无法将类型 'Promise<T>' 的值分配给类型 'Promise<_>?'

Question

我正在尝试 API 请求，但是我收到错误消息

Cannot assign value of type 'Promise<T>' to type 'Promise<_>?'

我不知道 'Promise<_>?' 在哪里出现。

看一下代码：在 'as! Promise'

处抛出错误

class Cache<T> {
    private let defaultMaxCacheAge: TimeInterval
    private let defaultMinCacheAge: TimeInterval
    private let endpoint: () -> Promise<T>
    private let cached = CachedValue<T>()
    private var pending: Promise<T>?

    // Always makes API request, without regard to cache
    func fresh() -> Promise<T> {
        // Limit identical API requests to one at a time
        if pending == nil {
            pending = endpoint().then { freshValue in
                self.cached.value = freshValue
                return .value(freshValue)
            }.ensure {
                self.pending = nil
            } as! Promise<T>
        }
        return pending!
    }
    //...More code...
}

和Cache的init

init(defaultMaxCacheAge: TimeInterval = .OneHour, defaultMinCacheAge: TimeInterval = .OneMinute, endpoint: @escaping () -> Promise<T>) {
    self.endpoint = endpoint
    self.defaultMaxCacheAge = defaultMaxCacheAge
    self.defaultMinCacheAge = defaultMinCacheAge
}

和缓存值

class CachedValue<T> {
    var date = NSDate.distantPast
    var value: T? { didSet { date = NSDate() as Date } }
}

Answer 1

您似乎对在 PromiseKit (migration guide to PMK6) 中使用 then/map 感到困惑：

then is fed the previous promise value and requires you return a promise.
map is fed the previous promise value and requires you return a non-promise, ie. a value.

因此，您应该将 then 更改为 map 到 return 和 T，而不是 Promise<T> in closure:

pending = endpoint()
    .map { freshValue in
        self.cached.value = freshValue
        return freshValue
    }.ensure {
        self.pending = nil
    }

无法将类型 'Promise<T>' 的值分配给类型 'Promise<_>?'

Cannot assign value of type 'Promise<T>' to type 'Promise<_>?'

swift

promisekit