在ARKit中，ARFaceAnchor包含了人脸识别的信息，包括顶点索引和顶点坐标。你可以使用ARFaceGeometry类来获取顶点索引。

以下是一个示例代码，展示了如何使用ARFaceAnchor和ARFaceGeometry来获取顶点索引：

import ARKit

// 在ARSessionDelegate中处理ARFaceAnchor更新
extension ViewController: ARSessionDelegate {
    func session(_ session: ARSession, didUpdate anchors: [ARAnchor]) {
        // 遍历所有ARFaceAnchor
        for anchor in anchors {
            if let faceAnchor = anchor as? ARFaceAnchor {
                // 获取ARFaceGeometry
                if let faceGeometry = faceAnchor.geometry {
                    // 获取顶点索引
                    let vertexBuffer = faceGeometry.buffer(with: .vertex)
                    let vertexCount = vertexBuffer.length / MemoryLayout.stride
                    let vertexBufferPointer = vertexBuffer.contents().bindMemory(to: simd_float3.self, capacity: vertexCount)
                    
                    // 遍历顶点索引
                    let indexBuffer = faceGeometry.buffer(with: .triangle)
                    let indexCount = indexBuffer.length / MemoryLayout.stride
                    let indexBufferPointer = indexBuffer.contents().bindMemory(to: Int32.self, capacity: indexCount)
                    
                    for i in 0..