NSDictionary如何处理NIL对象? [英] How does NSDictionary handle NIL objects?

问题描述

请考虑以下代码.本质上，我们得到2个字符串，然后将这些值添加到NSDictionary.

Consider the code below. In essence, we get 2 strings, then we add these values to the NSDictionary.

但是，我遇到了一个奇怪的错误.当fbAccessTokenKey为0x0(或nil)时，则不会同时添加twitterToken.

However, i hit a weird bug. When fbAccessTokenKey is 0x0 (or nil), then twitterToken would not be added as well.

NSString *fbAccessTokenKey=[[UserStockInfo sharedUserStockInfo] getFBAccessTokenKey];
NSString *twitterToken=[[UserStockInfo sharedUserStockInfo] getTwitterAccessTokenKey];

NSDictionary *params= [[NSDictionary alloc] initWithObjectsAndKeys:
                       fbAccessTokenKey, @"fb_access_token", 
                       twitterToken, @"twitter_access_token", 
                       nil
                       ];

为什么会这样?解决这个问题的一种好方法是什么?

Why is this happening, and what is a good way of resolving this?

推荐答案

nil用作标记参数结束"列表的哨兵".如果twitterToken为nil，则运行时将遍历您的参数，一旦到达twitterToken，它将认为这取决于对象和键列表的末尾.这是由于涉及列表参数的C/Obj-C的实现方式.

nil is used as a 'sentinel' for marking the "end of arguments" list. If twitterToken was nil, the runtime would go through your arguments, and once it got to twitterToken, it would think that it was up to the end of your list of objects and keys. This is due to the way that C/Obj-C is implemented when it comes to list arguments.

另一种安全的方法是使用NSMutableDictionary，并检查您的值是否为非nil，然后将它们添加到可变字典中，如下所示:

The alternative safe way to do it is to use an NSMutableDictionary, and check to see if your values are non-nil, then add them to the mutable dictionary like this:

NSString *fbAccessTokenKey = [[UserStockInfo sharedUserStockInfo] getFBAccessTokenKey];
NSString *twitterToken = [[UserStockInfo sharedUserStockInfo] getTwitterAccessTokenKey];

NSMutableDictionary *params = [NSMutableDictionary dictionary];
if (fbAccessTokenKey) [params setObject:fbAccessTokenKey forKey:@"fb_access_token"];
if (twitterToken) [params setObject:twitterToken forKey:@"twitter_access_token"];

---

有关更多技术信息，有一篇关于可可与爱的好文章: 查看全文