关于NSJSONReadingOptions参数的含义

JSONObjectWithData:options:error:方法来进行数据转换，这里的options是一个枚举值，官方文档定义是这样 
enum { 
    NSJSONReadingMutableContainers = (1UL << 0), 
    NSJSONReadingMutableLeaves = (1UL << 1), 
    NSJSONReadingAllowFragments = (1UL << 2) 
};

这三个的区别含义如下：

 

NSJSONReadingMutableContainers 
Specifies that arrays and dictionaries are created as mutable objects.    //  创建可变的数组或字典 接收 

NSJSONReadingMutableLeaves 
Specifies that leaf strings in the JSON object graph are created as instances of NSMutableString.   // 指定在JSON对象可变字符串被创建为NSMutableString的实例 
 
NSJSONReadingAllowFragments 
Specifies that the parser should allow top-level objects that are not an instance of NSArray or NSDictionary   //  指定解析器应该允许不属于的NSArray或NSDictionary中的实例顶层对象 

 

首先用代码来说明NSJSONReadingMutableContainers的作用：

 

    NSString *str = @"{\"name\":\"kaixuan_166\"}";
        
    NSMutableDictionary *dict = [NSJSONSerialization JSONObjectWithData:[str dataUsingEncoding:NSUTF8StringEncoding] options:0 error:nil];
        // 应用崩溃，不选用NSJSONReadingOptions，则返回的对象是不可变的，NSDictionary
        [dict setObject:@"male" forKey:@"sex"];
        
        NSMutableDictionary *dict = [NSJSONSerialization JSONObjectWithData:[str dataUsingEncoding:NSUTF8StringEncoding] options:NSJSONReadingMutableContainers error:nil];
        // 没问题，使用NSJSONReadingMutableContainers，则返回的对象是可变的，NSMutableDictionary
        [dict setObject:@"male" forKey:@"sex"];
        
        NSLog(@"%@", dict);

NSJSONReadingMutableContainers：返回可变容器，NSMutableDictionary或NSMutableArray。 
 
NSJSONReadingMutableLeaves：返回的JSON对象中字符串的值为NSMutableString，目前在iOS 7上测试不好用，应该是个bug，参见： 
http://stackoverflow.com/questions/19345864/nsjsonreadingmutableleaves-option-is-not-working 
 
NSJSONReadingAllowFragments：允许JSON字符串最外层既不是NSArray也不是NSDictionary，但必须是有效的JSON Fragment。例如使用这个选项可以解析 @“123” 这样的字符串。参见： 
http://stackoverflow.com/questions/16961025/nsjsonserialization-nsjsonreadingallowfragments-reading