FOUNDATION_EXPORT

.h

FOUNDATION_EXPORT NSString * const AFNetworkingReachabilityDidChangeNotification;

FOUNDATION_EXPORT NSString * const AFNetworkingReachabilityNotificationStatusItem;

.m

NSString * const AFNetworkingReachabilityDidChangeNotification = @"com.alamofire.networking.reachability.change";

NSString * const AFNetworkingReachabilityNotificationStatusItem = @"AFNetworkingReachabilityNotificationStatusItem";

FOUNDATION_EXPORT相当于对extern做了兼容处理。

NSObjCRuntime.h

与#define的对比：声明(.h)和实现(.m)可以分开。