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

objective - c

NSString *str = @"Hello World";
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:NO];

斯威夫特

let str = "Hello World"
let data = string.data(using: String.Encoding.utf8, allowLossyConversion: false)

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

在Swift中，有一个返回非可选参数的API

let str = "teststring"
let data = Data(str.utf8)

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

更新Swift 5.x

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