Xcode error: The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

Question

当使用 Xcode 13.2.1 和 SwiftUI 实现简单的幻灯片放映时，我遇到了一个编译时错误，其中 Xcode 在我的 M1 上花费了大约 5 分钟来决定它无法解析我的代码，并最终给我错误：

The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

我将其缩小到底部附近的 NavigationLink 行。如果我将其注释掉，它会快速编译，只会出现警告。

以下是我的最小可重现示例：

import SwiftUI
import Foundation

enum MarkerType: Double {
    case unlabeled = -99
    case end = -4
    case start  = -3
    case stop = -2
    case blank = -1
    case image = 1
}

class LabeledImage {
    let image: Image
    let marker: Double
    var appeared = false
    
    init(image: Image, marker: Double) {
        self.image = image
        self.marker = marker
    }
}

struct SlideShow {
    private let maxImages: Int = 10000
    var images = [LabeledImage]()
    var labels = [String]()
    var totalImages: Int { return self.images.count }
    private var fromFolder: URL
    
    init(fromURL: URL = Bundle.main.bundleURL.appendingPathComponent("Contents/Resources/DefaultImages")) {
        self.fromFolder = fromURL
    }
}

class AppState: ObservableObject {
    static var docDir: URL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
    @Published var isMainMenuActive = false
    @Published var loadFolder: URL = Bundle.main.bundleURL.appendingPathComponent("Contents/Resources/DefaultImages")
    @Published var intervalSeconds: Double = 0.6

    var saveFolder = URL(fileURLWithPath: "BCILab", relativeTo: docDir)
    var labels = [String]()
    var totalImages: Int = 0
    var saveIndex: Int = 0
}


struct minsample: View {
    @StateObject private var appState = AppState()
    @State private var slideshow = SlideShow()
    @State private var selection: Int = 0
    
    private func insertAppears(_ marker: Double) {
        let nmarker = marker + 100.0
    }
    
    var body: some View {
        NavigationView {
            ForEach(0..<slideshow.images.count-1, id: \.self) { i in
                let thisImage = slideshow.images[i].image
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .onAppear(perform: { insertAppears(slideshow.images[i].marker) })
                let nextImage = slideshow.images[i+1].image
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .onAppear(perform: { insertAppears(slideshow.images[i+1].marker) })

                NavigationLink(destination: nextImage, tag: i, selection: self.$selection) { thisImage }
            }
        }
    }
}

Answer 1

通常，在 ForEach 中使用 index-based 解决方案不是一个好主意。它破坏了 SwiftUI 的视图差异系统，并且往往会导致 compile-time 问题。

我先做 LabeledImage Identifiable:

class LabeledImage : Identifiable {
    var id = UUID()
    let image: Image
    let marker: Double
    var appeared = false
    
    init(image: Image, marker: Double) {
        self.image = image
        self.marker = marker
    }
}

（我也会将其设为 struct——稍后会详细介绍）

然后，由于您确实需要索引来实现您的 nextImage 功能，您可以在集合上使用 .enumerated：

struct MinSample: View {
    @StateObject private var appState = AppState()
    @State private var slideshow = SlideShow()
    @State private var selection: Int? = 0
    
    private func insertAppears(_ marker: Double) {
        let nmarker = marker + 100.0
    }
    
    var body: some View {
        NavigationView {
            ForEach(Array(slideshow.images.enumerated()), id: \.1.id) { (index,imageModel) in
                if index < slideshow.images.count - 1 {
                    let thisImage = imageModel.image
                        .resizable()
                        .aspectRatio(contentMode: .fit)
                        .onAppear(perform: { insertAppears(imageModel.marker) })
                    let nextImage = slideshow.images[index + 1].image
                        .resizable()
                        .aspectRatio(contentMode: .fit)
                        .onAppear(perform: { insertAppears(slideshow.images[index+1].marker) })

                    NavigationLink(destination: nextImage, tag: index, selection: self.$selection) { thisImage }
                } else {
                    EmptyView()
                }
            }
        }
    }
}

上面的代码在我的 M1 上编译很快，没有任何问题。

现在，不 directly-related 解决您的问题，但我会更改其他一些内容：

制作您的模型 structs，SwiftUI 在进行状态比较时通常可以更好地处理模型
不要存储对 SwiftUI Images 的引用——而是存储对路径的引用或以其他方式重新创建该图像。无论如何，这将使 LabeledImage 到 struct 的过渡更加容易。因此，您的模型可能如下所示：

struct LabeledImage : Identifiable {
    var id = UUID()
    let imageName: String
    let marker: Double
    var appeared = false
}

考虑您是否需要 NavigationLink 中的 tag 和 selection 参数——也许在最小示例中不清楚为什么要使用它们。

Xcode error: The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

Xcode error: The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

xcode

swift

swiftui