Can't get an objective-c object to respond to a message. What am I missing? - objective-c

Question

I'm trying to teach myself Objective-C and as an exercise, I'm trying to write an app with one button and one label. When I click on the button, I want to trigger a calculation then see the results in the label. The following code compiles and runs with no errors or warnings but as far as I can tell, the [object method] 'call' doesn't do anything. I've spent hours on this and just don't see what's wrong. Can anyone explain the problem? Thanks.
*** testMethodViewController.h ****
#import <UIKit/UIKit.h>
#import "testBrain.h"
#interface testMethodViewController : UIViewController
{
IBOutlet UILabel *display;
testBrain *model;
}
- (IBAction)cellPressed:(UIButton *)sender;
#end
*** testMethodViewController.m ****
#import "testMethodViewController.h"
#implementation testMethodViewController
- (testBrain *)model
{
if (!model) {model = [[testBrain alloc] init];}
return model;
}
- (IBAction)cellPressed:(UIButton *)sender
{
int x = [model check:3]; //This method call doesn't work. But gets no errors.
NSLog(#"Results from model: %i", x); //Says x = 0, but I expect 6
NSString *xAsString = [NSString stringWithFormat: #"testBrain: %i", x];
display.text = xAsString; //Label is updated and displays: testBrain: 0
} //I expect: testBrain: 6
#end
*** testBrain.h ****
#import <Foundation/Foundation.h>
#interface testBrain : NSObject {}
- (int) check:(int) anInteger;
#end
*** testBrain.m ****
#import "testBrain.h"
#implementation testBrain
- (int) check:(int) anInteger //3 passed as the parameter.
{
int r = anInteger + anInteger;
NSLog(#"inside check %i", r); //Debugging line: doesn't print.
return r;
}
#end

Answer 1

When this code runs:
int x = [model check:3];
model is nil. In Objective-C, messages sent to nil silently do nothing, and return 0. So, as you see, x is 0 and -check: is never called.
Apparently you were expecting this method to be called automatically:
- (testBrain *)model
{
if (!model) {model = [[testBrain alloc] init];}
return model;
}
However, that method will be called only if you do it yourself, by saying [self model] or self.model. So, this line would fix it:
int x = [[self model] check:3];
Try it and see.
Going a little further: It would be clearer to remove the model method entirely, and create the instance variable model when the UIViewController is created. That way, we can guarantee that model is valid anytime any code in the testMethodViewController class runs.
You would do that by overriding UIViewController's designated initializer:
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
// Now you can initialize your instance variables
model = [[testBrain alloc] init];
}
return self;
}

Answer 2

With your model method, you are halfway towards Lazy Instantiation, however to properly achieve this, you must always acess the lazily instantiated object through its accessor method. You aren't doing this in your button action, so your messages are going to nil, which is silently ignored.
This is one of the reasons you often see instance variables in objective-c declared with a leading or trailing underscore. If you then typed model anywhere in the rest of your class, it would be a compiler error, forcing you to use the accessor. Typically this is implemented with properties and the synthesize statement:
In your interface:
#property (nonatomic, strong) TestBrain* model;
In your implementation:
#synthesize model = model_;
Your model method would be:
-(TestBrain*)model
{
if (!model_)
model_ = [[TestBrain alloc] init];
return model_;
}
You would then use self.model instead of model throughout the rest of the class.
If you are just starting out, the Stanford iOS course on iTunes U is an excellent resource, a lot of this sort of material is covered.

Answer 3

int x = [model check:3];
This line should be:
int x = [self.model check:3];

Answer 4

you are almost there. You need to use #property and #synthesize in order to complete this. The #synthesize directive will direct the compiler to create the setters and getters for a particular property. The #synthesize directive tells the compiler that variable is a property. Properties allow you to use the dot syntax. i.e. self.model which will automatically the call the getter or setter method, depending on the context.
In your testMethodViewController.h file change it to look like this:
#interface testMethodViewController : UIViewController
{
IBOutlet UILabel *display;
testBrain *model;
}
#property (nonatomic,retain) testBrain *model;
- (IBAction)cellPressed:(UIButton *)sender;
#end
then in the .m implementation you need to use #synthesize after the #implementation. Like this:
#implementation testMethodViewController
#synthesize model; // tells the compiler to synthesize the setter and getter for you
- (testBrain *)model
{
if (!model) {model = [[testBrain alloc] init];}
return model;
}
then in your cellPressed: method, you need to use self.model in order for the getter to be called:
- (IBAction)cellPressed:(UIButton *)sender
{
int x = [self.model check:3]; //This method call doesn't work. But gets no errors.
NSLog(#"Results from model: %i", x); //Says x = 0, but I expect 6
NSString *xAsString = [NSString stringWithFormat: #"testBrain: %i", x];
display.text = xAsString; //Label is updated and displays: testBrain: 0
}
Hope this helps.

Answer 5

I dont see anywhere in the testMethodViewController.h file
IBOutlet UIButton *button;
Also check if u have properly connected all IBOutlet, IBAction & delegate, datasource.