Constants in Objective-C

The accepted (and correct) answer says that "you can include this [Constants.h] file... in the pre-compiled header for the project."

As a novice, I had difficulty doing this without further explanation -- here's how: In your YourAppNameHere-Prefix.pch file (this is the default name for the precompiled header in Xcode), import your Constants.h *inside the `#ifdef __OBJC__` block*.

#ifdef __OBJC__
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#import "Constants.h"
#endif

Also note that the Constants.h and Constants.m files should contain absolutely nothing else in them except what is described in the accepted answer. (No interface or implementation).

If you like namespace constant, you can leverage struct, [Friday Q&A 2011-08-19: Namespaced Constants and Functions](

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)

// in the header
extern const struct MANotifyingArrayNotificationsStruct
{
NSString *didAddObject;
NSString *didChangeObject;
NSString *didRemoveObject;
} MANotifyingArrayNotifications;

// in the implementation
const struct MANotifyingArrayNotificationsStruct MANotifyingArrayNotifications = {
.didAddObject = @"didAddObject",
.didChangeObject = @"didChangeObject",
.didRemoveObject = @"didRemoveObject"
};

If you want something like global constants; a quick an dirty way is to put the constant declarations into the `pch` file.

If you want to call something like this `NSString.newLine;` from objective c, and you want it to be static constant, you can create something like this in swift:

public extension NSString {
@objc public static let newLine = "\n"
}

And you have nice readable constant definition, and available from within a type of your choice while stile bounded to context of type.

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).