invalid context error when drawing a UIImage

Question

I am trying to draw a UIImage to the context of my UIView. I've used this code with the context stuff commented in and out ...

- (void)drawRect:(CGRect)rect
{
    //UIGraphicsBeginImageContextWithOptions(rect.size,YES,[[UIScreen mainScreen] scale]);
    //UIGraphicsBeginImageContext(rect.size);

    UIImage *newImage = [UIImage imageNamed:@"character_1_0001.png"];
    //[newImage drawAtPoint:CGPointMake(200, 200)];
    [newImage drawInRect:rect];

    //UIGraphicsEndImageContext();
}

As I understand it I shouldn't need to set the image context to do this and indeed without it I do see the image drawn in the view but I also get this error in the log ...

<Error>: CGContextSaveGState: invalid context 0x0

If I uncomment the UIGraphicsBeginImageContext lines I don't get anything drawn and no error.

Do I need to use the default graphics context and declare this somehow?

I need to draw the image because I want to add to it on the context and can't just generate a load of UIImageView objects.

You must have some other code elsewhere that is calling CGContextSaveGState? You are saying that with just the UIImage= and drawInRect lines, you get this error? — jrturton
If I uncomment the context lines in that code I DON'T get an error but I see nothing drawn in my view. As it is I see my image drawn but get the context error. Is very odd. — Lee Probert
And you don't call CGContextSaveGState yourself anywhere else? — jrturton
Nope. I'm testing this now. Will try just drawing some text. — Lee Probert
Also, you're not calling drawRect directly, are you? If so there may not be a valid context when you are drawing, which could give you that error. — jrturton

jrturton jrturton · Accepted Answer · 2011-10-04T08:55:27

It sounds like you are calling drawRect: directly. So it is getting called once from your call, and once from the genuine drawing loop.

With the context creation present, you do all the drawing in a new context then discard it, so you never see anything
With the context creation missing, you get the error from the manually called drawRect:, and the actual drawing takes place in the genuine drawRect: so you see your image.

Don't call drawRect: directly. Call setNeedsDisplay and drawRect: will be called for you at the appropriate point.

invalid context error when drawing a UIImage

2 Answers