Objective C - Unrecognized delegate - ios

Question

I am new to Objective C and trying to grasp the concept of a delegate. I have the following code and have made comments where a warning and an error occur with the build.I am using the latest version of Xcode and using a Storyboard.
From MainViewController.h
#import <UIKit/UIKit.h>
#import "FinalViewController.h"
#class MainViewController;
#protocol MainViewControllerDelegate <NSObject>
- (void)sayHello;
#end
#interface MainViewController : UIViewController
#property (nonatomic, strong) id <MainViewControllerDelegate> delegate;
- (void)writeToMe;
. . .
#end
From MainViewController.m
#import "MainViewController.h"
#import "FinalViewController.h"
#interface MainViewController ()
#end
#implementation MainViewController
- (void)viewDidLoad
{
[super viewDidLoad];
}
- (void)writeToMe
{
[self.delegate sayHello];
}
. . .
#end
From FinalViewController.h
#import <UIKit/UIKit.h>
#import "MainViewController.h"
#interface FinalViewController : UIViewController <NSCoding, MainViewController> //The ERROR below appears
//ERROR:Cannot find protocol declaration for ‘MainViewControllerDelegate’; did you mean ‘UIPageViewControllerDelegate’?
. . .
#end
From FinalViewController.m
#import "FinalViewController.h"
#interface FinalViewController ()
#end
#implementation FinalViewController
- (void)viewDidLoad
{
[super viewDidLoad];
MainViewController *mainVC;
mainVC.delegate = self; //causes WARNING below
//WARNING:Assigning to ‘id <MainViewControllerDelegate> from incompatible type ‘FinalViewController *const__strong’
[mainVC writeToMe];
. . .
}
- (void)sayHello
{
NSLog(#"sayHello called");
}
#end
I have researched problems and responses to a similar problem on this web site, but have not found a solution using Storyboard.

Answer 1

Change:
#import "MainViewController.h"
#import "FinalViewController.h"
#interface MainViewController ()
#end
#implementation MainViewController
- (void)viewDidLoad
{
[super viewDidLoad];
}
- (void)writeToMe
{
[self.delegate sayHello];
}
. . .
TO:
#import "MainViewController.h"
#interface MainViewController ()
#end
#implementation MainViewController
- (void)viewDidLoad
{
[super viewDidLoad];
}
- (void)writeToMe
{
[self.delegate sayHello];
}
. . .
ALSO:
#import <UIKit/UIKit.h>
#import "FinalViewController.h"
#class MainViewController;
#protocol MainViewControllerDelegate <NSObject>
- (void)sayHello;
#end
#interface MainViewController : UIViewController
#property (nonatomic, strong) id <MainViewControllerDelegate> delegate;
- (void)writeToMe;
. . .
#end
TO
#import <UIKit/UIKit.h>
#protocol MainViewControllerDelegate <NSObject>
- (void)sayHello;
#end
#interface MainViewController : UIViewController
#property (nonatomic, weak) id <MainViewControllerDelegate> delegate;
- (void)writeToMe;
. . .
#end
--------------------------- EDIT ------------------------------------
Change
#interface FinalViewController : UIViewController <NSCoding, MainViewController>
to
#interface FinalViewController : UIViewController <NSCoding, MainViewControllerDelegate>

Answer 2

This might be a typo but:
#interface FinalViewController : UIViewController <NSCoding, MainViewController>
should be:
#interface FinalViewController : UIViewController <NSCoding, MainViewControllerDelegate>
Also remove the import of "FinalViewController.h" from MainViewController.h.
And your other warning can be fixed by changing:
#interface FinalViewController ()
to:
#interface FinalViewController () < MFMailComposeViewControllerDelegate>
and change:
mainVC.delegate = self;
to:
mainVC.mailComposeDelegate = self;