Error when drawing simple shapes in Objective-C

Question

I have an app that loops through a strings of coordinates and draws a shape using UIBezierPath. When I run the app I get the following error:

<Error>: CGContextSetStrokeColorWithColor: invalid context 0x0.

Here is a cut down version of the code I have used to test, and to try and and resolve this error:

UIBezierPath *testPath = [UIBezierPath bezierPath];

[[UIColor blackColor] setStroke];
[[UIColor redColor] setFill];

CGContextRef testPathRef = UIGraphicsGetCurrentContext();

[testPath moveToPoint:CGPointMake(100, 100)];
[testPath addLineToPoint:CGPointMake(200, 100)];
[testPath addLineToPoint:CGPointMake(100, 200)];
[testPath fill];
[testPath stroke];
[testPath setLineWidth:5];
[testPath closePath];

Apple's own documentation seems to suggest that the above is all I need to draw a simple shape and to colour it in, it breifly mentions a 'context' but it being optional. I have looked at other questions involving this kind of error and some seem to suggest an object being null (0x0) but I can't work out what is going wrong.

EDIT: I have created a new classed called CreateButtons that is a sub class of UIView and called it but the drawRect method never seems to get fired.

-(void)drawShape:(NSString*)locationString buttonTagged:(NSInteger)buttonTag{

    CreateButtons *createButtonsInstance = [[CreateButtons alloc] init];

}

CreateButtons implementation:

#import "CreateButtons.h"

@implementation CreateButtons

-(id)initWithFrame:(CGRect)frame {

    self = [super initWithFrame:frame];

    if (self) {
        NSLog(@"initWithFrame fired");

    }
    return self;
}

-(void)drawRect:(CGRect) rect {
    NSLog(@"Draw rect fired");

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetLineWidth(context, 2.0);
    CGContextSetStrokeColorWithColor(context, [UIColor blueColor].CGColor);

    CGContextMoveToPoint(context, 100, 100);
    CGContextAddLineToPoint(context, 150, 150);
    CGContextAddLineToPoint(context, 100, 200);
    CGContextAddLineToPoint(context, 50, 150);
    CGContextAddLineToPoint(context, 100, 100);

    CGContextStrokePath(context);
}

Answer 1

Just make sure you calling this code when the Context is valid. Which is only in drawRect: method inside of UIView subclass. You may need to modify your code to work this way, as it is a basic iOS practice. In this case, the OS setups the correct CGContext for you.

Otherwise you might have to create your own CGContext.