PromiseKit ：将两个承诺合二为一

Question

给定以下方法：

func loadFromCache(url: URL) -> Promise<Array<StockItem>>
func loadFromNetwork(url: URL) -> Promise<Array<StockItem>>

我实现了一个方法，returns 一个新的承诺，首先尝试缓存，然后是网络

func load(skip: Int, query: String?, onlyInStock: Bool = false) -> Promise<Array<StockItem>> {

    let url = self.urlFor(skip: skip, query: query, onlyInStock: onlyInStock)
    print("Loading: \(url)")

    return Promise { fulfill, reject in
        self.loadFromCache(url: url).then { (items) in
            return fulfill(items)
        }.catch { (error) in
            self.loadFromNetwork(url: url).then { (items) in
                fulfill(items)
            }.catch { (error) in
                reject(error)
            }
        }
    }
}

我想上面的方法一定有更好的写法。在没有嵌套闭包的情况下，我如何将它写成一个合适的承诺链？

换句话说，如何将两个 promise 合二为一？

Answer 1

您要查找的是 recover 运算符。

func load(skip: Int, query: String?, onlyInStock: Bool = false) -> Promise<[StockItem]> {
    let url = urlFor(skip: skip, query: query, onlyInStock: onlyInStock)
    return loadFromCache(url: url).recover { _ in
        return loadFromNetwork(url: url)
    }
}

PromiseKit ：将两个承诺合二为一

PromiseKit : compose two promises into one

ios

swift

promisekit