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

首先，你应该使用dataUsingEncoding:而不是通过UTF8String。只有在需要该编码的C字符串时才使用UTF8String。

然后，对于UTF-16，在你的dataUsingEncoding:消息中传递NSUnicodeStringEncoding而不是NSUTF8StringEncoding。

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

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

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)