How to call method from one class in another (iOS)

Question

This a very basic question but I\'ve searched all over and been unable to find an answer that explains well enough for me to get my head around it.

What I want to do

Answer 1

You have two basic options. You can either create or pass-in an instance of the first class to the second class, or you can add a static method to the first class and call it directly using the class object.

For instance, say you have:

@interface ClassA : NSObject {
}

//instance methods
- (int) addNumber:(int)num1 withNumber:(int)num2;

//static/class methods
+ (int) add:(int)num1 with:(int)num2;
@end

@implementation ClassA
- (int) addNumber:(int)num1 withNumber:(int)num2 {
    return num1 + num2;
}

+ (int) add:(int)num1 with:(int)num2 {
    return num1 + num2;
}
@end

Then you can do:

#import "ClassA.h"

@interface ClassB : NSObject {
    ClassA* adder;
}

//constructors
- (id) init;  //creates a new instance of ClassA to use
- (id) initWithAdder:(ClassA*)theAdder;  //uses the provided instance of ClassA

//instance methods
- (int) add2To:(int)num;

//static/class methods
+ (int) add3To:(int)num;
@end

@implementation ClassB
- (id) init {
    if (self = [super init]) {
        adder = [[ClassA alloc] init];
    }
    return self;
}

- (id) initWithAdder:(ClassA*)theAdder {
    if (self = [super init]) {
        adder = theAdder;
    }
    return self;
}

- (int) add2To:(int)num {
    return [adder addNumber:2 withNumber:num];
}

+ (int) add3To:(int)num {
    return [ClassA add:3 with:num];
}
@end

Note that in most cases, you would use instance methods rather than static methods.