YouTipIos Delegates
Suppose object A calls object B to perform an operation, and once the operation is complete, object A must be notified that object B has finished its task, so that object A can carry out other necessary actions.
The key concepts in the above example are:
* A is the delegate of B
* B holds a reference to A
* A implements B’s delegate methods
* B notifies A via delegate methods
Creating a Delegate Object
1. Create a Single View Application
2. Then select File → New → File...
3. Then select Objective-C and click Next
4. Name the subclass of SampleProtocol as NSObject, as shown below
5. Then click Create
6. Add a protocol to the SampleProtocol.h file and update the code as follows:
```objc
#import
// Protocol definition
@protocol SampleProtocolDelegate
@required
- (void) processCompleted;
@end
// End of protocol definition
@interface SampleProtocol : NSObject{
// Delegate to respond back
id _delegate;
}
@property (nonatomic,strong) id delegate;
-(void)startSampleProcess; // Instance method
@end
7. Modify the SampleProtocol.m file to implement the instance method:
```objc
#import "SampleProtocol.h"
@implementation SampleProtocol
-(void)startSampleProcess{
[NSTimer scheduledTimerWithTimeInterval:3.0 target:self.delegate selector:@selector(processCompleted) userInfo:nil repeats:NO];
}
@end
8. Drag a Label from the Object Library onto the UIView to add a UILabel in ViewController.xib, as shown below:
9. Create an IBOutlet for the label named `myLabel`, then update the code as shown below and declare `SampleProtocolDelegate` in ViewController.h:
```objc
#import
#import "SampleProtocol.h"
@interface ViewController : UIViewController{
IBOutlet UILabel *myLabel;
}
@end
10. Implement the delegate method, create a `SampleProtocol` object, and call the `startSampleProcess` method. Update ViewController.m as shown below:
```objc
#import "ViewController.h"
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
;
SampleProtocol *sampleProtocol = [init];
sampleProtocol.delegate = self;
[myLabel setText:@"Processing..."];
;
// Do any additional setup after loading the view, typically from a nib.
}
- (void)didReceiveMemoryWarning {
;
// Dispose of any resources that can be recreated.
}
#pragma mark - Sample protocol delegate
-(void)processCompleted{
[myLabel setText:@"Process Completed"];
}
@end
11. You will see the output shown below. Initially, the label displays “Processing…”, and once the delegate method is invoked by the `SampleProtocol` object, the label’s text updates accordingly.
