AVFoundation 中的自动对焦模式默认是开启的，可以通过设置 AVCaptureDevice 自动对焦的模式来关闭或启用自动对焦功能。在以下示例中，我们使用 AVCaptureDevice 类来访问设备的硬件特性，并将其设置为手动对焦模式。

import AVFoundation

// 获取设备
let captureDevice = AVCaptureDevice.default(for: .video)

// 配置输入输出流
let captureSession = AVCaptureSession()
let videoInput = try? AVCaptureDeviceInput(device: captureDevice!)
let videoOutput = AVCaptureVideoDataOutput()

if captureSession.canAddInput(videoInput!) && captureSession.canAddOutput(videoOutput) {
    captureSession.addInput(videoInput!)
    captureSession.addOutput(videoOutput)
}

// 关闭自动对焦
do {
    try captureDevice!.lockForConfiguration()
    captureDevice!.focusMode = .locked
    captureDevice!.unlockForConfiguration()
} catch {
    // handle error
}

// 开启手动对焦
override func touchesBegan(_ touches: Set, with event: UIEvent?) {
    let touchPoint = touches.first!
    let screenSize = previewLayer.bounds.size
    let focusPoint = CGPoint(x: touchPoint.location(in: previewLayer).y / screenSize.height, y: 1.0 - touchPoint.location(in: previewLayer).x / screenSize.width)

    do {
        try captureDevice!.lockForConfiguration()
        captureDevice!.focusPointOfInterest = focusPoint
        captureDevice!.focusMode = .autoFocus
        captureDevice!.unlockForConfiguration()
    } catch {
        // handle error
    }
}