效果图：

第一种是使用camerax进行预览，android camerax预览官方文档，主要通过imageAnalysis，抓帧进行图片处理，然后通过android自带的图片人脸识别FaceDetector判断是否有人脸，有人脸保存当前抓拍的照片：

val file = File(filesDir, "head_tmp.png")
            val create = Observable.create<File> { emitter ->
                val intArray = IntArray(2)
                iv_scan.getLocationInWindow(intArray)
                val createBitmap = Bitmap.createBitmap(
                    bitmap, intArray[0], intArray[1], iv_scan.width, iv_scan.height
                )
                //必须是565才能识别
                val bitmap1: Bitmap = createBitmap.copy(Bitmap.Config.RGB_565, true)
                val faceDetector = FaceDetector(bitmap1.width, bitmap1.height, 1)
                val array = arrayOfNulls<FaceDetector.Face>(1)
                val faces = faceDetector.findFaces(bitmap1, array)
                if (faces > 0) {
                    Log.e(TAG, "检测到脸")
                    val fos = FileOutputStream(file.path)
                    createBitmap.compress(Bitmap.CompressFormat.PNG, 100, fos)
                    fos.flush()
                    fos.close()
                    emitter.onNext(file)
                } else {
                    Log.e(TAG, "未检测到脸")
                    emitter.onError(Throwable("未检测到脸"))
                }

            }
            var disposable: Disposable? = null
            val observer = object : Observer<File> {
                override fun onNext(t: File) {
                    disposable?.dispose()
                    isOne = false
                    setResult(Activity.RESULT_OK)
                    finish()
                }

                override fun onError(e: Throwable) {
                    isOne = true
                }

                override fun onComplete() {

                }

                override fun onSubscribe(d: Disposable) {
                    disposable = d
                }
            }
            create.subscribeOn(Schedulers.computation())//指定被观察者线程
                .observeOn(AndroidSchedulers.mainThread())//指定观察者线程
                .subscribe(observer)

第二种使用了免费的虹软识别人脸识别，主要判断指定识别框的rect和虹软人脸识别框的rect,比较两个rect，是否在它的范围内，如果在抓拍人脸：

if (drawInfoList.size > 0) {
                    for (i in drawInfoList.indices) {
                        val rect: Rect = drawInfoList[i].rect
                        val rect1 = Rect()
                        iv_scan.getGlobalVisibleRect(rect1)
                        if (rect1.contains(rect)) {
                            //为了美观，扩大rect截取注册图
                            val cropRect: Rect =
                                CommUtils.getBestRect(
                                    previewSize!!.width, previewSize!!.height, faceInfoList[i].rect
                                )
                            cropRect.left = cropRect.left and 3.inv()
                            cropRect.top = cropRect.top and 3.inv()
                            cropRect.right = cropRect.right and 3.inv()
                            cropRect.bottom = cropRect.bottom and 3.inv()

                            headBmp = CommUtils.getHeadImage(
                                nv21,
                                previewSize!!.width,
                                previewSize!!.height,
                                faceInfoList[i].orient,
                                cropRect,
                                ArcSoftImageFormat.NV21
                            )
                            headBmp?.apply {
                                cropBitmap(this)
                            }
                            break
                        }
                    }
                }

demo：https://github.com/withyi9223/facesb