public class NSNumber : NSValue
	public class NSDecimalNumber : NSNumber

• NSNumber 可以被赋值为各种数值类型。我们可以将一个 Int 类型的值直接传递给一个需要 NSNumber 类型参数或变量，Swift 会自动转换，如果两者类型不一致，会强制类型转换。

• 从 NSNumber 赋值给 Swift 数值类型是，数值类型会根据会更具声明类型将 NSNumber 强制转换。

1、Swift 数字类型 与 NSNumber 类型 的相互转换

	// Swift 数字 转 NSNumber
	    
		let nsNum1:NSNumber = 1
		    
		let nsNum2 = NSNumber(integer: 1)
		let nsNum3 = NSNumber(unsignedInteger: 1)
		let nsNum4 = NSNumber(float: 1.1)
		let nsNum5 = NSNumber(double: 3.2)
		let nsNum6 = NSNumber(bool: true)
			
	// NSNumber 转 Swift 数字
	    
		let num1 = nsNum1.integerValue
		let num2 = nsNum2.unsignedIntegerValue
		let num3 = nsNum3.floatValue
		let num4 = nsNum4.doubleValue
		let num5 = nsNum5.boolValue

2、比较两个 NSNumber 类型数据的大小

	let nsNum1:NSNumber = 10
	let nsNum2:NSNumber = 10.1
	    
	// 判断两个值是否相等
	let bl:Bool = nsNum1.isEqualToNumber(nsNum2)
	
	// 判断两个值的大小
	let result:NSComparisonResult = nsNum1.compare(nsNum2)

3、NSNumber 与 String 的相互转换

	// NSNumber 转换 NSString
	let str:String = NSNumber(float: 10.456).stringValue
	   	
	// NSString 转 NSNumber
	let nsNum:NSNumber = NSNumber(float: NSString(string: str).floatValue)

4、加、减、乘、除、n次方

	let dcNun1 = NSDecimalNumber(string:"15.99")
	let dcNun2 = NSDecimalNumber(string:"29.99")
	let dcNun3 = NSDecimalNumber(string:"5.00")
	let dcNun4 = NSDecimalNumber(string:".90")
	let dcNun5 = NSDecimalNumber(string:"2.0")
	    
	// 加     Adding
	let subtotal = dcNun1.decimalNumberByAdding(dcNun2)                         
	    
	// 减     Subtracting
	let afterCoupon = subtotal.decimalNumberBySubtracting(dcNun3)               
	    
	// 乘     Multiplying
	let afterDiscount = afterCoupon.decimalNumberByMultiplyingBy(dcNun4)        
	    
	// 平均   Dividing
	let average = afterDiscount.decimalNumberByDividingBy(dcNun5)               
	    
	// n 次方 RaisingToPower
	let averageSquared = average.decimalNumberByRaisingToPower(2)               

5、四舍五入 运算

	/*
		enum NSRoundingMode : UInt {
		
			case RoundPlain     // Round up on a tie  貌似取整
			case RoundDown      // Always down == truncate  只舍不入
			case RoundUp        // Always up  只入不舍
			case RoundBankers   // on a tie round so last digit is even  貌似四舍五入
		}
	*/
	    
	// 90.7049 + 0.22 然后四舍五入
	    
	let roundUp = NSDecimalNumberHandler(roundingMode: NSRoundingMode.RoundBankers, 
	                                            scale: 2, 
	                                 raiseOnExactness: false, 
	                                  raiseOnOverflow: false, 
	                                 raiseOnUnderflow: false, 
	                              raiseOnDivideByZero: true)
	    
	let subtotal = NSDecimalNumber(string:"90.7049")
	let discount = NSDecimalNumber(string:"0.22")
	    
	// 加 保留 2 位小数
	let total = subtotal.decimalNumberByAdding(discount, withBehavior: roundUp)