Question

I have subclassed a UIView and I have a drawRect implemented. I basically want this drawRect to be called after I’ve set the CGPoint, which is a property of this UIView. How do I do this? Here’s my code:

-(id) initWithCoder:(NSCoder *)aDecoder
{
    if(self = [super initWithCoder:aDecoder]) {
        point_.x = 0;
        self.page_ = [UIImage imageNamed:@"pages"];
    }
    return self;
}

// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    if (point_.x != 0){
        [self.page_ drawInRect:rect];
        CGContextRef context = UIGraphicsGetCurrentContext();
        CGContextSetStrokeColorWithColor(context, [UIColor blackColor].CGColor);
        CGContextSetLineWidth(context, 1.0f);
        CGContextMoveToPoint(context, 40, point_.y); 
        CGContextAddLineToPoint(context, 420, point_.y);
        CGContextStrokePath(context);
    }
}

When I’ve set the point of this UIView and I call drawRect in my other UIViewController like follows:

 self.page_.point_ = CGPointMake(-100, self.title_.frameHeight + self.title_.frameX + 40);
    [self.page_ drawRect:self.page_.frame];

It gives me all of these errors:

  <Error>: CGContextSaveGState: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextSetBlendMode: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextSetAlpha: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextTranslateCTM: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextScaleCTM: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextDrawImage: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextRestoreGState: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextSetStrokeColorWithColor: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextSetLineWidth: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextMoveToPoint: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextAddLineToPoint: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextDrawPath: invalid context 0x0

Why is this?

Answer 1

You should never, ever call -drawRect: directly.

Use -setNeedsDisplay instead:

self.page_.point_ = CGPointMake(-100, self.title_.frameHeight + self.title_.frameX + 40);
[self.page_ setNeedsDisplay];

If you need to customize the drawing area (aka the CGRect that will be passed to -drawRect:), you can use -setNeedsDisplayInRect:

As @rob mayoff has suggested, you can override the setter for the point_ property:

- (void)setPoint_:(CGPoint)point {
   if(!CGPointEqualToPoint(point_, point)) {
      point_ = point;
      [self setNeedsDisplay];
   }
}