New programmer here. I am having issues trying to use Core Graphics to draw a stroked arc around my touch location. I have the method to draw the circle working fine, and I have tested and am registering touches when I tap the screen, but when I try to call the method to draw the circle when I tap, I get the error "CGContextBlahBlah: invalid context 0x0"
I think that this is because I'm not calling the method in drawRect:().
So how am I able to call this method on a touch? Furthermore, how can I use the "CGPoint locationOfTouch" as a parameter in my draw method?
Here are the code chunks I am working with.
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
UITouch *touch = [touches anyObject];
CGPoint locationOfTouch = [touch locationInView:self];
[self drawTouchCircle:(locationOfTouch)];
[self setNeedsDisplay];
}
-(void)drawTouchCircle:(CGPoint)locationOfTouch
{
CGContextRef ctx= UIGraphicsGetCurrentContext();
CGContextSaveGState(ctx);
CGContextSetLineWidth(ctx,5);
CGContextSetRGBStrokeColor(ctx,0.8,0.8,0.8,1.0);
CGContextAddArc(ctx,locationOfTouch.x,locationOfTouch.y,30,0.0,M_PI*2,YES);
CGContextStrokePath(ctx);
}
Thanks in advance for the help!
Yes, you're right. The issue is that, rather than calling drawTouchCircle
yourself, you should implement a drawRect
method that calls it for you, and thus your touches
method only needs to call setNeedsDisplay
, and drawRect
will take care of the rest. You might, therefore, want to save the touch location in a class property, and then retrieve that in your drawRect
:
@interface View ()
@property (nonatomic) BOOL touched;
@property (nonatomic) CGPoint locationOfTouch;
@end
@implementation View
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
[super touchesBegan:touches withEvent:event];
self.touched = YES;
UITouch *touch = [touches anyObject];
self.locationOfTouch = [touch locationInView:self];
[self setNeedsDisplay];
}
- (void)drawTouchCircle:(CGPoint)locationOfTouch
{
CGContextRef ctx= UIGraphicsGetCurrentContext();
CGRect bounds = [self bounds];
CGPoint center;
center.x = bounds.origin.x + bounds.size.width / 2.0;
center.y = bounds.origin.y + bounds.size.height / 2.0;
CGContextSaveGState(ctx);
CGContextSetLineWidth(ctx,5);
CGContextSetRGBStrokeColor(ctx,0.8,0.8,0.8,1.0);
CGContextAddArc(ctx,locationOfTouch.x,locationOfTouch.y,30,0.0,M_PI*2,YES);
CGContextStrokePath(ctx);
}
- (void)drawRect:(CGRect)rect
{
if (self.touched)
[self drawTouchCircle:self.locationOfTouch];
}
@end