iOS swift5.0 NSNumber 转 Sting 遇到的问题（Swift中NSNumber转String）

编译器报错：Could not cast value of type ‘__NSCFNumber’ (0x…) to ‘NSString’ (0x…).

解决方式：
The value is an NSNumber, not an NSString. You can use stringValue to convert it:

if let a = d["a"] as? NSNumber {
    let aString = a.stringValue
    println(aString) // -1
}

If you’re sure it’s there, you can use forced unwrapping and string interpolation:

let a = d["a"]! as! NSNumber
let aString = "\(a)"

这样子也是直接nsnumber转nsstring了

2.另外的解决方式：

let num: Double = 123456789
let formattor = NumberFormatter()
formattor.numberStyle = .decimal
let str = formattor.string(from: NSNumber(value: num))
print(str)

要用到NumberFormatter这个类，相关属性如下：

open var numberStyle: NumberFormatter.Style

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