Swift如何通过变量名或索引动态修改Codable结构体的属性值

这种方案和硬编码性能完全一致，编译期即可做错误校验，是首选实现方式。
你只需要给Effects结构体扩展预定义属性映射和下标支持即可：

extension Effects {
    // 按你约定的索引顺序，预存所有ValueAndRange?类型属性的可写KeyPath
    static let valuePropertyKeyPaths: [WritableKeyPath<Effects, ValueAndRange?>] = [
        \.dryWetMix,
        \.resonance,
        \.cutoffFrequency
        // 后续新增属性只需按顺序追加到此处即可
    ]
    
    // 如需按属性名字符串访问，额外增加名称映射表
    static let valuePropertyNameMap: [String: WritableKeyPath<Effects, ValueAndRange?>] = [
        "dryWetMix": \.dryWetMix,
        "resonance": \.resonance,
        "cutoffFrequency": \.cutoffFrequency
    ]
    
    // 下标支持：通过索引访问属性
    subscript(valueIndex index: Int) -> ValueAndRange? {
        get {
            guard index >= 0, index < Self.valuePropertyKeyPaths.count else { return nil }
            return self[keyPath: Self.valuePropertyKeyPaths[index]]
        }
        set {
            guard index >= 0, index < Self.valuePropertyKeyPaths.count else { return }
            self[keyPath: Self.valuePropertyKeyPaths[index]] = newValue
        }
    }
    
    // 下标支持：通过属性名字符串访问属性
    subscript(valueName name: String) -> ValueAndRange? {
        get {
            guard let keyPath = Self.valuePropertyNameMap[name] else { return nil }
            return self[keyPath: keyPath]
        }
        set {
            guard let keyPath = Self.valuePropertyNameMap[name] else { return }
            self[keyPath: keyPath] = newValue
        }
    }
}

调用方式完全符合你的需求：

let setThisValue = 0.6
// 按索引访问
let effectProppertyIndex = 2
setConfig.instrumentsConfig[0].effects![0][valueIndex: effectProppertyIndex]?.setValue(value: setThisValue)

// 按属性名字符串访问
let effectPropperty: String = "resonance"
setConfig.instrumentsConfig[0].effects![0][valueName: effectPropperty]?.setValue(value: setThisValue)

如果你的属性变动非常频繁，不想每次新增属性都更新映射表，可以用这种动态实现方案，缺点是有编解码的性能开销，适合调用频率不高的场景：

extension Effects {
    mutating func setValue(_ value: Double, forPropertyName name: String) throws {
        // 序列化为字典修改
        var dict = try JSONSerialization.jsonObject(with: JSONEncoder().encode(self)) as? [String: Any] ?? [:]
        if var propDict = dict[name] as? [String: Any] {
            propDict["value"] = value
            dict[name] = propDict
        }
        // 重新解码为结构体实例
        self = try JSONDecoder().decode(Effects.self, from: JSONSerialization.data(withJSONObject: dict))
    }
}

调用方式：

let setThisValue = 0.6
let effectPropperty: String = "resonance"
try? setConfig.instrumentsConfig[0].effects![0].setValue(setThisValue, forPropertyName: effectPropperty)

内容的提问来源于stack exchange，提问作者Frans-Jan Wind