@dynamicMemberLookup attribute requires XXX to have a subscript(dynamicMember:) method that accepts either ExpressibleByStringLiteral or a keypath.

Dynamic Member Lookup 结合 KeyPath 能组合出无缝访问其他类型 Property 的效果。

不过极少提到的是，subscript 和属性一样可以被 keyPath 访问到！这样看来 subscript 和 Property 本质上非常相近，都是一对 setter 和 getter。

@dynamicMemberLookup
class MemberDelegater<ObjectType> {
    let object: ObjectType
    
    init(_ object: ObjectType) {
        self.object = object
    }
    
    subscript<Value>(dynamicMember keyPath: ReferenceWritableKeyPath<ObjectType, Value>) -> Value {
        get {
            object[keyPath: keyPath]
        }
        set {
            object[keyPath: keyPath] = newValue
        }
    }
}

class RunMebmberLookup: Runnable {
    class Writer {
        var result: String {
            get {
                return "Property Getter"
            }
            set {}
        }
        
        subscript(index i: String, index2 i2: String) -> String {
            get {
                return "Subscript Getter"
            }
            set {
                print("Subscript Setter")
            }
        }
    }
    static func run() {
        let writer = Writer()
        let member = MemberDelegater(writer)
        // 通过 Dynamic Member Lookup 访问 subscript
        let result = member[index: "123", index2: "123"]
        // 通过 Dynamic Member Lookup 访问属性
        let result2 = member.result
        print(result, result2)
    }
}

TCA 的 ViewStore.binding 就使用了这个技巧。