以防Swift开发者来这里，

将NSString / String转换为NSData

var _nsdata = _nsstring.dataUsingEncoding(NSUTF8StringEncoding)

对于Swift 3，你将主要从字符串转换为数据。

let myString = "test"
let myData = myString.data(using: .utf8)
print(myData) // Optional(Data)

NSString *str = @"hello";
NSData *data = [NSData dataWithBytes:str.UTF8String length:str.length];

NSString *str = @"helowrld";
// This converts the string to an NSData object
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];

你可以从这个链接得到参考

NSString *str = @"Banana";
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:true];

更新Swift 5.x

let str = "teststring"
let data = str.data(using: .utf8)