使用 savedInstanceState 保存和恢复 Kotlin lambda

Question

如何在屏幕旋转之前将 Kotlin lambda 保存在片段中？它可以在 Activity 中工作，但不能在片段中正确工作。执行 lambda 表达式时如果包含对 PermissionsFragment 子类的方法的调用会发生异常，为什么？

自身存储 lambda 的类：

class ActionKeeper(var action: ((isGranted: Boolean) -> Unit)? = null) : Serializable

在屏幕旋转时将 lambda 保存到 ActionKeeper 的片段：

abstract class PermissionsFragment : Fragment() {

    private var action: ((isGranted: Boolean) -> Unit)? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        if (savedInstanceState != null) {
            restoreState(savedInstanceState)
        }
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putSerializable("actionKeeper", ActionKeeper(action))
    }

    private fun restoreState(state: Bundle) {
        val keeper = state.getSerializable("actionKeeper") as ActionKeeper
        action = keeper.action
    }

    fun usePermission(permission: String, action: (isGranted: Boolean) -> Unit) {
        if (!isPermissionGranted(permission)) {
            this.action = action
            requestPermissions(arrayOf(permission), 1)
        } else {
            action(true)
        }
    }

   /* ........ */
}

从 PermissionsFragment 扩展的类：

class SamplePermissionsFragment : PermissionsFragment() {
    private var toast: Toast? = null

    private fun doWithPermission() {
        usePermission(Manifest.permission.SEND_SMS) { isGranted ->
            if (isGranted) {
                showToast("Fragment permission granted")
            } else {
                showToast("Fragment permission refused")
            }
        }
    }

    private fun showToast(text: String) {
        toast?.cancel()
        toast = Toast.makeText(context!!, text, Toast.LENGTH_SHORT).apply { show() }
    }
}

Logcat 扩展：

2018-09-27 15:57:17.068 5569-5569/com.alexchurkin.permissionsample E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.alexchurkin.permissionsample, PID: 5569
    java.lang.RuntimeException: Failure delivering result ResultInfo{who=@android:requestPermissions:, request=65537, result=-1, data=Intent { act=android.content.pm.action.REQUEST_PERMISSIONS (has extras) }} to activity {com.alexchurkin.permissionsample/com.alexchurkin.permissionsample.fragment.FragmentHostActivity}: kotlin.KotlinNullPointerException
        at android.app.ActivityThread.deliverResults(ActivityThread.java:4196)
        at android.app.ActivityThread.handleSendResult(ActivityThread.java:4239)
        at android.app.ActivityThread.-wrap20(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1599)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:165)
        at android.app.ActivityThread.main(ActivityThread.java:6365)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:883)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:773)
     Caused by: kotlin.KotlinNullPointerException
        at com.alexchurkin.permissionsample.fragment.SamplePermissionsFragment.showToast(SamplePermissionsFragment.kt:56)
        at com.alexchurkin.permissionsample.fragment.SamplePermissionsFragment.access$showToast(SamplePermissionsFragment.kt:13)
        at com.alexchurkin.permissionsample.fragment.SamplePermissionsFragment$doWithPermission$1.invoke(SamplePermissionsFragment.kt:35)
        at com.alexchurkin.permissionsample.fragment.SamplePermissionsFragment$doWithPermission$1.invoke(SamplePermissionsFragment.kt:13)
        at com.alexchurkin.fastpermissions.fragments.PermissionsFragment.onRequestPermissionsResult(PermissionsFragment.kt:38)
        at android.support.v4.app.FragmentActivity.onRequestPermissionsResult(FragmentActivity.java:860)
        at android.app.Activity.dispatchRequestPermissionsResult(Activity.java:7268)
        at android.app.Activity.dispatchActivityResult(Activity.java:7120)
        at android.app.ActivityThread.deliverResults(ActivityThread.java:4192)
        at android.app.ActivityThread.handleSendResult(ActivityThread.java:4239) 
        at android.app.ActivityThread.-wrap20(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1599) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:165) 
        at android.app.ActivityThread.main(ActivityThread.java:6365) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:883) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:773) 

Answer 1

您的代码中的问题是您的 lambda 中的引用将来会被释放（因此，无法访问）。保存状态时，Android 会将指针（引用）保存在该 lambda 中，因此它不能指向无效内存。

这里有更好的解释：How to save and restore lambdas in Android?

要修复它，在您的情况下，您可以在 lambda 参数之一中引用被调用者，即

private var action: ((Fragment, Boolean) -> Unit)? = null

Answer 2

您可能对 context

有疑问

试试下面的代码，让我知道：

private fun showToast(text: String) {
    toast?.cancel()
    activity?.let {
        toast = Toast.makeText(it, text, Toast.LENGTH_SHORT).apply { show() }
    }
}

编辑：

将您的 ActionKeeper 的对象另存为：

outState.putSerializable("actionKeeper", object: ActionKeeper(action))