關於NSJSONReadingOptions參數的含義

JSONObjectWithData:options:error:方法來進行數據轉換，這裏的options是一個枚舉值，官方文檔定義是這樣 
enum { 
    NSJSONReadingMutableContainers = (1UL << 0), 
    NSJSONReadingMutableLeaves = (1UL << 1), 
    NSJSONReadingAllowFragments = (1UL << 2) 
};

這三個的區別含義以下：

 

NSJSONReadingMutableContainers 
Specifies that arrays and dictionaries are created as mutable objects.    //  建立可變的數組或字典 接收 

NSJSONReadingMutableLeaves 
Specifies that leaf strings in the JSON object graph are created as instances of NSMutableString.   // 指定在JSON對象可變字符串被建立爲NSMutableString的實例 
 
NSJSONReadingAllowFragments 
Specifies that the parser should allow top-level objects that are not an instance of NSArray or NSDictionary   //  指定解析器應該容許不屬於的NSArray或NSDictionary中的實例頂層對象 

 

首先用代碼來講明NSJSONReadingMutableContainers的做用：

 

    NSString *str = @"{\"name\":\"kaixuan_166\"}";
        
    NSMutableDictionary *dict = [NSJSONSerialization JSONObjectWithData:[str dataUsingEncoding:NSUTF8StringEncoding] options:0 error:nil];
        // 應用崩潰，不選用NSJSONReadingOptions，則返回的對象是不可變的，NSDictionary
        [dict setObject:@"male" forKey:@"sex"];
        
        NSMutableDictionary *dict = [NSJSONSerialization JSONObjectWithData:[str dataUsingEncoding:NSUTF8StringEncoding] options:NSJSONReadingMutableContainers error:nil];
        // 沒問題，使用NSJSONReadingMutableContainers，則返回的對象是可變的，NSMutableDictionary
        [dict setObject:@"male" forKey:@"sex"];
        
        NSLog(@"%@", dict);

NSJSONReadingMutableContainers：返回可變容器，NSMutableDictionary或NSMutableArray。 
 
NSJSONReadingMutableLeaves：返回的JSON對象中字符串的值爲NSMutableString，目前在iOS 7上測試很差用，應該是個bug，參見： 
http://stackoverflow.com/questions/19345864/nsjsonreadingmutableleaves-option-is-not-working 
 
NSJSONReadingAllowFragments：容許JSON字符串最外層既不是NSArray也不是NSDictionary，但必須是有效的JSON Fragment。例如使用這個選項能夠解析 @「123」 這樣的字符串。參見： 
http://stackoverflow.com/questions/16961025/nsjsonserialization-nsjsonreadingallowfragments-reading