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使用泛型对象类型联合进行类型缩小

typescript

1

aleclarson · 技术社区 · 7 年前

以下类型在事件系统中用于对象更改。

我在基于某些条件缩小对象类型方面遇到了问题。例如,当 prop 属性为null,我们确信 deleted

失败案例如下所示: (playground link)

declare const change: Change<{ a: number; b: string }>

if (change.prop === 'a') {
    change.prop // OK
    change.newValue // Expected to be "number"
}

if (change.prop == null) {
    change.prop // Expected to be "null"
    change.deleted // Expected to be "false"
    change.newValue // Expected to be "{a: number; b: string}"
}

type Change<T = any> =
    | RootChange<T>
    | (T extends object ? NestedChange<T> : never)

type RootChange<T> = IChange & {
    prop: null
    oldValue: T
    newValue: T
    deleted: false
}

type NestedChange<T extends object = any, P extends keyof T = keyof T> =
    | (IChange & {
        prop: P
        oldValue: T[P]
        newValue: T[P]
        deleted: false
    })
    | (IChange & {
        prop: P
        oldValue: T[P]
        newValue: undefined
        deleted: true
    })

interface IChange {
    /** The property being changed. When null, this change is for the root value. */
    prop: keyof any | null
    /** The previous value */
    oldValue: unknown
    /** The next value */
    newValue: unknown
    /** Whether the property has been deleted */
    deleted: boolean
}

1 回复 | 直到 7 年前

1

Titian Cernicova-Dragomir 7 年前

问题在于 NestedChange . 定义的类型的方式 {a: number, b: string}

type NestedChange<{a: number, b: string}> =
    | (IChange & {
        prop: "a" | "b"
        newValue: number | string
        deleted: false
    })
    | (IChange & {
        prop: "a" | "b"
        newValue: undefined
        deleted: true
    })

因此属性类型和属性名称之间没有关系, prop:"a" 可以与 string 就编译器而言。

你想要一个更像这样的联盟:

 { prop: "a"; newValue: number; deleted: false; } | 
 { prop: "a"; newValue: undefined; deleted: true; } | 
 { prop: "b"; newValue: string; deleted: false; } | 
 { prop: "b"; newValue: undefined; deleted: true; }

可以使用联合类型的分布行为创建这样的联合(请参阅 here )。这意味着如果我们有一个裸类型参数,该参数包含 T

要引入新类型参数并在其上分布,我们使用两种条件类型: keyof T extends infer P ? P extends any ? ... : never: never . 在这两种类型中,条件都不重要,我们使用第一个条件( keyof T extends infer P )引入一个新的类型参数 P P extends any )触发分配行为。

declare const change: Change<{ a: number; b: string }>

if (change.prop === 'a') {
    change.prop // OK
    change.newValue // is number | undefined
    if (change.deleted) {
        change.newValue // undefined
    } else {
        change.newValue // number
    }
}

if (change.prop == null) {
    change.prop // is "null"
    change.deleted // is "false"
    change.newValue // is "{a: number; b: string}"
}

type Change<T = any> =
    | RootChange<T>
    | (T extends object ? NestedChange<T> : never)

type RootChange<T> = IChange & {
    prop: null
    newValue: T
    deleted: false
}

type NestedChange<T extends object> = keyof T extends infer P ?
    P extends any ?
    (IChange & {
        prop: P
        newValue: T[P]
        deleted: false
    })
    | (IChange & {
        prop: P
        newValue: undefined
        deleted: true
    })
    : never : never;
interface IChange {
    /** The property being changed. When null, this change is for the root value. */
    prop: keyof any | null
    /** The previous value */
    oldValue: unknown
    /** The next value */
    newValue: unknown
    /** Whether the property has been deleted */
    deleted: boolean
}