Question

I’m using the MVC model but I cannot get the data needed for the View, I am trying to use a data source since a View should never own its data. Typically, a protocol is used to create a data source.

• I have a MVC: CalculatorBrain.[hm] – CalculatorViewController.[hm] – (view is .xib)
• I also have MVC: GraphingView – GraphingViewController – (model is the data I can’t get)

The goal is: when I press a button on the calculator, it draws the function (e.g.: x+5=) that is currently in it’s display. The calculator part takes care of the expression/function, the display, etc while the graphing part should draw. CalculatorViewController should be the GraphingView’s source, but the data always stays null.

This is GraphingView.h with the declaration of the protocol :

@class GraphingView;

@protocol GraphingViewSource
- (float)getDataPoints:(GraphingView *)requestor;
@end

@interface GraphingView : UIView {
    id <GraphingViewSource> source;
}

@property (assign) id <GraphingViewSource> source;

@end

CalculatorViewController.m implements the protocol by implementing the getDataPoints: method. Header of CalculationViewController.h :

@interface CalculatorViewController : UIViewController <GraphingViewSource>

Pressing the button which should set up the data source :

- (IBAction)graphPressed:(UIButton *)sender
{
    GraphingViewController *graphingVC = [[GraphingViewController alloc] init];
    graphingVC.graphingView.source = self;
    [self.navigationController pushViewController:graphingVC animated:YES];
    [graphingVC release];
}

Pressing the button brings up the new view nicely etc, but following line of code only returns null (inside GraphingView.m) :

float result = [self.source getDataPoints:self];

It seems, I cannot access my data source…

Answer 1

From your comments it seems that graphingView is nil at the time you are trying to set its source.

If there is a xib for the view controller, you should be doing this:

GraphingViewController *graphingVC = [[GraphingViewController alloc] initWithNibName:@"GraphingViewController" bundle:nil];

Instead of a plain init.

This will create and instantiate all of your view objects so they are available to have properties set before you present the view controller.