NSDictionary/NSMutableDictionary

• 不可变字典NSDictionary

id obj = nil;
    
//dictionaryWithObjectsAndKeys，value可为nil，运行正常
NSDictionary *dict1 = [NSDictionary dictionaryWithObjectsAndKeys:@"value0",@"key0",obj,@"key1", nil];

//不可变字典NSDictionary，value为nil，crash！
NSDictionary *dict2 = [[NSDictionary alloc] init];
[dict2 setValue:obj forKey:@"key2"];

//冒号键值对写法，value为nil，crash！
NSDictionary *dict3 = @{@"key3":obj};

• 可变字典NSMutableDictionary

id obj = nil;

//可变字段NSMutableDictionary，语法糖写法，value可为nil，运行正常
NSMutableDictionary *dict4 = [[NSMutableDictionary alloc] init];
dict4[@"key4"] = obj;

//setValue，value可为nil，运行正常
//可变字典NSMutableDictionary，调用setValue实际是调用-setObject:forKey，但是当value为nil时，会调用removeObjectForKey，因为value可以为nil
NSMutableDictionary *dict5 = [[NSMutableDictionary alloc] init];
[dict5 setValue:obj forKey:@"key5"];

//setObject，value为nil，crash
NSMutableDictionary *dict6 = [[NSMutableDictionary alloc] init];
[dict6 setObject:obj forKey:@"key6"];  

总结：
1. NSDictionary，推荐使用dictionaryWithObjectsAndKeys；
2. NSMutableDictionary，推荐使用语法糖写法或setValue；
3. setObject: forKey:是NSMutableDictionary的特有方法；
4. setObject: forKey，value不可为nil；key为Id类型；
5. setValue: forKey，value可为nil，key必须为NSString；

setValue和setObject的区别