Protocol and Delegate between View and Controller

I'm having some trouble getting the protocol/delegate method to work between the Model (GraphView) and the Controller (GraphViewController). The NSLog in the drawRect: in GraphView.m tells me that expression is null

the variable expression holds a value (the NSLog in ViewDidLoad proves this). Am I missing something obvious?

GraphView.h

@class GraphView;

@protocol GraphViewDelegate
-(NSString *) expressionForGraphView:(GraphView *) requestor; 
@end

@interface GraphView : UIView
{
      id <GraphViewDelegate> delegate;
}

@property (assign) id <GraphViewDelegate> delegate;

@end

GraphView.m

#import "GraphView.h"

@implementation GraphView

@synthesize delegate;

- (void)drawRect:(CGRect)rect
{
      NSString *expression = [self.delegate expressionForGraphView:self];
      NSLog(@"%@", expression);

}

GraphViewController.h

#import <UIKit/UIKit.h>
#import "GraphView.h"

@interface GraphViewController : UIViewController <GraphViewDelegate>
{
    GraphView *graphView;
    NSString *expression;
}

@property (retain) IBOutlet GraphView *graphView;
@property (开发者_开发技巧retain) NSString *expression;

@end

GraphViewController.m

#import "GraphViewController.h"

@implementation GraphViewController

@synthesize graphView;
@synthesize expression;

-(NSString *) expressionForGraphView:(GraphView *) requestor
{
      NSString *tempString;
      if (requestor == self.graphView)
        tempString = self.expression;
      else 
        tempString = nil;

      return tempString;
}

- (void)viewDidLoad
{
     [super viewDidLoad];
     self.graphView.delegate = self;
     NSLog(@"%@", self.expression);
}

The clue is in the method name

- (void)viewDidLoad

The view has already loaded before you set the delegate. Therefore when the drawRect was called the delegate was nil.

Set up the delegate in the relevant initializer method or in IB