Quartz 2d trying to redraw a triangle

I'm developi开发者_开发百科ng 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?

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.

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.