UIImage from UIColor with alpha - ios

Question

I am trying to create a UIImage from this UIColor value I have. The UIImage is being created, but the alpha value is lost. The image remains opaque. I am using the following code:
#implementation GetImage
+ (UIImage *)imageWithColor:(UIColor *)color andSize:(CGSize)size
{
UIImage *img = nil;
CGRect rect = CGRectMake(0, 0, size.width, size.height);
UIGraphicsBeginImageContext(rect.size);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(context,
color.CGColor);
CGContextFillRect(context, rect);
img = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return img;
}
#end
Which I call like so:
-(void) setButtonColours{
UIImage *img1, *img2;
UIColor *red = [UIColor colorWithRed:205.0/255.0 green:68.0/255.0 blue:51.0/255.0 alpha:1.0],
*blue = [UIColor colorWithRed:71.0/255.0 green:97.0/255.0 blue:151.0/255.0 alpha:1.0];
img1 = [GetImage imageWithColor: blue andSize: self.dummyButton.frame.size];
img2 = [GetImage imageWithColor: red andSize: self.dummyButton.frame.size];
[self.dummyButton setBackgroundImage: img1 forState:UIControlStateNormal];
[self.dummyButton setBackgroundImage: img2 forState:UIControlStateSelected];
}
Any idea what I'm missing?

Answer 1

You are creating an opaque image context.
Replace this line:
UIGraphicsBeginImageContext(rect.size);
with:
UIGraphicsBeginImageContextWithOptions(rect.size, NO, 0);

Answer 2

In the event someone finds this question and they are looking for the Monotouch implementation:
public class ColorRGBA
{
public float Red=0;
public float Green=0;
public float Blue=0;
public float Alpha=0;
public ColorRGBA (float red, float green, float blue, float alpha)
{
Red = red;
Green = green;
Blue = blue;
Alpha = alpha;
}
}
//then from within a custom mt.d element
private ColorRGBA color;
public override UITableViewCell GetCell (UITableView tv)
{
//create square with color
var imageSize = new SizeF(30, 30);
var imageSizeRectF = new RectangleF (0, 0, 30, 30);
UIGraphics.BeginImageContextWithOptions (imageSize, false, 0);
var context = UIGraphics.GetCurrentContext ();
context.SetFillColor (color.Red, color.Green, color.Blue, color.Alpha);
context.FillRect (imageSizeRectF);
var image = UIGraphics.GetImageFromCurrentImageContext ();
UIGraphics.EndImageContext ();
var cell = base.GetCell(tv);
cell.Accessory = UITableViewCellAccessory.DisclosureIndicator;
cell.ImageView.Image = image;
return cell;
}

Answer 3

For those interested, here is a simple rewrite in Swift 4+:
import UIKit
extension UIImage {
class func colorAsImage(from color:UIColor, for size:CGSize) -> UIImage? {
var img:UIImage?
let rect = CGRect(x:0.0, y:0.0, width: size.width, height: size.height)
UIGraphicsBeginImageContextWithOptions(rect.size, false, 0)
guard let context = UIGraphicsGetCurrentContext() else { return nil }
context.setFillColor(color.cgColor)
context.fill(rect)
img = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
guard let colorImage = img else { return nil }
return colorImage
}
}