Question

Important:

I had figured out the problem but a moderator deleted it for some reason:

I tried calling the function directly from the button instead of having the function in the view controller.

I still don’t fully understand why it didn’t work, so I will accept any answer to that question.

Main Post

I am trying to do some drawing after pressing a UIButton. My drawing is done in a subclass of UIView. I have an instance of this subclass in my ViewController. I also have a UIButton in my ViewController that calls a function of the subclass. In that function I call

[self setNeedsDisplay];

When I load up my app, drawRect is called once at the when the view loads, but is never called again, although setNeedsDisplay is being called every time the button is pressed. I looked at many similar questions and I have not found a solution.

I will post all of my code here:

AEViewController.h

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

@interface AEViewController : UIViewController{

IBOutlet AEGraphView *theView;

}
-(IBAction)updateAEGraphView:(id)sender;
@end

AEViewController.m

#import "AEViewController.h"

@interface AEViewController ()

@end

@implementation AEViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    theView = [[AEGraphView alloc] init];
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone) {
        return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
    } 
    else {
        return YES;
    }
}
-(IBAction)updateAEGraphView:(id)sender{
    [theView UpdateView];
}

@end

AEGraphView.h

#import <UIKit/UIKit.h>

@interface AEGraphView : UIView{
    
    
    CGContextRef mainContext;
    int power;
    int multiplier;
    
    
}
-(void)prepareGraphics;
- (void) drawLineFrom:(CGPoint)p1 To:(CGPoint)p2;
-(void)UpdateView;
@end

AEGraphView.m

#import "AEGraphView.h"

@implementation AEGraphView

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    return self;
}


// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    [self prepareGraphics];
    // Drawing code
    NSLog(@"called");
    if (power == 0) {
        [self drawLineFrom:CGPointMake(100,100) To:CGPointMake(-50, -2)];
    }
    else {
        [self drawLineFrom:CGPointMake(0, 0) To:CGPointMake(150, 150)];
    }
    [super drawRect:rect];
    
}

-(void)prepareGraphics{
    mainContext = UIGraphicsGetCurrentContext();
    CGContextSetShouldAntialias(mainContext, TRUE);
    CGContextSetLineWidth(mainContext, 2);
    CGContextSetLineCap(mainContext, kCGLineCapRound);
    
}
- (void) drawLineFrom:(CGPoint)p1 To:(CGPoint)p2{
    NSLog(@"%f,%f  to %f,%f", p1.x,p1.y,p2.x,p2.y);
    // Begin a path.
    CGContextBeginPath(mainContext);
    
    // Add a line to the path.
    CGContextMoveToPoint(mainContext, p1.x, p1.y);
    CGContextAddLineToPoint(mainContext, p2.x, p2.y);
    
    // Stroke and reset the path.
    CGContextStrokePath(mainContext);
}
-(void)UpdateView{
    if (power == 0) {
        power = 1;
    }
    else {
        power = 0;
    }
    [self setNeedsDisplay];
}
@end

Answer 1

Solution was having the function called from the button instead of called from a function in the view controller that was called by the button.