example for delegate

let's assume an object a calls an object b to perform an action. once the action is complete, object a should know that b has completed the task and take necessary action. this is achieved with the help of delegates.

the key concepts in the above example are −

• a is a delegate object of b.

• b will have a reference of a.

• a will implement the delegate methods of b.

• b will notify a through the delegate methods.

steps in creating a delegate

step 1 − first, create a single view application.

step 2 − then select file → new → file...

step 3 − then select objective c class and click next.

step 4 − give a name to the class, say, sampleprotocol with subclass as nsobject as shown below.

step 5 − then select create.

step 6 − add a protocol to the sampleprotocol.h file and the updated code is as follows −

#import <foundation/foundation.h>

// protocol definition starts here 
@protocol sampleprotocoldelegate <nsobject>
@required
- (void) processcompleted;
@end

// protocol definition ends here
@interface sampleprotocol : nsobject {
   // delegate to respond back
   id <sampleprotocoldelegate> _delegate; 
}
@property (nonatomic,strong) id delegate;

-(void)startsampleprocess; // instance method
@end

step 7 − implement the instance method by updating the sampleprotocol.m file as shown below.

#import "sampleprotocol.h"

@implementation sampleprotocol

-(void)startsampleprocess {
   [nstimer scheduledtimerwithtimeinterval:3.0 target:self.delegate 
	selector:@selector(processcompleted) userinfo:nil repeats:no];
}
@end

step 8 − add a uilabel in the viewcontroller.xib by dragging the label from the object library to uiview as shown below.

step 9 − create an iboutlet for the label and name it as mylabel and update the code as follows to adopt sampleprotocoldelegate in viewcontroller.h.

#import <uikit/uikit.h>
#import "sampleprotocol.h"

@interface viewcontroller : uiviewcontroller<sampleprotocoldelegate> {
   iboutlet uilabel *mylabel;
}
@end

step 10 implement the delegate method, create object for sampleprotocol and call the startsampleprocess method. the updated viewcontroller.m file is as follows −

#import "viewcontroller.h"

@interface viewcontroller ()

@end

@implementation viewcontroller

- (void)viewdidload {
   [super viewdidload];
   sampleprotocol *sampleprotocol = [[sampleprotocol alloc]init];
   sampleprotocol.delegate = self;
   [mylabel settext:@"processing..."];
   [sampleprotocol startsampleprocess];
   // do any additional setup after loading the view, typically from a nib.
}

- (void)didreceivememorywarning {
   [super didreceivememorywarning];
   // dispose of any resources that can be recreated.
}

#pragma mark - sample protocol delegate
-(void)processcompleted {
   [mylabel settext:@"process completed"];
}
@end

step 11 we will see an output as follows. initially the label displays "processing...", which gets updated once the delegate method is called by the sampleprotocol object.