SimplePreviewActivity.kt

private var cameraDevice: CameraDevice? = null

private val cameraManager: CameraManager by lazy {
    getSystemService(Context.CAMERA_SERVICE) as CameraManager
}

private fun openCamera() {
    cameraManager.openCamera("0", object: CameraDevice.StateCallback() {
        override fun onOpened(camera: CameraDevice) {
            cameraDevice = camera
            createCameraPreviewSession()
        }

        override fun onDisconnected(camera: CameraDevice) {
            cameraDevice?.close()
            cameraDevice = null
        }

        override fun onError(camera: CameraDevice, p1: Int) {
            cameraDevice?.close()
            cameraDevice = null
        }
    }, null)
}