Swift中完成处理程序对闭包变量的赋值

我无意中发现了下面这段代码,我无法确切理解它是如何工作的。

AVCapturePhotoCaptureDelegate 被称为:

var photoCaptureCompletionBlock: ((UIImage?, Error?) -> Void)?

委托方法由以下代码触发:

func captureImage(completion: @escaping (UIImage?, Error?) -> Void) {

    let settings = AVCapturePhotoSettings()
    self.photoOutput?.capturePhoto(with: settings, delegate: self)
    self.photoCaptureCompletionBlock = completion
}

触发委托的行是:

self.photoOutput?.capturePhoto(with: settings, delegate: self)

紧接着 completion 变量被分配给 self.photoCaptureCompletionBlock

从概念上讲,我会理解相反的情况,即 自光捕获完成块 到 完成 而不是相反的方式(这是不可能没有 inout 自变量 是一个 let )。

这项任务背后的机制是什么?它是如何工作的?

func photoOutput(_ output: AVCapturePhotoOutput,
                                 didFinishProcessingPhoto photoSampleBuffer: CMSampleBuffer?,
                                 previewPhoto previewPhotoSampleBuffer: CMSampleBuffer?,
                                 resolvedSettings: AVCaptureResolvedPhotoSettings,
                                 bracketSettings: AVCaptureBracketedStillImageSettings?,
                                 error: Error?) {

    if let error = error {

        self.photoCaptureCompletionBlock?(nil, error)

    } else if let buffer = photoSampleBuffer,
        let data = AVCapturePhotoOutput.jpegPhotoDataRepresentation(forJPEGSampleBuffer: buffer, previewPhotoSampleBuffer: nil) {

        let image = UIImage(data: data)
        self.photoCaptureCompletionBlock?(image, nil)

    } else {

        self.photoCaptureCompletionBlock?(nil, CameraControllerError.unknown)

    }

}