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NSJSONReadingOptions
typedef NS_OPTIONS(NSUInteger, NSJSONReadingOptions) {
NSJSONReadingMutableContainers = (1UL << 0),
NSJSONReadingMutableLeaves = (1UL << 1),
NSJSONReadingAllowFragments = (1UL << 2)
} API_AVAILABLE(macos(10.7), ios(5.0), watchos(2.0), tvos(9.0));
NSJSONReadingMutableContainers = (1UL << 0), // 返回可变容器，NSMutableDictionary或NSMutableArray
NSJSONReadingMutableLeaves = (1UL << 1), // 不仅返回的最外层是可变的, 内部的子数值或字典也是可变对象
NSJSONReadingAllowFragments = (1UL << 2) // 返回允许JSON字符串最外层既不是NSArray也不是NSDictionary，但必须是有效的JSON Fragment.可以是如 "10"
NSJSONWritingOptions
typedef NS_OPTIONS(NSUInteger, NSJSONWritingOptions) {
NSJSONWritingPrettyPrinted = (1UL << 0),
/* Sorts dictionary keys for output using [NSLocale systemLocale]. Keys are compared using NSNumericSearch. The specific sorting method used is subject to change.
*/
NSJSONWritingSortedKeys API_AVAILABLE(macos(10.13), ios(11.0), watchos(4.0), tvos(11.0)) = (1UL << 1)
} API_AVAILABLE(macos(10.7), ios(5.0), watchos(2.0), tvos(9.0));
NSJSONWritingPrettyPrinted = (1UL << 0) //是将生成的json数据格式化输出，这样可读性高，不设置则输出的json字符串就是一整行。
NSJSONWritingSortedKeys //输出的json字符串就是一整行
