Constants in Objective-C
原文: http://stackoverflow.com/questions/538996/constants-in-objective-c
You should create a header file like
// Constants.h
FOUNDATION_EXPORT NSString const MyFirstConstant;
FOUNDATION_EXPORT NSString const MySecondConstant;
//etc.
(you can use extern instead of FOUNDATION_EXPORT if your code will not be used in mixed C/C++ environments or on other platforms)
You can include this file in each file that uses the constants or in the pre-compiled header for the project.
You define these constants in a .m file like
// Constants.m
NSString const MyFirstConstant = @"FirstConstant";
NSString const MySecondConstant = @"SecondConstant";
Constants.m should be added to your application/framework's target so that it is linked in to the final product.
The advantage of using string constants instead of #define'd constants is that you can test for equality using pointer comparison (stringInstance == MyFirstConstant) which is much faster than string comparison ([stringInstance isEqualToString:MyFirstConstant]) (and easier to read, IMO).