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

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)

objective - c:

NSString *str = @"test string";
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:str];
NSString *thatStr = [NSKeyedUnarchiver unarchiveObjectWithData:data];

迅速:

let str = "test string"
let data = NSKeyedArchiver.archivedData(withRootObject: str)
let thatStr = NSKeyedUnarchiver.unarchiveObject(with: data) as! String

更新Swift 5.x

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

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

以防Swift开发者来这里，

将NSString / String转换为NSData

var _nsdata = _nsstring.dataUsingEncoding(NSUTF8StringEncoding)