函数抛出AND返回可选..可以有条件地解开一行？

Question

我正在使用SQLite库,其中查询返回可选值以及可能抛出错误.我想有条件地解包该值,或者如果它返回错误则收到nil.我不完全确定怎么说这个,这段代码会解释,这就是它的样子:

func getSomething() throws -> Value? {
    //example function from library, returns optional or throws errors
}


func myFunctionToGetSpecificDate() -> Date? {
    if let specificValue = db!.getSomething() {
         let returnedValue = specificValue!
         // it says I need to force unwrap specificValue, 
         // shouldn't it be unwrapped already?

         let specificDate = Date.init(timeIntervalSinceReferenceDate: TimeInterval(returnedValue))
         return time
    } else {
         return nil
    }

}

有没有办法避免不得不强行打开那里？在更新到Swift3之前,我没有被迫在这里强行打开包装.

以下是实际代码.只是想从所有条目中获取最新的时间戳:

func getLastDateWithData() -> Date? {
    if let max = try? db!.scalar(eventTable.select(timestamp.max)){

        let time = Date.init(timeIntervalSinceReferenceDate: TimeInterval(max!))

        // will max ever be nil here? I don't want to force unwrap!
        return time

    } else {
        return nil
    }
}

Answer 1

如果函数抛出并返回一个可选项

if let val = try? getSomething() {
    // ...
}

然后try?返回该类型的"双重可选",nil你必须解开两次:

func getSomething() throws -> Value? { ... }

这里先绑定val成功,如果函数没有抛出,而第二结合try? getSomething()成功,如果返回值不是Value??.

这可以通过let optval = ...模式匹配缩短

if let optval = try? getSomething(), let val = optval {

}

哪里let val = optval是快捷方式nil.