iOS CGContext error

[kevinrichardsuk]

Hi everyone,

I'm trying to draw a PNG file to my iPhone screen and, although I've managed, the results are a little confusing..

Here is some code;

- (void)drawRect:(CGRect)rect {

    [self reDraw];
}

- (void)reDraw {

    CGContextRef cgContext = UIGraphicsGetCurrentContext();

    CGImageRef imageRef = [[UIImage imageNamed:[self getImgFilename] ] CGImage];
    CGRect frame = [self bounds];

    CGContextDrawImage(cgContext, frame, imageRef);

    NSLog([self getImgFilename]);
}

- (NSMutableString *)getImgFilename {

    NSMutableString *newImage;

    if (state=1)
    {
        newImage = [NSMutableString stringWithString:@"image1.png"];
    }
    else if (state=2)
    {
        newImage = [NSMutableString stringWithString:@"image2.png"];
    }

    return newImage;
}

So far, so basic.

The problem I have is that I get "<Error>: CGContextDrawImage: invalid context" whenever I try and call reDraw a second time.

The image is displayed when the app starts, if I call the reDraw function again then I get the error.

My other issue is that the image is flipped and mirrored, but that looks easily fixable compared to the error message.

Any ideas?? Thanks, Kevin

 

[robbieduncan]

You should move all the code from reDraw into drawRect:

I imagine you've done this as the documentation says something like "drawRect: should not be called directly". This does not mean that you should create a new method and call that: the reason that you should not call drawRect: directly is that it won't work for the same reason that your reDraw won't work.

When drawRect: gets called by the system it does a lot of work in advance including setting up the context for drawing. You have not done that which is why your context is invalid.

The correct way to get a view to redraw is to tell it that it needs redrawing via the setNeedsDisplay: method. This will cause the system to call drawRect: with a valid context.

 

[kevinrichardsuk]

ok - thanks for that. I now understand why it doesn't work - so the error doesn't appear at startup (and it consequently works) because drawRect is called by the system and manages the context.

I tried what you suggested but my UIView class doesn't recognise the setNeedsDisplay method, and any attempt to call it bombs the emulator..

 

[robbieduncan]

ok - thanks for that. I now understand why it doesn't work - so the error doesn't appear at startup (and it consequently works) because drawRect is called by the system and manages the context.

I tried what you suggested but my UIView class doesn't recognise the setNeedsDisplay method, and any attempt to call it bombs the emulator..

setNeedsDisplay: is an NSView method. I assume there is a similar UIView method. Note the :. That's important. It's called something like:

[myView setNeedsDisplay:YES];

That's for an NSView. A quick Google search indicates that for a UIView it doesn't take the argument. Check the documentation...

 

[kevinrichardsuk]

You beat me to it, I found that removing the :YES argument fixes the problem.

Thanks again