iOS - Delegates

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Example for delegate

Let's assume an object A calls 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 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

1. First, create a single view application.

2. Then select File -> New -> File...

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3. Then select Objective C Class and click Next.

4. Give the name for the class say SampleProtocol with subclass as NSObject as shown below.

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5. Then select create.

6. Add a protocol to the SampleProtocol.h file and 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

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

8. Add an UILabel in the ViewController.xib by dragging the label from the object library to UIView as shown below.

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9. Create an IBOutlet for the label and name it as myLabel and update the code as follow to adopt SampleProtocolDelegate in ViewController.h


#import <UIKit/UIKit.h>
#import "SampleProtocol.h"

@interface ViewController : UIViewController<SampleProtocolDelegate>
{
    IBOutlet UILabel *myLabel;
}
@end

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

11. We will see an output as follows, initially the label will be processing which gets updated once the delegate method is called by the SampleProtocol object.

iOS Tutorial

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