iphone - Setting A CGContext Transparent Background

Question

I am still struggling with drawing a line with CGContext. I have actually go to line to draw, but now I need the background of the Rect to be transparent so the existing background shows thru. Here's my test code:

(void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [UIColor clearColor].CGColor);
    CGContextSetAlpha(context,0.0);
    CGContextFillRect(context, rect);

    CGContextSetStrokeColorWithColor(context, [UIColor whiteColor].CGColor);
    CGContextSetLineWidth(context, 5.0);
    CGContextMoveToPoint(context, 100.0,0.0);
    CGContextAddLineToPoint(context,100.0, 100.0);
    CGContextStrokePath(context);
}

Any ideas?

Answer 1

After UIGraphicsGetCurrentContext() call CGContextClearRect(context,rect)

Edit: Alright, got it.

Your custom view with the line should have the following:

- (id)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        [self setBackgroundColor:[UIColor clearColor]];
    }
    return self;
}

- (void)drawRect:(CGRect)rect {
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextClearRect(context, rect);
    CGContextSetStrokeColorWithColor(context, [UIColor whiteColor].CGColor);
    CGContextSetLineWidth(context, 5.0);
    CGContextMoveToPoint(context, 100.0,0.0);
    CGContextAddLineToPoint(context,100.0, 100.0);
    CGContextStrokePath(context);
}

My test used this as a very basic UIViewController:

- (void)viewDidLoad {
    [super viewDidLoad];
    UIImageView *v = [[UIImageView alloc] initWithFrame:self.view.bounds];
    [v setBackgroundColor:[UIColor redColor]];
    [self.view addSubview:v];
    TopView *t = [[TopView alloc] initWithFrame:self.view.bounds];
    [self.view addSubview:t];
    [v release];
    [t release];
}