Class 作为 NSMutableDictionary 的 key


Class clazz = [NSString class];
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
[dict setObject:@"" forKey:clazz];

今天看到相关代码，跟同事讨论了一下，为什么Class可以作为NSMutableDictionary的key，而不会发生如下崩溃？


*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController copyWithZone:]: unrecognized selector sent to instance 0x7ffad4d1ba40'

首先，一个Class也是一个NSObject，即是一个对象。


NSLog(@"isKindOfClass %d", [clazz isKindOfClass:[NSObject class]]);
// isKindOfClass 1

然后，这个对象是有遵循NSCopying协议的。


NSLog(@"conformsToProtocol %d", [clazz conformsToProtocol:@protocol(NSCopying)]);
// conformsToProtocol 1

下一个符号断点+[NSObject copyWithZone:]

这时发现执行了一个对key的copy操作，也是符合预期的。

后来才发现原来官方文档已经说明得很详细了。

Apple Developer Documentation

https://developer.apple.com/documentation/objectivec/nsobject/1571953-copywithzone

👉

This method exists so class objects can be used in situations where you need an object that conforms to the NSCopying protocol. For example, this method lets you use a class object as a key to an NSDictionary object. You should not override this method.