Friday, August 28, 2020

How to wire up a delegate method in Objective-C

This post covers how to wire up a delegate method in Objective-C. I'm always forgetting how to do this, so this post will serve as a reference for myself going forward, and hopefully may be useful to others as well!

A delegate method is useful when a given object needs to call a method on an object to which it does not have a reference. This typically comes up when the calling object is the child in a parent-child relationship in the application's object model. 

A practical example of when I've used this pattern is in Vigil RPG, after a combat has concluded, the combat screen ViewController notifies the parent ViewController (either the world map, or the dungeon level) that the combat has concluded, so that the parent can resume playing its background music.

In the parent class (the CALLEE): 

In the .h file:

Add the delegate as an interface in the class declaration:

@interface ParentClass: NSObject<myDelegate>


(There's no need to declare the callee method in the class's public interface.)

 

In the .m file:

1. Wire up the delegate

This can be done in the init method, or in any location where a reference to the child object is available which gets called only once:

self.myChild.myDelegate = self;

2. Define the callee method

- (void) myDelegateMethod
{

 // Implementation of the method that we want to call 

// from the child class goes here.
}


In the child class (the CALLER):

In the .h file:

1. Define the delegate protocol

(This can alternatively be done in a standalone .h file if the delegate is going to be used from more than a single callee class.)

 

This can go just before the @interface:


@protocol myDelegate<NSObject>
- (void) myDelegateMethod;
@end


2. Declare the delegate property

(This goes inside the @interface, along with any other @property items)

@property id<myDelegate> myDelegate;


In the .m file:

Add calls to the delegate methods 

 

These go in whatever are the appropriate place(s) in the class method implementations: 


if (self.myDelegate != nil)
{
// Calls myDelegateMethod on the parent class
    [self.myDelegate myDelegateMethod];
}



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