如何在 Swift 中使可变性敏感 Class？

Question

我有自定义数组-class，例如：

public typealias JsonValue = AnyHashable;

public class Json {
    public private(set) var raw: NSMutableDictionary

// ...

    public subscript(key: String) -> JsonValue? {
        get {
            return self.raw[key];
        }
        set(value) {
            raw[key] = value;
        }
    }
}

以后像这样使用它：

var myVariable = Json();
myVariable["key"] = "value";

但是 Swift 警告如下：

Variable 'myVariable' was never mutated; consider changing to 'let' constant

如何告诉 Swift 我正在变异 myVariable？

I already tried mutating keyword, but that resulted in error (telling not allowed on setter).

I am using Swift 5 on Xcode 12.

Answer 1

您的变量具有引用 (class) 类型，因此该变量仅包含对 class 实例的 引用，并且未发生变化.

struct（值类型）和class（引用类型）有很大区别。在这种情况下，您应该将声明更改为：

let myVariable = Json()

要用引用类型改变变量，您必须分配一个新实例，例如

myVariable = Json()

You seem to want value semantics for your Json type. If so, just make it a struct. For more info, see the Swift Guide