使用 PromiseKit 6 在 Swift 4.2 中缓存

Posted 2023-02-24

【中文标题】使用 PromiseKit 6 在 Swift 4.2 中缓存

【英文标题】：Caching in Swift 4.2 using PromiseKit 6

【发布时间】：2018-11-02 23:41:13

【问题描述】：

我遇到了两个错误：

pending = endpoint().then  freshValue in

导致错误：“无法推断复杂的闭包返回类型；添加显式类型以消除歧义”

return Guarantee(cachedValue) as! Guarantee<t>

导致错误：“无法将类型 'T' 的值转换为预期的参数类型 'PMKUnambiguousInitializer'”

这在 Swift 2 中工作（此后略有编辑），但是自从更新到 Swift 4.2 后，此代码开始中断。我承认，我仍在尝试找出从 Promisekit 4 到 6 的所有变化。

下面是剩下的代码：

import Foundation
import PromiseKit

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


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

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

    // If cache too old (invalid), returns nil
    func cachedOrNil(maxCacheAge: TimeInterval) -> T? 
        // maxCacheAge is maximum cache age before cache is deleted
        if NSDate().timeIntervalSince(cached.date) > maxCacheAge 
            cached.value = nil
        
        return cached.value
    

    // If cache nil too old (stale), makes API request
    func cachedOrFresh(maxCacheAge: TimeInterval, minCacheAge: TimeInterval) -> Guarantee<T> 
        // minCacheAge is minimum cache age before API request is made
        if let cachedValue = cachedOrNil(maxCacheAge: maxCacheAge) 
            if NSDate().timeIntervalSince(cached.date) < minCacheAge 
                return Guarantee(cachedValue) as! Guarantee<T>
            
        
        return fresh()
    
    /// ... More code in file...

【参考方案1】：

这里你需要通过指定具体类型来提供then块的返回类型，例如MyClass，如下所示，

pending = endpoint().then  freshValue -> Guarantee<MyClass> in ...