A modifier to mark SwiftUI views in the view hierarchy and get a hierarchical textual description of the View - meant for unit testing

HierarchicalViewDescription.swift

import SwiftUI

/**
 A modifier to do textual asserts for SwiftUI Views in unit tests.
 Thought to be used in tandem with https://github.com/pointfreeco/swift-snapshot-testing , but with a self-built textual description.

 Mark SwiftUI views with the .trace("Example") modifier:

 HStack {
    Text("Foo")
        .trace("Foo")
    Text("Bar")
        .trace("Foo")
 }
 .trace("HStack")

 Get a hierarchical textual description from that:

 view
    .onPreferenceChange(ViewTracePreferenceKey.self) { value in
        print(value.description)
    }

 """
 HStack
     Foo
     Bar
 """

 (for this to work, the View needs to be brought to a UIWindow)
 */

#if DEBUG
    public struct ViewTraceElement: Equatable {
        var name: String
        var indent: Int
    }

    public struct ViewTracePreferenceKey: PreferenceKey {
        public static let defaultValue: [ViewTraceElement] = []

        public static func reduce(value: inout [ViewTraceElement], nextValue: () -> [ViewTraceElement]) {
            value = value + nextValue()
        }
    }

    public struct ViewTraceModifier: ViewModifier {
        let name: String

        public init(name: String) {
            self.name = name
        }

        public func body(content: Content) -> some View {
            content.transformPreference(ViewTracePreferenceKey.self) { value in
                if value.isEmpty {
                    value = [ViewTraceElement(name: name, indent: 0)]
                } else {
                    // This captures the View hierarchy
                    value = [ViewTraceElement(name: name, indent: 1)] + value + [ViewTraceElement(name: name, indent: -1)]
                }
            }
        }
    }

    public extension [ViewTraceElement] {
        var description: String {
            var result = ""
            let indent = "    "
            var currentIndent = 0

            for e in self {
                if e.indent >= 0 {
                    if !result.isEmpty {
                        result += "\n"
                    }
                    result += String(repeating: indent, count: currentIndent) + e.name
                }
                currentIndent = currentIndent + e.indent
                assert(currentIndent >= 0)
            }

            return result
        }
    }
#endif

public extension View {
    @inline(__always) func trace(_ name: String) -> some View {
        #if DEBUG
            self.modifier(ViewTraceModifier(name: name))
        #else
            self
        #endif
    }
}