按下返回键关闭App时DetectionFragment触发NullPointerException求助

我开发了一个采用单Activity多Fragment架构的Android应用，其中包含一个DetectionFragment用于检测功能，核心代码如下：

private val PERMISSIONS_REQUIRED = arrayOf(Manifest.permission.CAMERA)
class DetectionFragment : Fragment(), ObjectDetectorHelper.DetectorListener {
    private var _binding: FragmentDetectionBinding? = null
    private val binding get() = _binding!!
    // 其余代码省略...
    override fun onResults(
        results: MutableList<Detection>?,
        inferenceTime: Long,
        imageHeight: Int,
        imageWidth: Int
    ) {
        activity?.runOnUiThread {
            binding.bottomSheetLayout.inferenceTimeVal.text =
            String.format("%d ms", inferenceTime)
            // 其余代码省略...
        }
    }
}

当在该Fragment页面按下返回键并立即关闭应用时，触发致命异常，错误栈如下：

FATAL EXCEPTION: main
Process: id.naufalfajar.go, PID: 5383
at id.naufalfajar.go.view.detection.DetectionFragment.getBinding(DetectionFragment.kt:34)
at id.naufalfajar.go.view.detection.DetectionFragment.onResults$lambda$14(DetectionFragment.kt:316)
// 其余错误栈省略...

错误指向private val binding get() = _binding!!及onResults中访问binding的代码行。推测原因是后台线程仍在运行，按下返回键后Fragment的_binding已设为null，导致触发NullPointerException。

解决方案 #

1. 回调中先校验Fragment状态与Binding非空 #

在访问Binding前，先确认Fragment是否仍处于活跃状态，同时用安全调用替代非空断言，避免直接崩溃：

override fun onResults(
    results: MutableList<Detection>?,
    inferenceTime: Long,
    imageHeight: Int,
    imageWidth: Int
) {
    activity?.runOnUiThread {
        // 校验Fragment是否已绑定视图且处于活跃状态
        if (!isAdded || _binding == null) return@runOnUiThread
        
        _binding?.bottomSheetLayout?.inferenceTimeVal?.text =
            String.format("%d ms", inferenceTime)
        // 其余代码省略...
    }
}

2. Fragment销毁时主动停止后台任务 #

从根源上避免无用回调，在Fragment生命周期销毁阶段停止检测任务，确保后续不再触发onResults：

private lateinit var objectDetectorHelper: ObjectDetectorHelper

override fun onDestroyView() {
    super.onDestroyView()
    // 调用检测器的停止方法终止后台任务
    objectDetectorHelper.stopDetection()
    _binding = null
}

如果你的ObjectDetectorHelper没有stopDetection方法，需要补充实现：比如取消协程作用域、中断检测线程、释放模型资源等，彻底终止回调逻辑。

3. 用生命周期感知组件绑定回调 #

让检测器感知Fragment的生命周期变化，仅在Fragment活跃时发送回调：
首先改造ObjectDetectorHelper，添加生命周期监听：

class ObjectDetectorHelper(
    private val lifecycleOwner: LifecycleOwner,
    // 其他初始化参数
) : DefaultLifecycleObserver {
    private var detectorListener: DetectorListener? = null

    init {
        lifecycleOwner.lifecycle.addObserver(this)
    }

    fun setListener(listener: DetectorListener) {
        this.detectorListener = listener
    }

    // 仅在Fragment处于STARTED及以上状态时发送回调
    private fun sendResults(results: MutableList<Detection>?, inferenceTime: Long) {
        if (lifecycleOwner.lifecycle.currentState.isAtLeast(Lifecycle.State.STARTED)) {
            detectorListener?.onResults(results, inferenceTime, ...)
        }
    }

    override fun onDestroy(owner: LifecycleOwner) {
        super.onDestroy(owner)
        // 销毁时清空监听、停止任务
        detectorListener = null
        stopDetection()
        owner.lifecycle.removeObserver(this)
    }

    private fun stopDetection() {
        // 实现停止检测的逻辑
    }
}

然后在Fragment中初始化时传入自身作为生命周期所有者：

override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View {
    _binding = FragmentDetectionBinding.inflate(inflater, container, false)
    val objectDetectorHelper = ObjectDetectorHelper(this)
    objectDetectorHelper.setListener(this)
    return binding.root
}

内容的提问来源于stack exchange，提问作者Zen Ryuu