Question

I’m trying to create a CGRect inside of a custom view (rectView) that will move up and down as you move a slider.

My slider’s IBAction calls the following method: (which get’s called fine)

- (void)moveRectUpOrDown:(int)y
{
    self.verticalPositionOfRect += y;
    [self setNeedsDisplay];
}

My drawRect method:

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGFloat size = 100;
    self.rect = CGRectMake((self.bounds.size.width / 2) - (size / 2), 
                                 self.verticalPositionOfRect - (size / 2), 
                                 size, 
                                 size);
    CGContextAddRect(context, self.rect);
    CGContextFillPath(context);
}

My custom view’s initWithFrame calls the drawRect method using setNeedsDisplay, but for some reason the moveRectUpOrDown won’t call drawRect.

Any ideas what I’m doing wrong?

For clarity the entire implementation is below:

//ViewController.h
#import <UIKit/UIKit.h>
#import "rectView.h"

@interface ViewController : UIViewController
@property (strong, nonatomic) IBOutlet rectView *rectView;
- (IBAction)sliderChanged:(id)sender;
@end

//ViewController.m
#import "ViewController.h"

@implementation ViewController
@synthesize rectView;

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.rectView = [[rectView alloc] initWithFrame:self.rectView.frame];
}

- (void)viewDidUnload
{
    [super viewDidUnload];
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
}

- (IBAction)sliderChanged:(id)sender 
{
    UISlider *slider = sender;
    CGFloat sliderValue = slider.value;
    [self.rectView moveRectUpOrDown:sliderValue];
}
@end

//rectView.h
#import <UIKit/UIKit.h>

@interface rectView : UIView
- (void)moveRectUpOrDown:(int)y;
@end

//rectView.m
#import "rectView.h"

@interface rectView ()
@property CGRect rect;
@property int verticalPositionOfRect;
@end

@implementation rectView
@synthesize rect, verticalPositionOfRect;

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

        self.verticalPositionOfRect = (self.bounds.size.height / 2);
        [self setNeedsDisplay];
    }
    return self;
}

- (void)moveRectUpOrDown:(int)y
{
    self.verticalPositionOfRect += y;
    [self setNeedsDisplay];
}

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGFloat size = 100.0;
    self.rect = CGRectMake((self.bounds.size.width / 2) - (size / 2), 
                                 self.verticalPositionOfRect - (size / 2), 
                                 size, 
                                 size);
    CGContextAddRect(context, self.rect);
    CGContextFillPath(context);
}

@end

Thanks for the help 🙂

Answer 1

OK so the answer is the view is never actually added to the viewController‘s view..

You have an IBOutlet to your rectView – so I assume you dragged a UIView component onto the view in Interface Builder and then changed it’s class to rectView, which is all fine BUT in viewDidLoad you wipe over this object with a new one

self.rectView = [[rectView alloc] initWithFrame:self.rectView.frame];

So this is now no longer the object that was loaded from the xib.

Subsequently this view is never actually added to the view hierarchy so it’s never going to draw itself as it does not make sense.

So the solution is to remove the line I highlighted above.

---

Notes

I would probably still go with my other answer for such an implementation but it’s helpful to get tripped up and learn new things.

rectView does not follow naming conventions. Ideally you should start your class name with a 2-3 letter prefix (possibly your initial’s or company name) followed by a camel cased named.