Question

I have a UITableView where I am trying to get the best possible scrolling performance.

The cell does not contain any subviews, all is done in the drawRect method.

Each cell has a white line at the top, a grey line at the bottom and a linear gradient between top and bottom. On top of this background is some text and an image.

I can see from using instruments that the main portion (>50%) of the runtime when scrolling comes from the drawing of the linear gradient. Actually almost all of that comes from the call to CGContextDrawLinearGradient.

Is there a good way to optimize this? I am thinking, either cache the linear gradient some way, or maybe draw it in another way… Maybe draw a one pixel wide and then stretch it?

Here is some of my code:

- (void)drawRect:(CGRect)rect  
{  
  [self drawCellBackground:rect];  
  [self drawSeparator:rect];  
  ....
}

- (void)drawCellBackground:(CGRect)rect
{
  CGContextRef context = UIGraphicsGetCurrentContext();

  CGColorRef notQuiteWhiteColor = [UIColor colorWithRed:245.0/255.0 green:245.0/255.0 
                                             blue:245.0/255.0 alpha:1.0].CGColor;
  CGColorRef lightGrayColor = [UIColor colorWithRed:235.0/255.0 green:235.0/255.0 
                                             blue:235.0/255.0 alpha:1.0].CGColor;

  drawLinearGradient(context, rect, notQuiteWhiteColor, lightGrayColor);
}

From a separate .c file:

void drawLinearGradient(CGContextRef context, CGRect rect, CGColorRef startColor, CGColorRef  endColor)
{
  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
  CGFloat locations[] = { 0.0, 1.0 };

  NSArray *colors = [NSArray arrayWithObjects:(id)startColor, (id)endColor, nil];

  CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, 
                                                  (CFArrayRef) colors, locations);

  CGPoint startPoint = CGPointMake(CGRectGetMidX(rect), CGRectGetMinY(rect));
  CGPoint endPoint = CGPointMake(CGRectGetMidX(rect), CGRectGetMaxY(rect));

  CGContextSaveGState(context);
  CGContextAddRect(context, rect);
  CGContextClip(context);
  CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);
  CGContextRestoreGState(context);

  CGGradientRelease(gradient);
  CGColorSpaceRelease(colorSpace);
}

Answer 1

If you absolutely want to stick to using drawRect: (which I believe gives better performance than separate views on older devices), why not prerender your gradient into a CGImage or UIImage, then use that to draw in -drawRect: ?

Also, you can create your gradient ahead of time:

static CGGradientRef GetCellBackgroundGradient()
{
    static CGGradientRef gradient = NULL ;
    if ( !gradient )
    {
        gradient = // create your CGGradientRef here
    }

    return gradient;
}

For high performance code you want to preallocate as many things (memory/objects) as possible.