iOS 以编程方式渐变

Question

如何以编程方式实现与附加图像完全相同的渐变？

Answer 1

答案很大程度上取决于设计师用来创建渐变的内容。有一些工具(例如 Invision)可以为开发人员提供有用的信息，例如渐变颜色和点。但是，如果他们只是在 Photoshop 中创建它，则很难以编程方式重现，也许您应该只要求提供渐变图像。

另一种解决方案是使用 @IBDesignable Xcode 中的渐变 View ；许多开发人员创建了它们，包括我，很大程度上是因为过去不得不处理这个问题。这种方法的一个主要优点是调整颜色非常容易，如果需要，甚至可以对其进行动画处理。

我有一个开源示例 available on GitHub ，另外我写了一个extended explanation of how it works .

我快速尝试使用我的项目制作像您一样的渐变，这就是它在 Xcode 中的外观:-



关键部分是 Attributes Inspector 面板的顶部，您可以在其中轻松设置开始和结束渐变颜色以及渐变角度 - 您的角度看起来约为 310 度。



您需要的主要代码粘贴在下面。按照上面的链接了解它的工作原理。

import UIKit
@IBDesignable
class LDGradientView: UIView {

    // the gradient start colour
    @IBInspectable var startColor: UIColor? {
        didSet {
            updateGradient()
        }
    }

    // the gradient end colour
    @IBInspectable var endColor: UIColor? {
        didSet {
            updateGradient()
        }
    }

    // the gradient angle, in degrees anticlockwise from 0 (east/right)
    @IBInspectable var angle: CGFloat = 270 {
        didSet {
            updateGradient()
        }
    }

    // the gradient layer
    private var gradient: CAGradientLayer?

    // initializers
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        installGradient()
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
        installGradient()
    }

    // Create a gradient and install it on the layer
    private func installGradient() {
        // if there's already a gradient installed on the layer, remove it
        if let gradient = self.gradient {
            gradient.removeFromSuperlayer()
        }
        let gradient = createGradient()
        self.layer.addSublayer(gradient)
        self.gradient = gradient
    }

    // Update an existing gradient
    private func updateGradient() {
        if let gradient = self.gradient {
            let startColor = self.startColor ?? UIColor.clear
            let endColor = self.endColor ?? UIColor.clear
            gradient.colors = [startColor.cgColor, endColor.cgColor]
            let (start, end) = gradientPointsForAngle(self.angle)
            gradient.startPoint = start
            gradient.endPoint = end
        }
    }

    // create gradient layer
    private func createGradient() -> CAGradientLayer {
        let gradient = CAGradientLayer()
        gradient.frame = self.bounds
        return gradient
    }

    // create vector pointing in direction of angle
    private func gradientPointsForAngle(_ angle: CGFloat) -> (CGPoint, CGPoint) {
        // get vector start and end points
        let end = pointForAngle(angle)
        //let start = pointForAngle(angle+180.0)
        let start = oppositePoint(end)
        // convert to gradient space
        let p0 = transformToGradientSpace(start)
        let p1 = transformToGradientSpace(end)
        return (p0, p1)
    }

    // get a point corresponding to the angle
    private func pointForAngle(_ angle: CGFloat) -> CGPoint {
        // convert degrees to radians
        let radians = angle * .pi / 180.0
        var x = cos(radians)
        var y = sin(radians)
        // (x,y) is in terms unit circle. Extrapolate to unit square to get full vector length
        if (fabs(x) > fabs(y)) {
            // extrapolate x to unit length
            x = x > 0 ? 1 : -1
            y = x * tan(radians)
        } else {
            // extrapolate y to unit length
            y = y > 0 ? 1 : -1
            x = y / tan(radians)
        }
        return CGPoint(x: x, y: y)
    }

    // transform point in unit space to gradient space
    private func transformToGradientSpace(_ point: CGPoint) -> CGPoint {
        // input point is in signed unit space: (-1,-1) to (1,1)
        // convert to gradient space: (0,0) to (1,1), with flipped Y axis
        return CGPoint(x: (point.x + 1) * 0.5, y: 1.0 - (point.y + 1) * 0.5)
    }

    // return the opposite point in the signed unit square
    private func oppositePoint(_ point: CGPoint) -> CGPoint {
        return CGPoint(x: -point.x, y: -point.y)
    }

    // ensure the gradient gets initialized when the view is created in IB
    override func prepareForInterfaceBuilder() {
        super.prepareForInterfaceBuilder()
        installGradient()
        updateGradient()
    }
}