在Swift 2.0版本中:

class func jsonStringWithJSONObject(jsonObject: AnyObject) throws -> String? {
    let data: NSData? = try? NSJSONSerialization.dataWithJSONObject(jsonObject, options: NSJSONWritingOptions.PrettyPrinted)

    var jsonStr: String?
    if data != nil {
        jsonStr = String(data: data!, encoding: NSUTF8StringEncoding)
    }

    return jsonStr
}

注:这个答案是在iOS 5发布之前给出的。

获取json-framework并执行以下操作:

#import "SBJsonWriter.h"

...

SBJsonWriter *jsonWriter = [[SBJsonWriter alloc] init];

NSString *jsonString = [jsonWriter stringWithObject:myDictionary];  

[jsonWriter release];

myDictionary就是你的字典。

将NSDictionary转换为NSString:

NSError * err;
NSData * jsonData = [NSJSONSerialization dataWithJSONObject:myDictionary options:0 error:&err]; 
NSString * myString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];

在Swift 2.0版本中:

class func jsonStringWithJSONObject(jsonObject: AnyObject) throws -> String? {
    let data: NSData? = try? NSJSONSerialization.dataWithJSONObject(jsonObject, options: NSJSONWritingOptions.PrettyPrinted)

    var jsonStr: String?
    if data != nil {
        jsonStr = String(data: data!, encoding: NSUTF8StringEncoding)
    }

    return jsonStr
}

这是Swift 4的版本

extension NSDictionary{

func toString() throws -> String? {
    do {
        let data = try JSONSerialization.data(withJSONObject: self, options: .prettyPrinted)
        return String(data: data, encoding: .utf8)
    }
    catch (let error){
        throw error
    }
}

}

使用的例子

do{
    let jsonString = try dic.toString()
    }
    catch( let error){
        print(error.localizedDescription)
    }

或者如果你确定它是有效的字典，那么你可以使用

let jsonString = try? dic.toString()

苹果在iOS 5.0和Mac OS X 10.7中添加了JSON解析器和序列化器。看到NSJSONSerialization。

要从NSDictionary或NSArray中生成JSON字符串，你不再需要导入任何第三方框架。

以下是如何做到这一点:

NSError *error; 
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:dictionaryOrArrayToOutput 
                                                   options:NSJSONWritingPrettyPrinted // Pass 0 if you don't care about the readability of the generated string
                                                     error:&error];

if (! jsonData) {
    NSLog(@"Got an error: %@", error);
} else {
    NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
}