定义了一些常量，引入之后还是报找不到的错误，最后是因为没有放到预编译文件里面。
Constants.hFOUNDATION_EXPORTNSString*constMyFirstConstant;FOUNDATION_EXPORTNSString*constMySecondConstant;//etc.
(you can useexterninstead ofFOUNDATION_EXPORTif 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.mNSString*constMyFirstConstant=@"FirstConstant";NSString*constMySecondConstant=@"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).
