Question

I’m trying to draw a line between one object and where the user touched. I’ve tried subclassing and I can’t get the “-(void)drawrect” to update itself every time the user touches the screen. I deleted those files and tried putting the code right into the “-(void)touchesbegan”, but it does not WORK:

    - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event

    UITouch *touch = [touches anyObject];
    CGPoint locationOfTouch = [touch locationInView:nil];
    // You can now use locationOfTouch.x and locationOfTouch.y as the user's coordinates



Int xpos = (int)(starShip.center.x);
int ypos = (int)(starShip.center.y);

    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5.0);
       CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0.0, 0.0, 0.0, 1.0);
    CGContextBeginPath(UIGraphicsGetCurrentContext());
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), starShip.center.x, starShip.center.y);
    //draws a line to the point where the user has touched the screen
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), locationOfTouch.x, locationOfTouch.y);
    CGContextStrokePath(UIGraphicsGetCurrentContext());
}

Answer 1

drawRect: is only called when required. It’s called automatically the first time the view is displayed and every time the view is resized. If you want it to be called after a touch you have to call [self setNeedsDisplay];.

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    [self setNeedsDisplay];
}

This will make sure the drawRect: method will be called. You don’t directly call drawRect:, because it you called it multiple times in one frame, the view would redraw itself multiple times. Instead you flag it as needing a redraw using setNeedsDisplay. This way drawRect: will only be called once, no matter how often you call setNeedsDisplay.