getToDay 何日前かを指定して日付をstringで取得、DateManager 日付と体重の組を管理（コンストラクタで乱数組作成）

day.swift

func getToDay(day : Double = 0) -> String {
    
    let now = Date( timeInterval: 60*60*24*day, since: Date() as Date )
    let formatter = DateFormatter()
    formatter.dateFormat = "yyyy-MM-dd"
    return formatter.string(from: now as Date)

}


class DateManager {
    
    var dateToWeight : Dictionary<String,String> = [:]

    static let singleton = DateManager()
    private init(){
        for i in 10...20 {
            dateToWeight[getToDay(day:(Double)(-i))] = String((Int)(arc4random()) % 50+50);
        }
    }
    
    func saveWeight(day :String , weight : String){
        dateToWeight[day] = weight;
    }
    
    func ToString() -> String {
        var str : String = "";
        for (key, value) in dateToWeight {
            str += key + value + "\n";
        }
        return str;
    }
    
    func getWeight( day : String ) -> String {
        if dateToWeight.contains(where:{
            key,value in
            return key.contains(day)}){
            return dateToWeight[day]!;
        }
        return "";
    }
}