:6198)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2859)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2386)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1524)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7398)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1092)
        at android.view.Choreographer.doCallbacks(Choreographer.java:888)
        at android.view.Choreographer.doFrame(Choreographer.java:819)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1078)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:201)
        at android.app.ActivityThread.main(ActivityThread.java:6815)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)

现在，我不确定这是否与Sceneform相关的错误，还是我替换片段的方式有问题。

我想澄清一下，除非我在替换完成后双击应用程序概述按钮，否则替换工作得非常完美，并且我没有在CustomArFragment的生命周期方法中执行任何可能干扰替换的操作。因此，如果我按概述按钮，然后等待一两秒钟再次按下它以打开应用程序，则所有内容都按预期工作而不会崩溃。

非常感谢您的帮助，我已经无法想出可能引起此问题的原因！

- A. Claesson

Answer 1

崩溃似乎与某些ARCore/Sceneform资源在删除旧片段后仍保持运行一段时间有关，但仍然引用已删除的片段（我解决这个问题已经有一段时间了，所以我不记得确切的原因，但我认为是这个原因）。

无论如何，为了避免崩溃，我发现唯一有效的方法是将已删除的片段添加到返回堆栈中。这样，sceneform/ARCore就不会遇到空指针。

    val oldFragment = (supportFragmentManager.findFragmentById(R.id.custom_ar_fragment)) as? CustomArFragment ?: return
    oldFragment.arSceneView.pause()
    val newArFragment = CustomArFragment()

    val transaction = supportFragmentManager.beginTransaction().remove(oldFragment).add(R.id.sceneform_fragment, newArFragment).addToBackStack(null)
    transaction.commit()
    supportFragmentManager.executePendingTransactions()

当然，将旧片段保存到后退栈中并不理想，因为我从未打算返回它们。但是，由于我假设片段的重置次数很少，所以这对我来说已经足够好了。但是，如果您找到了更好的重置ArFragment的解决方案，请告诉我。

- A. Claesson