How to create UIImage with vertical gradient using “from-color” and “to-color”

Question

How to create new image just with gradient colors, using \"from-color\" and \"to-color\"?

Answer 1

UPDATED TO Swift 3

I wrote UIImage extension for Swift but you can also use it from Objective-C:

import UIKit

private let ChannelDivider: CGFloat = 255

public class RGBA: NSObject {
    var red: CGFloat
    var green: CGFloat
    var blue: CGFloat
    var alpha: CGFloat

    init(red: CGFloat, green: CGFloat, blue: CGFloat, alpha: CGFloat) {
        self.red = red
        self.green = green
        self.blue = blue
        self.alpha = alpha
    }

    init(intRed: Int, green: Int, blue: Int, alpha: Int) {
        self.red = CGFloat(intRed)/ChannelDivider
        self.green = CGFloat(green)/ChannelDivider
        self.blue = CGFloat(blue)/ChannelDivider
        self.alpha = CGFloat(alpha)/ChannelDivider
    }
}

public class Grayscale: NSObject {
    var white: CGFloat
    var alpha: CGFloat

    init(white: CGFloat, alpha: CGFloat) {
        self.white = white
        self.alpha = alpha
    }
}

public class GradientPoint: NSObject {
    var location: CGFloat
    var color: C

    init(location: CGFloat, color: C) {
        self.location = location
        self.color = color
    }
}

extension UIImage {

    public class func image(withGradientPoints gradientPoints: [GradientPoint<[CGFloat]>], colorSpace: CGColorSpace, size: CGSize) -> UIImage? {
        UIGraphicsBeginImageContextWithOptions(size, false, 0);
        guard
            let context = UIGraphicsGetCurrentContext(),
            let gradient = CGGradient(colorSpace: colorSpace,
                                      colorComponents: gradientPoints.flatMap { $0.color },
                                      locations: gradientPoints.map { $0.location }, count: gradientPoints.count) else {
                                        return nil
        }

        context.drawLinearGradient(gradient, start: CGPoint.zero, end: CGPoint(x: 0, y: size.height), options: CGGradientDrawingOptions())
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return image
    }

    public class func image(withRGBAGradientPoints gradientPoints: [GradientPoint], size: CGSize) -> UIImage? {
        return image(withGradientPoints: gradientPoints.map {
            GradientPoint(location: $0.location, color: [$0.color.red, $0.color.green, $0.color.blue, $0.color.alpha])
        }, colorSpace: CGColorSpaceCreateDeviceRGB(), size: size)
    }

    public class func image(withRGBAGradientColors gradientColors: [CGFloat: RGBA], size: CGSize) -> UIImage? {
        return image(withRGBAGradientPoints: gradientColors.map {  GradientPoint(location: $0, color: $1)}, size: size)
    }

    public class func image(withGrayscaleGradientPoints gradientPoints: [GradientPoint], size: CGSize) -> UIImage? {
        return image(withGradientPoints: gradientPoints.map {
            GradientPoint(location: $0.location, color: [$0.color.white, $0.color.alpha]) },
                     colorSpace: CGColorSpaceCreateDeviceGray(), size: size)
    }

    public class func image(withGrayscaleGradientColors gradientColors: [CGFloat: Grayscale], size: CGSize) -> UIImage? {
        return image(withGrayscaleGradientPoints: gradientColors.map { GradientPoint(location: $0, color: $1) }, size: size)
    }
}

You can create gradient image using RGBA colors:

// Objective-C
RGBA *startColor = [[RGBA alloc] initWithRed:1 green:0 blue:0 alpha:1];
RGBA *endColor = [[RGBA alloc] initWithIntRed:0 green:255 blue:0 alpha:255];
UIImage *gradientImage = [UIImage imageWithRGBAGradient: @{ @0: startColor, @1: endColor} size: CGSizeMake(32, 64)];

// Swift
let startColor = RGBA(red: 1, green: 0, blue: 0, alpha: 1)
let endColor = RGBA(intRed: 0, green: 255, blue: 0, alpha: 255)
let gradientImage = UIImage.image(withRGBAGradientPoints: [0: startColor, 1: endColor], size: CGSizeMake(32, 64))

And grayscale colors:

// Objective-C
Grayscale *startColor = [[Grayscale alloc] initWithWhite:1 alpha:1];
Grayscale *endColor = [[Grayscale alloc] initWithWhite:0 alpha: 0.5];
UIImage *gradientImage = [UIImage imageWithGrayscaleGradient: @{ @0: startColor, @1: endColor} size: CGSizeMake(32, 64)];

// Swift
let startColor = Grayscale(white: 1, alpha: 1)
let endColor = Grayscale(white:0, alpha: 0.5)
let gradientImage = UIImage.image(withGrayscaleGradientPoints: [0: startColor, 1: endColor], size: CGSizeMake(32, 64))

If you are not going to use this code from Objective-C than you can remove NSObject inheritance from RGBA, Grayscale and GradientPoint.