Message Forwarding is not working (Objective-C)

Question

I am writing a sample code in order to understand message forwarding in Objective C (iOS).

I have two classes (class A and class B). I want to create an instance of class B and set a class A instance variable to it. I am calling the Forward Invocation method (Message Forwarding) in the following code.

//  ViewController.h
//  TestInvocation
#import <UIKit/UIKit.h>
@interface ViewController : UIViewController
@end

//  ViewController.m
//  TestInvocation
#import "ViewController.h"
#import "TestInvocation.h"
@interface ViewController ()
@end

@implementation ViewController
- (void)viewDidLoad {
    [TestInvocation testRun];
    [super viewDidLoad];
}
@end

//  TestInvocation.h
//  TestInvocation
#import <Foundation/Foundation.h>
@interface TestInvocation : NSObject
{}
+(void)testRun;
@end

//  TestInvocation.m
//  TestInvocation
#import "TestInvocation.h"
#import "ClassB.h"
#import "ClassA.h"
@implementation TestInvocation
+(void)testRun
{
    ClassB* diplomat = [[ClassB alloc] init];
    NSLog(@"value = %d",[diplomat value]);// Error shows up here on running: 
//No visible @interface for 'ClassB' declares the selector 'value'
}
   @end

//  ClassA.h
//  TestInvocation
#import <Foundation/Foundation.h>
@interface ClassA : NSObject
{
    int  value;
}
@property(readwrite,assign) int  value;
@end

//  ClassA.m
//  TestInvocation
#import "ClassA.h"
@implementation ClassA
@synthesize value;
@end

//  ClassB.h
//  TestInvocation
#import <Foundation/Foundation.h>
@interface ClassB : NSObject
{}
@end

//  ClassB.m
//  TestInvocation
#import "ClassB.h"
#import "ClassA.h"
@implementation ClassB
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    return [ClassA instanceMethodSignatureForSelector:aSelector];
}
-(void)forwardInvocation:(NSInvocation *)anInvocation
{
    ClassA* negotiate = [[ClassA alloc] init];
negotiate.value = 15;
    [anInvocation invokeWithTarget:negotiate];
}
@end

I am expecting the above code to work. But instead I get the following build time error:

ARC Semantic Issue TestInvocation.m:19:35: No visible @interface for 'ClassB' declares the selector 'value'

Answer 1

Class B should have the property in interface at least. But you can make it @dynamic in implementation if you want to call forwardInvocation.

@interface ClassB : NSObject
@property (readwrite, assign) int value;
@end

@implementation ClassB
@dynamic value;
...
@end

I think it should work for you.