Take a look at the tests we have for this project. You can find them in UnitTests/Tests/NSNumber+FISMathSpec.m
You'll see that they attempt to import NSNumber+FISMath.h. You're getting errors right now because that file doesn't exist. So let's make our first category:
File -> New File... -> Source: Objective-C File
Name your category FISMath
Select File Type: Category
Select Class: NSNumber <- this is the class that we're adding a category
to, in this case NSNumber
Awesome! So the file that gets created is an empty category called FISMath that will allow us to add to the functionality of NSNumber.
Declare 4 methods in the FISMath category interface and define them in the implementation file. For now, assume the NSNumbers all contain NSIntegers.
add:(NSNumber *)number that returns the NSNumber sum of the receiver and the NSNumber that is passed in as a parameter
subtract:(NSNumber *)number that returns the NSNumber difference of the receiver and the NSNumber that is passed in as a parameter
multiplyBy:(NSNumber *)number that returns the NSNumber product of the receiver and the NSNumber that is passed in as a parameter
divideBy:(NSNumber *)number that returns an NSNumber that is the receiver divided by the NSNumber that is passed in as a parameter
For more information, examine the tests in the NSNumber+FISMathSpec.m file
Make the tests pass!
Advanced
First add in the ADVANCED pre-processor macro to turn on the advanced tests.
- In Xcode, switch to the file browser in the left pane. Select the project itself (the first item, "CategoryMath", with the blue icon). Now, in the main pane, select "UnitTests" under "Targets", and switch to the "Build Settings" tab. Search in there for "preprocessor", and double click the column after "Prepocessor Macros". In the window that pops up, hit the plus, enter "ADVANCED", and hit enter. See the picture below for more clarity:
Now let's make our category work a bit better. Right now it only works on integers because we converted everything to an NSInteger before doing the math. The problem is this should work for any type of NSNumber, from floating point to unsigned integer. There is a method on NSNumber called objCType that will tell you what type of number a given NSNumber instance contains. Use that so that our category works on all types of numbers.
- The advanced tests assume you will write these methods in a category called FISMathAdvanced, and name them advancedAdd:, advancedSubtract:, and so on.
