Question

I am fairly new at programming (I have no education in programming – everything I know have been obtained by reading tutorials) and completely new at XCode and iOS developing. So far I understand the basics in developing an iOS app, but one thing I can’t figure out how works is delegates. I understand the idea behind using delegates, but I don’t know what I am doing wrong when trying to implement delegates. I have created a small example (Single-view application) to illustrate how I am implementing a custom delegate and I hope you can tell me what I am doing wrong.

I am using XCode 4.5.2, iOS6.0 with ARC enabled.

In the example I create a simple NSObject subclass (TestClassWithDelegate). TestClassWithDelegate.h looks like this:

@protocol TestDelegate <NSObject>

-(void)stringToWrite:(NSString *)aString;

@end

@interface TestClassWithDelegate : NSObject

@property (weak, nonatomic) id<TestDelegate> delegate;

-(TestClassWithDelegate *)initWithString:(NSString *)theString;

@end

TestClassWithDelegate.m looks like this:

#import "TestClassWithDelegate.h"

@implementation TestClassWithDelegate

@synthesize delegate;

-(TestClassWithDelegate *)initWithString:(NSString *)theString
{
    self=[super init];

    [delegate stringToWrite:theString];

    return self;
}

@end

The view controller (ViewController) consists of a UILabel which I want to write some text to.
ViewController.h looks like this:

#import "TestClassWithDelegate.h"

@interface ViewController : UIViewController <TestDelegate>

@property (weak, nonatomic) IBOutlet UILabel *testlabel;

@end

ViewController.m looks like this:

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

@synthesize testlabel;

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.testlabel.text = @"Before delegate";
    TestClassWithDelegate *dummy = [[TestClassWithDelegate alloc]    initWithString:@"AfterDelegate"]; //This should init the TestClassWithDelegate which should "trigger" the stringToWrite method.
}

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

#pragma mark Test delegate
- (void)stringToWrite:(NSString *)aString
{
    self.testlabel.text = aString;
}
@end

Problem with above example is that the label on the view only writes “Before delegate” where I want it to write “AfterDelegate”.

All help is much appreciated. Happy New Year.

Answer 1

You haven’t set the delegate anywhere, so it will be nil. You’d either need to initWithString:delegate: instead of initWithString: or (better) just create the object, set the delegate, and send the string separately.

You may have made a (common) mistake whereby you assume that @synthesize actually creates an object within your code and assigns a value to it. It does not. It is a (now mostly redundant!) instruction to the compiler to create accessor methods for a property.

Below is a slightly reworked example of your delegate class, and some example usage:

.h file:

@interface TestClassWithDelegate : NSObject

@property (weak, nonatomic) id<TestDelegate> delegate;
-(void)processString:(NSString*)string

@end

.m file:

@implementation TestClassWithDelegate

-(void)processString:(NSString *)theString
{
   [delegate stringToWrite:theString];
}

@end

Usage:

TestClassWithDelegate *test = [TestClassWithDelegate new];
[test processString:@"Hello!"]; // Nothing will happen, there is no delegate
test.delegate = self;
[test processString:@"Hello!"]; // Delegate method will be called.