What's wrong with UIGraphicsGetCurrentContext()? - objective-c

Question

I'm building up a game so in my structure I have the Game class asking to elements to draw themselves. This is done by a class inherited from UIView which calls the draw method on an instance of Game class
//jeu is an istance of Game
- (void)drawRect:(CGRect)rect {
// Drawing code
CGContextRef context = UIGraphicsGetCurrentContext();
[jeu draw:context];
}
//so the Game method ask all objects to draw themselves
-(void) draw: (CGContextRef) gc{
GameElement* element ;
for( element in drawArr )
{
[element draw:(CGContextRef) gc];
}
}
some elements (a ball and two borders which are all inherited by super class GameElement) correctly draw but some border doesn't
here's the draw method of borders
- (void) draw: (CGContextRef) gc
{
CGFloat black[4] = {0.0f, 0.0f, 0.0f, 1.0f}; // R G B + Alpha
CGPoint p [2];
p[0].x = xStart ;
p[0].y = yStart ;
p[1].x = xEnd ;
p[1].y = yEnd ;
CGContextSetStrokeColor(gc, black); //Definie la couleur
CGContextStrokeLineSegments(gc, p, 2 ) ;
}
So on the console log I read:
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextDrawImage: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextSetStrokeColor: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextStrokeLineSegments: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextSetStrokeColor: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextStrokeLineSegments: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextSetStrokeColor: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextStrokeLineSegments: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextSetStrokeColor: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextStrokeLineSegments: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextSetStrokeColor: invalid context 0x0
Thu Mar 24 21:53:59 MacBook-Air-di-Michele-Giarratana.local IVBricker[381] <Error>: CGContextStrokeLineSegments: invalid context 0x0
And I have no idea of what the hell it means. If the graphic context created in ViewDesigner is the same for all why certains objects draw themselves while others don't?
Thank you all

Answer 1

We know your context is becoming invalid. Check if gc == nil in each GameElement draw method, until you find the first one with a invalid context.
UIView will setup the current context before calling drawRect:, but don't call drawRect manually, use the setNeedsDisplay method instead.
Other ways the context can die, calling UIGrhicsPushContext()
I think after determining where it's invalidating, you'll have a clue.