问CIImage内存未释放

EN

static func blendImages(blendFrames: Int, blendMode: CIImage.BlendMode, imagePaths: [URL], completion: @escaping (_ progress: Double) -> Void) {
    var currentIndex = 0

    while currentIndex + blendFrames < imagePaths.count {            
        let currentSubset = Array(imagePaths[currentIndex..<(currentIndex + blendFrames)])

        var currentSubsetIndex = 0

        var firstImage: CIImage?

        while currentSubset.count > currentSubsetIndex + 1 {                
            if firstImage == nil { firstImage = CIImage(contentsOf: currentSubset[currentSubsetIndex]) } //First image allocated in memory

            if let secondImage = CIImage(contentsOf: currentSubset[currentSubsetIndex + 1]) {//Second image allocated in memory
                firstImage = firstImage?.blend(with: secondImage, blendMode: blendMode)
            } //Would expect secondImage to be freed from memory at this point, but it does not happen

            currentSubsetIndex += 1
        } 

        if let finalImage = firstImage {
            let bitMapRep = NSBitmapImageRep(ciImage: finalImage)
            let data = bitMapRep.representation(using: .jpeg, properties: [:])

            do {
                try data?.write(to: URL(fileURLWithPath: "Final-" + String(format: "%03d", currentIndex) + ".jpg"))
            } catch let error as NSError {
                print("Failed to write to disk: \(error)")
            }

        }

        firstImage = nil //Would expect firstImage to be freed from memory at this point (or at beginning of next cycle in while-loop), but it does not happen

        currentIndex += 1
    }
}

extension CIImage {

    enum BlendMode: String {
        case Lighten = "CILightenBlendMode"
        case Darken = "CIDarkenBlendMode"
    }

    func blend(with image: CIImage, blendMode: BlendMode) -> CIImage? {
        let filter = CIFilter(name: blendMode.rawValue)!
        filter.setDefaults()

        filter.setValue(self, forKey: "inputImage")
        filter.setValue(image, forKey: "inputBackgroundImage")

        return filter.outputImage
    }
}