Perhaps a little late, so I hope this is still useful for you. I have listed the code for a simple subclass of UIView that draws a circle using the shading you described. Code with comments should be clear.
@implementation CGShadingCircle // This is the callback of our shading function. // info: a pointer to our NSMutableArray of UIColor objects // inData: contains a single float that gives is the current position within the gradient // outData: we fill this with the color to display at the given position static void CGShadingCallback(void* info, const float* inData, float* outData) { // Our colors NSMutableArray* colors = (NSMutableArray*)info; // Position within the gradient, ranging from 0.0 to 1.0 CGFloat position = *inData; // Find the color that we want to used based on the current position; NSUInteger colorIndex = position * [colors count]; // Account for the edge case where position == 1.0 if (colorIndex >= [colors count]) colorIndex = [colors count] - 1; // Get our desired color from the array UIColor* color = [colors objectAtIndex:colorIndex]; // Copy the 4 color components (red, green, blue, alpha) to outData memcpy(outData, CGColorGetComponents(color.CGColor), 4 * sizeof(CGFloat)); } // Set up our colors and shading function - (void)initInternal { _colors = [[NSMutableArray alloc] init]; // Creating the colors in this way ensures that the underlying color space is UIDeviceRGBColorSpace // and thus has 4 color components: red, green, blue, alpha [_colors addObject:[UIColor colorWithRed:1.0f green:0.0f blue:0.0f alpha:1.0f]]; // Red [_colors addObject:[UIColor colorWithRed:0.0f green:1.0f blue:0.0f alpha:1.0f]]; // Green [_colors addObject:[UIColor colorWithRed:0.0f green:0.0f blue:1.0f alpha:1.0f]]; // Blue [_colors addObject:[UIColor colorWithRed:1.0f green:1.0f blue:1.0f alpha:1.0f]]; // White [_colors addObject:[UIColor colorWithRed:0.0f green:0.0f blue:0.0f alpha:1.0f]]; // Black // Define the shading callbacks CGFunctionCallbacks callbacks; callbacks.version = 0; // Defaults to 0 callbacks.evaluate = CGShadingCallback; // This is our color selection function callbacks.releaseInfo = NULL; // Not used // As input to our function we want 1 value in the range [0.0, 1.0]. // This is our position within the 'gradient'. size_t domainDimension = 1; CGFloat domain[2] = {0.0f, 1.0f}; // The output of our function is 4 values, each in the range [0.0, 1.0]. // This is our selected color for the input position. // The 4 values are the red, green, blue and alpha components. size_t rangeDimension = 4; CGFloat range[8] = {0.0f, 1.0f, 0.0f, 1.0f, 0.0f, 1.0f, 0.0f, 1.0f}; // Create the shading function _shadingFunction = CGFunctionCreate(_colors, domainDimension, domain, rangeDimension, range, &callbacks); } - (id)initWithFrame:(CGRect)frame { self = [super initWithFrame:frame]; if (self) { [self initInternal]; } return self; } - (id)initWithCoder:(NSCoder*)decoder { self = [super initWithCoder:decoder]; if (self) { [self initInternal]; } return self; } - (void)dealloc { [_colors release]; CGFunctionRelease(_shadingFunction); [super dealloc]; } - (void)drawRect:(CGRect)rect { CGRect b = self.bounds; CGContextRef ctx = UIGraphicsGetCurrentContext(); // Create a simple elliptic path CGContextAddEllipseInRect(ctx, b); // Set the current path as the clipping path CGContextClip(ctx); // Create our shading using the function that was defined earlier. CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB(); CGShadingRef shading = CGShadingCreateAxial(colorspace, CGPointMake(CGRectGetMinX(b), CGRectGetMidY(b)), CGPointMake(CGRectGetMaxX(b), CGRectGetMidY(b)), _shadingFunction, true, true); // Draw the shading CGContextDrawShading(ctx, shading); // Cleanup CGShadingRelease(shading); CGColorSpaceRelease(colorspace); } @end
This gives me the following result:
Hope this helps!