Swift4.0 Dictionary 遍历keys问题

swift4.0遍历key有点麻烦，请问有没有更好的方法？
数据存储方式为：

let sourceDic: [string: [string]] = [
  "Asia":["China", "Japan"],
  "Africa":["Egypt", "Morocco"]
]

使用方式为，key为table.sectionName, value 为section.cell 显示内容。
在cellForRow方法中代码如下

        let key = self.sourceDic.keys[indexPath.section]
        let list = self.sourceDic[key]
        let text = list![indexPath.row]
        
        let cell = tableView.dequeueReusableCell(withIdentifier: cellIdentifier)
        cell?.textLabel?.text = text

得到编译Error:

Cannot subscript a value of type 'Dictionary.Keys' with an index of type 'Int'

意思是，不能对Dictionary类型使用Int下标，然而在我的认知里，keys是数组而不是Dictionary。

查Dictionary.keys API，发现果然返回是字典，懵逼脸。

 @available(swift 4.0)
    public var keys: Dictionary.Keys { get }

附带有用法说明：

    ///     let countryCodes = ["BR": "Brazil", "GH": "Ghana", "JP": "Japan"]
    ///     print(countryCodes)
    ///     // Prints "["BR": "Brazil", "JP": "Japan", "GH": "Ghana"]"
    ///
    ///     for k in countryCodes.keys {
    ///         print(k)
    ///     }
    ///     // Prints "BR"
    ///     // Prints "JP"
    ///     // Prints "GH"

然而我并不需要如此遍历，很不方便。
进一步查询官方教程 Apple Inc. “The Swift Programming Language (Swift 4)”
给出例子如下：

“var airports: [String: String] = ["YYZ": "Toronto Pearson", "DUB": "Dublin"]”

“let airportCodes = [String](airports.keys)
// airportCodes is ["YYZ", "LHR"]
 
let airportNames = [String](airports.values)”
// airportNames is ["Toronto Pearson", "London Heathrow"]

摘录来自: Apple Inc. “The Swift Programming Language (Swift 4)”

所以，必须使用强转喽？于是代码变成如下模样，说好swift的优雅呢？

        let key = ([String](self.sourceDic.keys))[indexPath.section]
        let list = self.sourceDic[key]
        let text = list![indexPath.row]