如何让Swift KVO对AVCaptureDevice静态成员portraitEffectEnabled生效？

我有一个UIViewController，代码如下。希望监听控制中心内人像效果（portrait effect）值的变化，尝试了AVCaptureDevice.isPortraitEffectEnabled和.portraitEffectEnabled两种方式，但都遇到相同问题：observeValue()从未被调用。我已确认该值确实会变化，且文档表明该属性支持KVO。

测试时，我通过调用AVCaptureDevice.showSystemUserInterface(.videoEffects)来切换portraitEffectEnabled的开关，期望KVO能触发。

@objc class EventSettingsCaptureViewController : UIViewController, ... {

    required init(...) {
        super.init(nibName: nil, bundle: nil)

        if #available(iOS 15.0, *) {
            AVCaptureDevice.self.addObserver(self, forKeyPath: "portraitEffectEnabled", options: [.new], context: nil)
        }
    }

    deinit {
        if #available(iOS 15.0, *) {
            AVCaptureDevice.self.removeObserver(self, forKeyPath: "portraitEffectEnabled", context: nil)
        }
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {

        // Breakpoint set here: never hits
        if keyPath == "portraitEffectEnabled" {
            guard let object = object as? AVCaptureDevice.Type else { return }

            if #available(iOS 15.0, *) {
                WLog("isPortraitEffectEnabled changed: \(object.isPortraitEffectEnabled)")
            }

        } else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }
    }
}

问题原因与解决方案 #

问题核心在于：你监听的是AVCaptureDevice类本身，但portraitEffectEnabled的KVO通知是发送给具体的AVCaptureDevice实例的，类对象不会触发这个属性的变更通知。

正确的做法是获取实际的视频捕获设备实例（比如前置摄像头，人像效果通常用于前置），然后给这个实例添加KVO监听，具体步骤如下：

• 获取当前用于视频捕获的设备实例（以前置摄像头为例）
• 给该实例添加KVO监听，而非AVCaptureDevice.self
• 在deinit中移除该实例的监听

修改后的代码示例：

@objc class EventSettingsCaptureViewController : UIViewController {

    private var videoDevice: AVCaptureDevice?

    required init(...) {
        super.init(nibName: nil, bundle: nil)
        
        if #available(iOS 15.0, *) {
            // 获取前置摄像头设备
            videoDevice = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .front)
            // 给设备实例添加监听
            videoDevice?.addObserver(self, forKeyPath: "portraitEffectEnabled", options: [.new], context: nil)
        }
    }

    deinit {
        if #available(iOS 15.0, *) {
            // 移除设备实例的监听
            videoDevice?.removeObserver(self, forKeyPath: "portraitEffectEnabled", context: nil)
        }
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "portraitEffectEnabled" {
            guard let device = object as? AVCaptureDevice else { return }
            
            if #available(iOS 15.0, *) {
                WLog("portraitEffectEnabled changed: \(device.portraitEffectEnabled)")
            }
        } else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }
    }
}

额外注意事项 #

• 确保获取到的videoDevice不为nil，否则监听不会生效
• 如果场景中存在设备切换（比如前后摄像头切换），需要在切换前移除旧设备的监听，再给新设备添加监听

内容的提问来源于stack exchange，提问作者xaphod