从字符串到数据再回到字符串的Swift版本:

Xcode 10.1•Swift 4.2.1

extension Data {
    var string: String? {
        return String(data: self, encoding: .utf8)
    }
}

extension StringProtocol {
    var data: Data {
        return Data(utf8)
    }
}

extension String {
    var base64Decoded: Data? {
        return Data(base64Encoded: self)
    }
}

操场上

let string = "Hello World"                                  // "Hello World"
let stringData = string.data                                // 11 bytes
let base64EncodedString = stringData.base64EncodedString()  // "SGVsbG8gV29ybGQ="
let stringFromData = stringData.string                      // "Hello World"

let base64String = "SGVsbG8gV29ybGQ="
if let data = base64String.base64Decoded {
    print(data)                                    //  11 bytes
    print(data.base64EncodedString())              // "SGVsbG8gV29ybGQ="
    print(data.string ?? "nil")                    // "Hello World"
}

let stringWithAccent = "Olá Mundo"                          // "Olá Mundo"
print(stringWithAccent.count)                               // "9"
let stringWithAccentData = stringWithAccent.data            // "10 bytes" note: an extra byte for the acute accent
let stringWithAccentFromData = stringWithAccentData.string  // "Olá Mundo\n"

有时候，其他答案中的方法不管用。在我的例子中，我用我的RSA私钥生成了一个签名，结果是NSData。我发现这似乎很管用:

objective - c

NSData *signature;
NSString *signatureString = [signature base64EncodedStringWithOptions:0];

斯威夫特

let signatureString = signature.base64EncodedStringWithOptions(nil)

如果数据不是以空结束的，你应该使用-initWithData:encoding:

NSString* newStr = [[NSString alloc] initWithData:theData encoding:NSUTF8StringEncoding];

如果数据是以空结束的，你应该使用-stringWithUTF8String:来避免在结尾出现额外的\0。

NSString* newStr = [NSString stringWithUTF8String:[theData bytes]];

(注意，如果输入不是正确的utf -8编码，你将得到nil。)

斯威夫特变体:

let newStr = String(data: data, encoding: .utf8)
// note that `newStr` is a `String?`, not a `String`.

如果数据以空结束，你可以采用安全的方法，即删除空字符，或者采用类似于上面Objective-C版本的不安全方法。

// safe way, provided data is \0-terminated
let newStr1 = String(data: data.subdata(in: 0 ..< data.count - 1), encoding: .utf8)
// unsafe way, provided data is \0-terminated
let newStr2 = data.withUnsafeBytes(String.init(utf8String:))

从字符串到数据再回到字符串的Swift版本:

Xcode 10.1•Swift 4.2.1

extension Data {
    var string: String? {
        return String(data: self, encoding: .utf8)
    }
}

extension StringProtocol {
    var data: Data {
        return Data(utf8)
    }
}

extension String {
    var base64Decoded: Data? {
        return Data(base64Encoded: self)
    }
}

操场上

let string = "Hello World"                                  // "Hello World"
let stringData = string.data                                // 11 bytes
let base64EncodedString = stringData.base64EncodedString()  // "SGVsbG8gV29ybGQ="
let stringFromData = stringData.string                      // "Hello World"

let base64String = "SGVsbG8gV29ybGQ="
if let data = base64String.base64Decoded {
    print(data)                                    //  11 bytes
    print(data.base64EncodedString())              // "SGVsbG8gV29ybGQ="
    print(data.string ?? "nil")                    // "Hello World"
}

let stringWithAccent = "Olá Mundo"                          // "Olá Mundo"
print(stringWithAccent.count)                               // "9"
let stringWithAccentData = stringWithAccent.data            // "10 bytes" note: an extra byte for the acute accent
let stringWithAccentFromData = stringWithAccentData.string  // "Olá Mundo\n"

你可以调用这个方法

+(id)stringWithUTF8String:(const char *)bytes.

我谦卑地提交了一个分类，让这个不那么烦人:

@interface NSData (EasyUTF8)

// Safely decode the bytes into a UTF8 string
- (NSString *)asUTF8String;

@end

and

@implementation NSData (EasyUTF8)

- (NSString *)asUTF8String {
    return [[NSString alloc] initWithData:self encoding:NSUTF8StringEncoding];    
}

@end

(注意，如果你不使用ARC，你需要一个自动释放。)

现在，我不再赘述了:

NSData *data = ...
[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

你可以:

NSData *data = ...
[data asUTF8String];