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。

NSMutableDictionary *contentDictionary = [[NSMutableDictionary alloc]init];
        [contentDictionary setValue:@"a" forKey:@"b"];
        [contentDictionary setValue:@"c" forKey:@"d"];
        NSData *data = [NSJSONSerialization dataWithJSONObject:contentDictionary options:NSJSONWritingPrettyPrinted error:nil];
        NSString *jsonStr = [[NSString alloc] initWithData:data
                                                  encoding:NSUTF8StringEncoding];

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()

public func jsonPrint(_ o: NSObject, spacing: String = "", after: String = "", before: String = "") {
    let newSpacing = spacing + "    "
    if o.isArray() {
        print(before + "[")
        if let a = o as? Array<NSObject> {
            for object in a {
                jsonPrint(object, spacing: newSpacing, after: object == a.last! ? "" : ",", before: newSpacing)
            }
        }
        print(spacing + "]" + after)
    } else {
        if o.isDictionary() {
            print(before + "{")
            if let a = o as? Dictionary<NSObject, NSObject> {
                for (key, val) in a {
                    jsonPrint(val, spacing: newSpacing, after: ",", before: newSpacing + key.description + " = ")
                }
            }
            print(spacing + "}" + after)
        } else {
            print(before + o.description + after)
        }
    }
}

这个非常接近最初的Objective-C打印风格

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];
}