Objective-C: @class Directive before @interface?

Question

What is the difference between these two class declarations? I don\'t understand why @class is utilized here. Thanks.

@class TestClass;

@interface TestCl         


        

Answer 1

@class is pretty handy when you need to define a protocol for an object that will usually interact with the object whose interface you are also defining. Using @class, you can keep the protocol definition in header of your class. This pattern of delegation is often used on Objective-C, and is often preferable to defining both "MyClass.h" and "MyClassDelegate.h". That can cause some confusing import problems

@class MyClass;

@protocol MyClassDelegate

- (void)myClassDidSomething:(MyClass *)myClass
- (void)myClass:(MyClass *)myClass didSomethingWithResponse:(NSObject *)reponse
- (BOOL)shouldMyClassDoSomething:(MyClass *)myClass;
- (BOOL)shouldMyClass:(MyClass *)myClass doSomethingWithInput:(NSObject *)input

@end

// MyClass hasn't been defined yet, but MyClassDelegate will still compile even tho
// params mention MyClass, because of the @class declaration.
// You're telling the compiler "it's coming. don't worry".
// You can't send MyClass any messages (you can't send messages in a protocol declaration anyway),
// but it's important to note that @class only lets you reference the yet-to-be-defined class. That's all.
// The compiler doesn't know anything about MyClass other than its definition is coming eventually.

@interface MyClass : NSObject

@property (nonatomic, assign) id delegate;

- (void)doSomething;
- (void)doSomethingWithInput:(NSObject *)input

@end

Then, when you are using the class, you can both create instances of the class as well as implement the protocol with a single import statement

#import "MyClass.h"

@interface MyOtherClass()

@property (nonatomic, strong) MyClass *myClass;

@end

@implementation MyOtherClass

#pragma mark - MyClassDelegate Protocol Methods

- (void)myClassDidSomething:(MyClass *)myClass {

    NSLog(@"My Class Did Something!")

}

- (void)myClassDidSomethingWithResponse:(NSObject *)response {

    NSLog(@"My Class Did Something With %@", response);

}

- (BOOL)shouldMyClassDoSomething {

    return YES;

- (BOOL)shouldMyClassDoSomethingWithInput:(NSObject *)input {

    if ([input isEqual:@YES]) {

        return YES;

    }

    return NO;

}


- (void)doSomething {

    self.myClass = [[MyClass alloc] init];
    self.myClass.delegate = self;
    [self.myClass doSomething];
    [self.myClass doSomethingWithInput:@0];

}