How to create a colored 1x1 UIImage on the iPhone dynamically?

I would like to create a 1x1 UIImage dynamically based on a UIColor.

I suspect this can quickly be done with Quartz2d, and I'm poring over the documentation trying to get a grasp of the fundamentals. However, it looks like there are a lot of potential pitfalls: not identifying the numbers of bits and bytes per things correctly, not specifying the right flags, not releasing unused data, etc.

How can this be safely done with Quartz 2d (or another simpler way)?

You can use CGContextSetFillColorWithColor and CGContextFillRect for this:

Swift

extension UIImage {
    class func image(with color: UIColor) -> UIImage {
        let rect = CGRectMake(0.0, 0.0, 1.0, 1.0)
        UIGraphicsBeginImageContext(rect.size)
        let context = UIGraphicsGetCurrentContext()

        CGContextSetFillColorWithColor(context, color.CGColor)
        CGContextFillRect(context, rect)

        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return image
    }
}

Swift3

extension UIImage {
    class func image(with color: UIColor) -> UIImage {
        let rect = CGRect(origin: CGPoint(x: 0, y:0), size: CGSize(width: 1, height: 1))
        UIGraphicsBeginImageContext(rect.size)
        let context = UIGraphicsGetCurrentContext()!
        
        context.setFillColor(color.cgColor)
        context.fill(rect)
        
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return image!
    }
}

Objective-C

+ (UIImage *)imageWithColor:(UIColor *)color {
    CGRect rect = CGRectMake(0.0f, 0.0f, 1.0f, 1.0f);
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSetFillColorWithColor(context, [color CGColor]);
    CGContextFillRect(context, rect);

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return image;
}

Here's another option based on Matt Stephen's code. It creates a resizable solid color image such that you could reuse it or change it's size (e.g. use it for a background).

+ (UIImage *)prefix_resizeableImageWithColor:(UIColor *)color {
	CGRect rect = CGRectMake(0.0f, 0.0f, 3.0f, 3.0f);
	UIGraphicsBeginImageContext(rect.size);
	CGContextRef context = UIGraphicsGetCurrentContext();

	CGContextSetFillColorWithColor(context, [color CGColor]);
	CGContextFillRect(context, rect);

	UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
	UIGraphicsEndImageContext();

	image = [image resizableImageWithCapInsets:UIEdgeInsetsMake(1, 1, 1, 1)];

	return image;
}

Put it in a UIImage category and change the prefix.

I used Matt Steven's answer many times so made a category for it:

@interface UIImage (mxcl)
+ (UIImage *)squareImageWithColor:(UIColor *)color dimension:(int)dimension;
@end

@implementation UIImage (mxcl)
+ (UIImage *)squareImageWithColor:(UIColor *)color dimension:(int)dimension {
    CGRect rect = CGRectMake(0, 0, dimension, dimension);
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSetFillColorWithColor(context, [color CGColor]);
    CGContextFillRect(context, rect);

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return image;
}
@end

Using Apple's latest UIGraphicsImageRenderer the code is pretty small:

import UIKit

extension UIImage {
    static func from(color: UIColor) -> UIImage {
        let size = CGSize(width: 1, height: 1)
        return UIGraphicsImageRenderer(size: size).image(actions: { (context) in
            context.cgContext.setFillColor(color.cgColor)
            context.fill(.init(origin: .zero, size: size))
        })
    }
}

To me, a convenience init feels neater in Swift.

extension UIImage {
    
    convenience init?(color: UIColor, size: CGSize = CGSize(width: 1, height: 1)) {
        
        let rect = CGRect(x: 0, y: 0, width: size.width, height: size.height)
        UIGraphicsBeginImageContext(rect.size)
        guard let context = UIGraphicsGetCurrentContext() else {
            return nil
        }
        
        context.setFillColor(color.cgColor)
        context.fill(rect)

        guard let image = context.makeImage() else {
            return nil
        }
        UIGraphicsEndImageContext()
        
        self.init(cgImage: image)
    }

}

Ok, this won't be exactly what you want, but this code will draw a line. You can adapt it to make a point. Or at least get a little info from it.

Making the image 1x1 seems a little weird. Strokes ride the line, so a stroke of width 1.0 at 0.5 should work. Just play around.

- (void)drawLine{

UIGraphicsBeginImageContext(CGSizeMake(320,300));

CGContextRef ctx = UIGraphicsGetCurrentContext();

float x = 0;
float xEnd = 320;
float y = 300;

CGContextClearRect(ctx, CGRectMake(5, 45, 320, 300));

CGContextSetGrayStrokeColor(ctx, 1.0, 1.0);

CGContextSetLineWidth(ctx, 1);
CGPoint line[2] = { CGPointMake(x,y), CGPointMake(xEnd, y) };

CGContextStrokeLineSegments(ctx, line, 2);

UIImage *theImage=UIGraphicsGetImageFromCurrentImageContext();
	UIGraphicsEndImageContext();
}