Protocol 作为 NSMutableDictionary 的 key

上次讨论了Class 作为 NSMutableDictionary 的 key的情况，

这次来看看Protocol作为NSMutableDictionary的key，会怎样。


Protocol *protocol = @protocol(UIScrollViewDelegate);

NSLog(@"class %@", NSStringFromClass([(id)protocol class]));
NSLog(@"isKindOfClass %d", [(id)protocol isKindOfClass:[NSObject class]]);
NSLog(@"conformsToProtocol %d", [(id)protocol conformsToProtocol:@protocol(NSCopying)]);

NSMutableDictionary *dict = [NSMutableDictionary dictionary];
[dict setObject:@"" forKey:protocol];

很遗憾，尽管Protocol *也是个NSObject对象，但并不遵循NSCopying协议。所以就发生了崩溃。


class Protocol
isKindOfClass 1
conformsToProtocol 0
-[Protocol copyWithZone:]: unrecognized selector sent to instance 0x7fff8a490608

当然了，通过NSStringFromProtocol，我们还是可以达到目的。

iOS 使用 Protocol 作为组件化解耦与通信的方式，大部分用的就是这种NSMutableDictionary+NSStringFromProtocol的方案。比如BeeHive。

alibaba/BeeHive

honeybee: BeeHive is a solution for iOS Application module programs, it absorbed the Spring Framework API service concept to avoid coupling between modules. - alibaba/BeeHive

https://github.com/alibaba/BeeHive/blob/master/BeeHive/BHServiceManager.m

问题也不大，只是对于这种底层框架，还是需要尽可能优化性能。

于是改用CoreFoudation的CFDictionary。写个简单DEMO，对比一下性能。

自定义3个Protocol。


@protocol DWJProtocol0 <NSObject>
@end
@protocol DWJProtocol1 <NSObject>
@end
@protocol DWJProtocol2 <NSObject>
@end

对NSMutableDictionary进入3次写入，并读取30万次，统计读取耗时。


- (void)testNSDicionary
{
    NSMutableDictionary *dict = [NSMutableDictionary dictionary];
    [dict setObject:@"" forKey:NSStringFromProtocol(@protocol(DWJProtocol0))];
    [dict setObject:@"" forKey:NSStringFromProtocol(@protocol(DWJProtocol1))];
    [dict setObject:@"" forKey:NSStringFromProtocol(@protocol(DWJProtocol2))];
    CFAbsoluteTime startTime = CFAbsoluteTimeGetCurrent();
    for (int i = 0; i < 100000; i++) {
        NSString *object0 = [dict objectForKey:NSStringFromProtocol(@protocol(DWJProtocol0))];
        NSString *object1 = [dict objectForKey:NSStringFromProtocol(@protocol(DWJProtocol1))];
        NSString *object2 = [dict objectForKey:NSStringFromProtocol(@protocol(DWJProtocol2))];
    }
    CFAbsoluteTime endTime = CFAbsoluteTimeGetCurrent();
    NSLog(@"%@", @((endTime - startTime) * 1000));
}

同样对CFMutableDictionaryRef进入3次写入，并读取30万次，统计读取耗时。


- (void)testCFDictionary
{
    CFMutableDictionaryRef dict = CFDictionaryCreateMutable(CFAllocatorGetDefault(), 0, NULL, &kCFTypeDictionaryValueCallBacks);
    CFDictionarySetValue(dict, (__bridge const void *)(@protocol(DWJProtocol0)), (__bridge const void *)(@""));
    CFDictionarySetValue(dict, (__bridge const void *)(@protocol(DWJProtocol1)), (__bridge const void *)(@""));
    CFDictionarySetValue(dict, (__bridge const void *)(@protocol(DWJProtocol2)), (__bridge const void *)(@""));
    CFAbsoluteTime startTime = CFAbsoluteTimeGetCurrent();
    for (int i = 0; i < 100000; i++) {
        NSString *object0 = CFDictionaryGetValue(dict, (__bridge const void *)(@protocol(DWJProtocol0)));
        NSString *object1 = CFDictionaryGetValue(dict, (__bridge const void *)(@protocol(DWJProtocol1)));
        NSString *object2 = CFDictionaryGetValue(dict, (__bridge const void *)(@protocol(DWJProtocol2)));
    }
    CFAbsoluteTime endTime = CFAbsoluteTimeGetCurrent();
    NSLog(@"%@", @((endTime - startTime) * 1000));
    CFRelease(dict);
}

直接跑Time Profiler，使用Debug模式运行在iOS 14.4.2的iPhone 8，对比耗时，以及观察调用堆栈。

总耗时CFMutableDictionaryRef大概是NSMutableDictionary的1/25，效果可观。原因如下

不需要NSString的转换字符串操作

不需要对key的copy操作，甚至没有对key的retain操作（keyCallBacks为NULL）

当然了，这里有个前提是，这个Protocol *的对象，在运行时里是唯一生成不变的。所以没必要转换为NSString。