black background when overriding drawRect in UIScrollView

Question

So I am trying to override drawRect in my UIScrolLView, however it gives me this black background instead of the background color that I've specified for my UIScrollView. Why is this? If I remove the drawRect code then everything is fine:

- (void)drawRect:(CGRect)rect
{
    [super drawRect:rect];
    if (shouldDrawVerticalLineForProfile){

        CGContextRef context = UIGraphicsGetCurrentContext();
        CGColorRef separatorColor = [UIColor colorWithRed:47.0/255.0 green:47.0/255.0 
                                                     blue:47.0/255.0 alpha:1.0].CGColor;

        // Add at bottom
        CGPoint startPoint = CGPointMake(60, 0);
        CGPoint endPoint = CGPointMake(60, 10000);

        CGContextSaveGState(context);
        CGContextSetLineCap(context, kCGLineCapSquare);
        CGContextSetStrokeColorWithColor(context, separatorColor);
        CGContextSetLineWidth(context, 5.0);
        CGContextMoveToPoint(context, startPoint.x + 0.5, startPoint.y + 0.5);
        CGContextAddLineToPoint(context, endPoint.x + 0.5, endPoint.y + 0.5);
        CGContextStrokePath(context);
        CGContextRestoreGState(context);  

    }

}

Answer 1

I guess what you are searching for is:

myScrollViewInstance.opaque = NO

After that the background of your scroll view should not be black anymore.

Answer 2

I ran into a similar situation when overriding drawRect in my UIScrollView subclass.

Simply overriding with:

- (void)drawRect:(CGRect)rect
{
    [super drawRect:rect];
}

resulted in a black background instead of the desired clear background I'd get without overriding.

I found it was because the View Background was set to Default in Interface Builder and that setting it to Clear Color resolved the issue for me.

I'm not sure if this is related to your problem but thought I'd share in case it might help.

Answer 3

The answer here is very clearly documented in the AppleDocs for UIView's drawRect: method. The first sentence is:

The default implementation of this method does nothing.

So, the suggestions here to call super are not going to help. The rest of the answer is contained further down in the discussion:

... if the opaque property of your view is set to YES, your drawRect: method must totally fill the specified rectangle with opaque content.

Meaning that if you don't intend to have a transparent background, you need to actually draw the background. The most correct way to do this is to use the following drawRect: template:

- (void)drawRect:(CGRect)rect
{
    [super drawRect:rect]; // optional if a direct UIView-subclass, should be called otherwise.

    CGContextRef context = UIGraphicsGetCurrentContext();
    // Fill the background color, if needed
    if (self.opaque) {
        UIGraphicsPushContext(context);
        CGContextSetFillColorWithColor(context, self.backgroundColor.CGColor);
        CGContextFillRect(context, rect);
        UIGraphicsPopContext();
    }

    // Your code here...
}

Answer 4

This should help

CGContextSetFillColorWithColor(context, colorBack);
CGContextFillRect(context, self.bounds); 

// Choose bounds and colorBack accordingly