Objective C terminology: outlets & delegates

cocoa-design-patterns, delegates, iphone, objective-c, outlet

Solution

Outlets (in Interface Builder) are member variables in a class where objects in the designer are assigned when they are loaded at runtime. The `IBOutlet` macro (which is an empty `#define`) signals Interface Builder to recognise it as an outlet to show in the designer.

For example, if I drag out a button, then connect it to the `aButton` outlet (defined in my interface .h file), the loading of the NIB file at runtime will assign `aButton` the pointer to that `UIButton` instantiated by the NIB.

@interface MyViewController : UIViewController {
    UIButton *aButton;
}

@property(nonatomic, retain) IBOutlet UIButton *aButton;

@end

Then in the implementation:

@implementation MyViewController

@synthesize aButton; // Generate -aButton and -setAButton: messages

-(void)viewDidAppear {
    [aButton setText:@"Do Not Push. No, seriously!"];
}

@end

This eliminates the need to write code to instantiate and assign the GUI objects at runtime.

As for Delegates, they are event receiving objects used by another object (usually a generalised API class such as a table view). There's nothing inherently special about them. It's more of a design pattern. The delegate class may define several of the expected messages such as:

-(void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath

...and the API object calls this message on the delegate when it wants to notify it of the event. For example:

-(void)update:(double)time {
    if (completed) {
        [delegate process:self didComplete:totalTimeTaken];
    }
}

And the delegate defines the message:

-(void)process:(Process *)process didComplete:(double)totalTimeTaken {
    NSString *log = [NSString stringWithFormat:@"Process completed in %2.2f seconds.", totalTimeTaken];
    NSLog(log);
}

Such use might be:

Process *proc = [Process new];
[proc setDelegate:taskLogger];
[proc performTask:someTask];

// Output:
// "Process completed in 21.23 seconds."

Problem

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