iOS / Swift / Closures：完成处理程序，处理多个API调用

Question

我想比较两个单独的服务器调用的结果，并且在如何使用闭包格式进行操作方面感到困惑。

例如，如果一个天气api一次返回一个城市的温度，并且我想比较两个城市的温度，那么我就在如何等待两个调用完成方面苦苦挣扎。

我正在使用的，从单个服务器调用改编的代码可以等待第一个结果。但是，我不知道该如何等待第二个结果。

这是我的代码：

func compareWeather (onecity:String, anothercity:String,completion:@escaping (_ response:String)->()){
    let city1:String = "Boston"
    let city2:Strng = "Palo Alto"
    self.getWeatherForCity(city: city1){//open 2
                answer in
    let temp1:String = answer
    }
    self.getWeatherForCity(city: city2){//open 2
                answer in
    let temp2:String = answer
    }

    //THIS DOES NOT WORK Since it is called before we here back from APIS BUT THIS IS WHAT I WANT TO accomplish

    completion("the temperature in Boston is ",temp1, "but the temperature in Palo Alto is ",temp2)
}