Question

I am trying to draw a graph which moves up and down depending on the value of a slider.
My graph is being drawn in a view which belongs to a custom class GraphView.
There is one ViewController for the project and the slider calls a method moveLine.
This has a property endXPoint which I have set such that:
endXPoint = mySlider.value

My problem is that I don’t know how to reference this value from inside the drawRect method of my GraphView.

I have tried creating a reference to GraphView in the ViewController and setting the property there but it does not work:
GraphView *myGraphView = (GraphView *)self.view;
myGraphView.endXPoint = mySlider.value;

Answer 1

You have to set a property for the GraphView class like that:

@interface GraphView : UIView
@property float endXPoint;

Then from the ViewController you set the GraphView variable like:

[myGraphView setendXPoint: [mySlider value]];
[myGraphView setNeedsDisplay];

The last line ask GraphView to update the View, calling the drawRect method.
In the drawRect method you can use endXPoint directly because it is a class property.

This is the correct version:

//ViewController.h

#import <UIKit/UIKit.h> 
#import "GraphView.h" //import headers in the header file

@interface ViewController : UIViewController  

@property (strong, nonatomic) IBOutlet UISlider *mySlider;  
@property (strong, nonatomic) IBOutlet UILabel *myLabel;  
@property (strong, nonatomic) IBOutlet GraphView *myGraphView; //Connect this with the IB

- (IBAction)moveLine:(id)sender; 
- (IBAction)setLabelText:(id)sender;  
@end  

//ViewController.m

#import "ViewController.h"   

@implementation ViewController  
@synthesize mySlider;  
@synthesize myLabel;  
@synthesize myGraphView;



- (IBAction)moveLine:(id)sender {  
    [myGraphView setendXPoint:[mySlider value]];  
    [myGraphView setNeedsDisplay];  
}  

@end 

//GraphView.h 

#import <UIKit/UIKit.h>  

@interface GraphView : UIView  
@property float endXPoint;  


@end

//GraphView.m  

#import "GraphView.h"   

@implementation GraphView  
@synthesize endXPoint;  


- (void)drawRect:(CGRect)rect  
{

    CGContextRef ctx = UIGraphicsGetCurrentContext(); //get the graphics context  
    CGContextSetRGBStrokeColor(ctx, 1.0, 0, 0, 1);   
    CGContextMoveToPoint(ctx, 0, 0);  
    //add a line from 0,0 to the point 100,100;   
    CGContextAddLineToPoint( ctx, endXPoint,100);  
    //"stroke" the path  
    CGContextStrokePath(ctx);  
}  


@end