Quartz 2d trying to redraw a triangle - iphone

Question

I'm developing an iPhone app.
I have the following class:
#import "Triangulo.h"
#implementation Triangulo
#synthesize minusValue;
- (id)initWithFrame:(CGRect)frame {
if ((self = [super initWithFrame:frame])) {
// Initialization code
}
return self;
}
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect {
CGSize size = rect.size;
CGPoint origin = {10.0f, size.height - 10.0f};
[self drawLineAtOrigin:origin withLength:size.width - 10.0f];
CGPoint origin2 = {10.0f, size.height - 20.0f};
CGSize size2 = {size.width - minusValue, 20.0f};
[self drawTriangleAtOrigin:origin2 withSize: size2 inRect: rect];
}
- (void)drawLineAtOrigin:(CGPoint)origin withLength:(float)length {
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 2.0);
CGContextSetStrokeColorWithColor(context, [UIColor blackColor].CGColor);
CGContextMoveToPoint(context, origin.x, origin.y);
CGContextAddLineToPoint(context, length, origin.y);
CGContextStrokePath(context);
}
- (void)drawTriangleAtOrigin:(CGPoint)origin withSize:(CGSize)size inRect:(CGRect)rect {
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextClearRect(UIGraphicsGetCurrentContext(), rect);
CGContextSetLineWidth(context, 2.0);
CGContextSetStrokeColorWithColor(context, [UIColor redColor].CGColor);
CGContextSetFillColorWithColor(context, [UIColor redColor].CGColor);
CGContextMoveToPoint(context, origin.x, origin.y);
CGContextAddLineToPoint(context, size.width, origin.y);
CGContextAddLineToPoint(context, size.width, origin.y - size.height);
CGContextAddLineToPoint(context, origin.x, origin.y);
CGContextFillPath(context);
CGContextStrokePath(context);
}
- (void)dealloc {
[super dealloc];
}
#end
This UIView is part of UIViewController.view. I've add in UIViewController.h as:
IBOutlet Triangulo *triangulo;
...
#property (nonatomic, retain) IBOutlet Triangulo *triangulo;
On UIViewController I have a button to modify Triangulo:
- (IBAction)btnDrawTriangleClicked:(id)sender {
triangulo.minusValue += 10.0f;
CGRect rect = CGRectMake(0, 0, 280, 50);
[triangulo drawRect: rect];
}
But I can't draw anything because UIGraphicsGetCurrentContext is always nil.
Any advice?

Answer 1

Add this code to your Triangulo class:
- (void)setMinusValue:(double)aMinusValue {
minusValue = aMinusValue;
[self setNeedsDisplay];
}
And then the btnDrawTriangleClicked: method to the following:
- (IBAction)btnDrawTriangleClicked:(id)sender {
triangulo.minusValue += 10.0f;
}
Now when you alter the minusValue, Triangulo automatically flags itself as needing redisplay. (See The View Drawing Cycle for more info). As Deepak stated, you never call drawRect: yourself (unless you're within the drawRect: method itself, and you're calling [super drawRect:frame];). The drawRect: method is called automatically by the system, and it always makes sure that a valid CGGraphicsContext is set up beforehand. Since you were calling it directly, that preparation work wasn't being done, so UIGraphicsGetCurrentContext() usually returned NULL.

Answer 2

You should never call drawRect: directly. You should call either setNeedsDisplay or setNeedsDisplayInRect: whenever you want to redraw the view. rect that is passed to drawRect: is not a random value that is passed. It tells you the portion of the view's bounds that need to be redrawn.