iphone - 创建NSDictionary的NSDictionary

Question

信不信由你，在问这个问题之前，我已经搜索了互联网。令人难以置信的是，我还没有找到一个很好的清晰示例，说明如何创建NSDictionaries 的 NSDictionary。

到目前为止，这是我的代码，但是它显示null。有任何想法吗？

// Here I am creating the dictionaries in the code until I start getting them from the server ;)
NSArray *keys = [NSArray arrayWithObjects:@"mission", @"target", @"distance",@"status", nil];


NSArray *objectsA = [NSArray arrayWithObjects:@"tiger", @"bill", @"5.4km", @"unknown", nil];
NSDictionary *tiger = [NSDictionary dictionaryWithObjects:objectsA 
                                                  forKeys:keys];

NSArray *objectsB = [NSArray arrayWithObjects:@"bull", @"roger", @"10.1km", @"you are dead",  nil];
NSDictionary *bull = [NSDictionary dictionaryWithObjects:objectsB 
                                                 forKeys:keys];

NSArray *objectsC = [NSArray arrayWithObjects:@"peacock", @"geoff", @"1.4km", @"target liquidated", nil];
NSDictionary *peacock = [NSDictionary dictionaryWithObjects:objectsC 
                                                    forKeys:keys];

// activeMissions = [NSArray arrayWithObjects:tiger, bull, peacock, nil];


[activeMissions setObject:tiger forKey:@"tiger"];
[activeMissions setObject:bull forKey:@"bull"];
[activeMissions setObject:peacock forKey:@"peacock"];

NSLog(@"active Missions %@", activeMissions);

Answer 1

您没有初始化activeMissions，这就是为什么NSLog语句输出null的原因(将消息发送到ObjC中的nil对象返回nil)。

在分配给activeMissions之前，先把它放进去:

NSMutableDictionary *activeMissions = [NSMutableDictionary dictionaryWithCapacity:3];

否则，如果您希望拥有一个不可更改的NSDictionary，则可以执行以下操作:

NSDictionary *activeMissions = [NSDictionary dictionaryWithObjects: [NSArray arrayWithObjects:tiger, bull, peacock, nil]
                                                  forKeys: [NSArray arrayWithObjects:@tiger, @"bull", @"peacock", nil]];

(请记住，该文件是自动发布的，您必须保留某种方式)。