Xcode development model-view-controller (MVC) issue - iphone

Question

I have been reading numerous books on iPhone development and doing the examples but I notice the idea of MVC is not really being taught correctly (although the authors do say that Xcode "lends itself" to the MVC way of coding).
A quick example. I want to make a simple calculator app (as many who are starting out do).
I have a working version with all of my code inside the xxxxxViewController.m file. Actions and Outlets all working well. The trouble with this approach is if I want to have multiple views (normal calculator and scientific calculator) I would have copy and paste my code so I now have two versions. I am clearly trying to avoid this.
So, I have created my own class (based on NSObject) as my CalculatorEngine.
Trouble is when trying to allocate and initialise my CalculatorEngine I receive errors such as "Redefinition of CalculatorEngine with a different type" and "Type specifier missing, defaults to int".
I guess I am missing something obvious.
Can you point me in the direction of a sample of any kind where a separate class is being used as an "engine" rather than having the code inside the xxxxViewController?
At this point the code below does not actually do anything. I am just trying to get the CalculatorEngine object useable in CalculatorViewController.m. This is where I receive the error.
// CalculatorAppDelegate.h
// Calculator
#import <UIKit/UIKit.h>
#class CalculatorViewController, CalculatorEngine;
#interface CalculatorAppDelegate : NSObject <UIApplicationDelegate>
#property (nonatomic, retain) IBOutlet UIWindow *window;
#property (nonatomic, retain) IBOutlet CalculatorViewController *viewController;
#end
// CalculatorAppDelegate.m
// Calculator
#import "CalculatorAppDelegate.h"
#import "CalculatorViewController.h"
#implementation CalculatorAppDelegate
#synthesize window = _window;
#synthesize viewController = _viewController;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
}
- (void)applicationWillResignActive:(UIApplication *)application
{
}
- (void)applicationDidEnterBackground:(UIApplication *)application
{
}
- (void)applicationWillEnterForeground:(UIApplication *)application
{
}
- (void)applicationDidBecomeActive:(UIApplication *)application
{
}
- (void)applicationWillTerminate:(UIApplication *)application
{
}
- (void)dealloc
{
}
#end
// CalculatorViewController.h
// Calculator
#import <UIKit/UIKit.h>
#interface CalculatorViewController : UIViewController
#end
// CalculatorViewController.m
// Calculator
#import "CalculatorViewController.h"
#import "CalculatorEngine.h"
#implementation CalculatorViewController
// This was wrong here. Now moved to viewDidLoad().
//CalculatorEngine *CalcEng;
//CalcEng = [[CalculatorEngine alloc] init];
// trouble here. CalcEng is unknow.
-(IBAction)buttonPressed:(id)sender {
[CalcEng setRegisterX:1];
}
- (void)viewDidLoad
{
CalculatorEngine *CalcEng;
CalcEng = [[CalculatorEngine alloc] init];
[super viewDidLoad]
}
- (void)didReceiveMemoryWarning
{
}
- (void)viewDidUnload
{
}
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
}
#end
// CalculatorEngine.h
// Calculator
#import <Foundation/Foundation.h>
#interface CalculatorEngine : NSObject
#end
// CalculatorEngine.m
// Calculator
#import "CalculatorEngine.h"
#implementation CalculatorEngine
- (id)init
{
self = [super init];
if (self) {
// Initialization code here.
}
return self;
}
#end

Answer 1

This code is in the wrong location:
CalculatorEngine *CalcEng;
CalcEng = [[CalculatorEngine alloc] init];
Put that into -(void)viewDidLoad.
UPDATE
You cannot call your method because your view controller is not keeping a reference to the CalcEngine (by the way, variables like this should be camel cased to keep in line with naming conventions, so it would be calcEngine). To keep a reference you need to add an iVar, or more appropriately, a property called CalcEngine. To do this, your CalculatorViewController header would look like this:
// CalculatorViewController.h
// Calculator
#import <UIKit/UIKit.h>
#interface CalculatorViewController : UIViewController {
CalculatorEngine *CalcEngine;
}
#property (retain) CalculatorEngine *CalcEngine;
#end
Your implementation would look like this:
// CalculatorViewController.m
// Calculator
#import "CalculatorViewController.h"
#import "CalculatorEngine.h"
#implementation CalculatorViewController
// This was wrong here. Now moved to viewDidLoad().
//CalculatorEngine *CalcEng;
//CalcEng = [[CalculatorEngine alloc] init];
// trouble here. CalcEng is unknow.
-(IBAction)buttonPressed:(id)sender {
[CalcEng setRegisterX:1];
}
- (void)viewDidLoad
{
CalcEng = [[CalculatorEngine alloc] init];
[super viewDidLoad]
}
- (void)didReceiveMemoryWarning
{
}
- (void)viewDidUnload
{
}
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
}
#end
Don't take this the wrong way, but you should spend some time reading Apple's Objective C guide. The problems you are having have nothing to do with MVC, but with objective-c.

Answer 2

Did you?
#class CalculatorEngine;
But not?
#import "CalculatorEngine.h"