属性初始化器在self前执行时，如何调用实例方法生成CGImage？

错误原因 #

你遇到的报错核心逻辑是：类的存储属性初始化代码会在self实例完全创建之前执行，而makeRandomImage是实例方法，必须依托已初始化完成的self才能调用，因此直接在属性初始化阶段调用该方法会触发这个错误。

解决方案 #

以下是三种可行的修复方式，根据你的业务场景选择即可：

方案1：使用懒加载属性（推荐）

将cgImage和uiImage声明为lazy var，这样它们会在第一次被访问时才完成初始化，此时self已经完全可用，就能正常调用实例方法：

import SwiftUI
import Accelerate

class ImageClassifier: ObservableObject {
    
    @Published private var classifier = Classifier()
    
    var imageClass: String? {
        classifier.results
    }
    
    func makeRandomImage(width: Int, height: Int) -> CGImage? {
        let imageFormat = vImage_CGImageFormat(
            bitsPerComponent: 8,
            bitsPerPixel: 8 * 3,
            colorSpace: CGColorSpaceCreateDeviceRGB(),
            bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue))!
        
        let arrayDescriptor = BNNSNDArrayDescriptor.allocate(
            randomIn: Pixel_8(0) ... 255,
            shape: .matrixRowMajor(width * imageFormat.componentCount,
                                   height))
        
        let imageBuffer = vImage_Buffer(data: arrayDescriptor.data,
                                        height: vImagePixelCount(height),
                                        width: vImagePixelCount(width),
                                        rowBytes: width)
        
        return try? imageBuffer.createCGImage(format: imageFormat)
    }
    
    // 延迟初始化，第一次访问时self已完成初始化
    lazy var cgImage = makeRandomImage(width: 1024, height: 1024)!
    lazy var uiImage = UIImage(cgImage: cgImage)
    
    // MARK: Intent(s)
    // 修正参数类型为UIImage（原代码写的uiImage是变量名，不是类型）
    func detect(uiImage: UIImage) {
        guard let ciImage = CIImage(image: uiImage) else { return }
        classifier.detect(ciImage: ciImage)
    }
}

方案2：在初始化方法中赋值

如果需要在实例创建时就完成属性初始化，可以把属性声明为隐式解包可选类型，然后在init方法中调用赋值：

import SwiftUI
import Accelerate

class ImageClassifier: ObservableObject {
    
    @Published private var classifier = Classifier()
    
    var imageClass: String? {
        classifier.results
    }
    
    // 声明为隐式解包可选类型
    var cgImage: CGImage!
    var uiImage: UIImage!
    
    func makeRandomImage(width: Int, height: Int) -> CGImage? {
        // 原方法代码不变
        let imageFormat = vImage_CGImageFormat(
            bitsPerComponent: 8,
            bitsPerPixel: 8 * 3,
            colorSpace: CGColorSpaceCreateDeviceRGB(),
            bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue))!
        
        let arrayDescriptor = BNNSNDArrayDescriptor.allocate(
            randomIn: Pixel_8(0) ... 255,
            shape: .matrixRowMajor(width * imageFormat.componentCount,
                                   height))
        
        let imageBuffer = vImage_Buffer(data: arrayDescriptor.data,
                                        height: vImagePixelCount(height),
                                        width: vImagePixelCount(width),
                                        rowBytes: width)
        
        return try? imageBuffer.createCGImage(format: imageFormat)
    }
    
    init() {
        // init方法中self已完成初始化，可调用实例方法
        cgImage = makeRandomImage(width: 1024, height: 1024)!
        uiImage = UIImage(cgImage: cgImage)
    }
    
    func detect(uiImage: UIImage) {
        guard let ciImage = CIImage(image: uiImage) else { return }
        classifier.detect(ciImage: ciImage)
    }
}

方案3：将方法改为静态方法

如果makeRandomImage不需要访问任何实例属性，可以把它改成静态方法，这样无需依赖self就能直接调用：

import SwiftUI
import Accelerate

class ImageClassifier: ObservableObject {
    
    @Published private var classifier = Classifier()
    
    var imageClass: String? {
        classifier.results
    }
    
    // 改为static静态方法
    static func makeRandomImage(width: Int, height: Int) -> CGImage? {
        let imageFormat = vImage_CGImageFormat(
            bitsPerComponent: 8,
            bitsPerPixel: 8 * 3,
            colorSpace: CGColorSpaceCreateDeviceRGB(),
            bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue))!
        
        let arrayDescriptor = BNNSNDArrayDescriptor.allocate(
            randomIn: Pixel_8(0) ... 255,
            shape: .matrixRowMajor(width * imageFormat.componentCount,
                                   height))
        
        let imageBuffer = vImage_Buffer(data: arrayDescriptor.data,
                                        height: vImagePixelCount(height),
                                        width: vImagePixelCount(width),
                                        rowBytes: width)
        
        return try? imageBuffer.createCGImage(format: imageFormat)
    }
    
    // 调用静态方法，无需self
    let cgImage = ImageClassifier.makeRandomImage(width: 1024, height: 1024)!
    let uiImage = UIImage(cgImage: cgImage)
    
    func detect(uiImage: UIImage) {
        guard let ciImage = CIImage(image: uiImage) else { return }
        classifier.detect(ciImage: ciImage)
    }
}

内容的提问来源于stack exchange，提问作者ColorTheory42