Question

I have a delegate method of a UIVIew and in the drawRect method I add in a UIBezierPath to show a shadow on a square.

 //// General Declarations
 CGContextRef context = UIGraphicsGetCurrentContext();

 //// Shadow Declarations
 UIColor* shadow = [UIColor blackColor];
 CGSize shadowOffset = CGSizeMake(0, -0);
 CGFloat shadowBlurRadius = 15;


 //// Rectangle Drawing
 rectanglePath = [UIBezierPath bezierPathWithRect: CGRectMake(8, 8, 44, 44)];
 CGContextSaveGState(context);
 CGContextSetShadowWithColor(context, shadowOffset, shadowBlurRadius, shadow.CGColor);
 [[UIColor whiteColor] setFill];
 [rectanglePath fill];
 CGContextRestoreGState(context);

I then want to change the colour of the shadow based on certain criteria so I mave a method called makeRed.

- (void)makeRed {
  NSLog(@"makeRed");
  CGContextRef context = UIGraphicsGetCurrentContext();
  // Shadow Declarations
  UIColor* shadow = [UIColor redColor];
  CGSize shadowOffset = CGSizeMake(0, -0);
  CGFloat shadowBlurRadius = 15;
  CGContextSaveGState(context);
  CGContextSetShadowWithColor(context, shadowOffset, shadowBlurRadius, shadow.CGColor);
  [[UIColor whiteColor] setFill];
  [rectanglePath fill];
  CGContextRestoreGState(context);
}

But when I call the method I get the message:

: CGContextSaveGState: invalid context 0x0

any ideas how I can either set the right context or change the shadow colour in a different way?

Note the initial drawing of the shadow works perfectly, as there are other attributes to the delegate i.e. some fancy animations using the .layer method of creating shadows wont work.

Cheers

Answer 1

In the UIView docs you can see drawRect:

By the time this method is called, UIKit has configured the drawing environment appropriately for your view and you can simply call whatever drawing methods and functions you need to render your content.

So the drawing you do inside drawRect: is correct as the drawing context is setup correctly etc, this is not the case in your makeRed method.

I would suggest having an ivar shadowColor and then use this inside your drawRect: method.
Your makeRed would then look like this

- (void)makeRed;
{
  self.shadowColor = [UIColor redColor];
  [self setNeedsDisplay];
}

and then modify the line in drawRect: to

CGContextSetShadowWithColor(context, shadowOffset, shadowBlurRadius, self.shadowColor.CGColor);

setNeedsDisplay is used to tell UIKit that you would like your view to be redrawn, which then results in drawRect: being called again.

You will of course have to initialize _shadowColor = [UIColor blackColor] in the init* method.