Prabhu Sunderaraman

The equivalent of interfaces in Java and C# in Objective-C are the Protocols and not the @interface that you use to declare the methods and variables. A protocol is defined using @protocol directive. It can have mandatory and optional members. A class needs to implement the mandatory methods declared in the protocol, if the “interface” of the class follows it. A simple protocol Shape with area and draw methods is shown below.

@protocol Shape <NSObject>
    @required
    -(NSInteger) area;

    @optional
    -(void) draw;
@end

As you can Shape is a protocol with area and draw methods. Shape protocol itself conforms to NSObject protocol.

Let’s create an interface Circle that uses the Shape protocol as shown below.

#import "Shape.h"

@interface Circle : NSObject<Shape> {
    NSInteger radius;
}
@property NSInteger radius;
@end

Circle interface inherits NSObject and follows the Shape protocol. The protocol list is specified within the angular brackets < >. The Circle implementation is shown below

#import "Circle.h"

@implementation Circle
    @synthesize radius;

-(NSInteger)area{
    return 3.14 * radius * radius;
}

//optional
-(void)draw{
    NSLog(@"Drawing circle");
}
@end

Circle implementation needs to implement the area method, though the draw method is optional. The main function is shown below.

int main (int argc, const char * argv[])
{
    Circle *circle = [Circle alloc];
    circle.radius = 22;
    NSLog(@"Area: %ld",[circle area]);
   if([circle conformsToProtocol:@protocol(Shape)])
       NSLog(@"Circle conforms to Shape protocol");
}

We have created a Circle object and also used the conformsToProtocol method that checks if circle object conforms to Shape protocol. It’s easier to understand protocols if you have worked with interfaces in Java/.NET languages.

Methods in Objective-C support named parameters. You can specify the name of the parameter and assign value to it while invoking the method.
Let’s discuss this with a class Calculator that has a method add with three parameters num1, num2 and num3.

//Calculator.h
@interface Calculator : NSObject {
}
-(int)add:(int)num1 : (int)num2 : (int)num3;
@end

The add method accepts three parameters num1 an (int), num2 an (int) and num3 an (int). The parameter list is separated by colon (:). Let’s implement this method in Calculator.m and invoke it in the main function

//Calculator.m
@implementation Calculator
-(int) add:(int)num1 : (int)num2 : (int)num3{
    return num1+num2+num3;
}
@end

//main.m
int main(int argc, const char * argv[]){
 Calculator *calc = [Calculator alloc];
 int sum = [calc add:12 :12 :12];
}

In the line [calc add:12 :12 :12] you’re invoking add method by passing 12,12 and 12 as arguments. If you have a method that accepts 4-5 parameters method invocation code will not be readable at all. It’ll be better if you can specify the parameter name and pass a value to it, while invoking the method.
Let’s modify the add method to accept a name for the second and third parameters

//Calculator.h
@interface Calculator : NSObject { 
}
-(int)add:(int)num1  number2:(int)num2 number3:(int)num3;
@end

//Calculator.m
@implementation Calculator
-(int) add:(int)num1 number2:(int)num2 number3:(int)num3{
    return num1+num2+num3;
}
@end

We have introduced names for the second and third parameters number2 and number3. The syntax can be confusing to begin with. The method parameters are delimited by a colon(:) and you add a name before every colon. This is not the case for the first parameter of a method, though. You can invoke the add method in the main function as shown below.

int main (int argc, const char * argv[])
{
    Calculator *calc = [Calculator alloc];
    int sum = [calc add:12 number2:12 number3:12];
}

The add method call is a lot more readable now.
Understanding the named parameters concept in Objective-C is very essential to comfortably work with iOS applications.