要从iPhone / iPad带LiDAR导出OBJ文件，可以使用ARKit中的ARMeshAnchor和ARSessionDelegate来获取网格数据并将其导出为OBJ文件。下面是一个代码示例，演示了如何实现这一过程：

首先，确保在项目的Info.plist文件中添加以下键值对：

NSCameraUsageDescription
We need access to your device's camera to enable AR functionality.
NSMicrophoneUsageDescription
We need access to your device's microphone to enable AR functionality.

然后，在ViewController类中，添加以下代码：

import ARKit

class ViewController: UIViewController, ARSessionDelegate {
    var sceneView: ARSCNView!
    var objExporter: OBJExporter!

    override func viewDidLoad() {
        super.viewDidLoad()

        sceneView = ARSCNView(frame: view.frame)
        view.addSubview(sceneView)

        let configuration = ARWorldTrackingConfiguration()
        configuration.sceneReconstruction = .meshWithClassification
        sceneView.session.delegate = self
        sceneView.session.run(configuration)
        
        objExporter = OBJExporter()
    }

    func session(_ session: ARSession, didUpdate anchors: [ARAnchor]) {
        guard let frame = session.currentFrame else { return }
        
        for anchor in anchors {
            if let meshAnchor = anchor as? ARMeshAnchor {
                let meshGeometry = meshAnchor.geometry
                let vertices = meshGeometry.vertices
                let normals = meshGeometry.normals
                let faces = meshGeometry.faces

                objExporter.exportMesh(vertices: vertices, normals: normals, faces: faces)
            }
        }
    }
}

接下来，在项目中创建一个名为OBJExporter的新类：

class OBJExporter {
    func exportMesh(vertices: [SIMD3], normals: [SIMD3], faces: [ARFaceAnchor.Geometry.TriangleIndices]) {
        var objString = ""

        for vertex in vertices {
            let vertexString = "v \(vertex.x) \(vertex.y) \(vertex.z)\n"
            objString.append(vertexString)
        }

        for normal in normals {
            let normalString = "vn \(normal.x) \(normal.y) \(normal.z)\n"
            objString.append(normalString)
        }

        for face in faces {
            let faceString = "f \(face.vertexIndexA+1)//\(face.normalIndexA+1) \(face.vertexIndexB+1)//\(face.normalIndexB+1) \(face.vertexIndexC+1)//\(face.normalIndexC+1)\n"
            objString.append(faceString)
        }

        let fileURL = getDocumentsDirectory().appendingPathComponent("mesh.obj")
        
        do {
            try objString.write(to: fileURL, atomically: true, encoding: .utf8)
            print("Exported OBJ file: \(fileURL.absoluteString)")
        } catch {
            print("Failed to export OBJ file: \(error.localizedDescription)")
        }
    }
    
    private func getDocumentsDirectory() -> URL {
        let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
        return paths[0]
    }
}

此代码将导出一个包含网格顶点、法线和面的OBJ文件。OBJExporter类包含一个exportMesh方法，它将网格数据转换为OBJ格式的字符串，并将其写入设备的文档目录中的mesh.obj文件中。

请注意，此示例假定您已经设置了ARSCNView并运行了ARSession。

希望这可以帮助您实现从iPhone / iPad带LiDAR导出OBJ文件的功能！