Angled Gradient Layer

问题:

I have custom UIView class that renders a gradient in Swift 2. I'm struggling with making an angled gradient so that it draws from the top-left to the bottom-right. Can somebody help me a bit?

import UIKit

class GradientView: UIView {

    let gradientLayer = CAGradientLayer()

    override func awakeFromNib() {
        // 1
        self.backgroundColor = ColorPalette.White

        // 2
        gradientLayer.frame = self.bounds

        // 3
        let color1 = ColorPalette.GrdTop.CGColor as CGColorRef
        let color2 = ColorPalette.GrdBottom.CGColor as CGColorRef
        gradientLayer.colors = [color1, color2]

        // 4
        gradientLayer.locations = [0.0, 1.0]

        // 5
        self.layer.addSublayer(gradientLayer)
    }

}

I suspect this should be something else but whatever I input nothing changes.

gradientLayer.locations = [0.0, 1.0]

回答1:

You don't want to use locations to specify the direction of the gradient. Instead use startPoint and endPoint for that.

The locations array is used when one wants to specify where, in between startPoint and endPoint, the gradient should to take place. For example, if you want the colors to only take place in the middle 10% of the range from the start and end points, you'd use:

locations = [0.45, 0.55]

The locations array doesn't dictate the direction. The startPoint and endPoint do. So, for a diagonal gradient from upper left to lower right, you would set startPoint of CGPoint(x: 0, y: 0) and an endPoint to CGPoint(x: 1, y: 1).

For example:

@IBDesignable
class GradientView: UIView {

    override class var layerClass: AnyClass { return CAGradientLayer.self }

    private var gradientLayer: CAGradientLayer { return layer as! CAGradientLayer }

    @IBInspectable var color1: UIColor = .white { didSet { updateColors() } }
    @IBInspectable var color2: UIColor = .blue  { didSet { updateColors() } }

    override init(frame: CGRect = .zero) {
        super.init(frame: frame)
        configureGradient()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        configureGradient()
    }

    private func configureGradient() {
        gradientLayer.startPoint = CGPoint(x: 0, y: 0)
        gradientLayer.endPoint = CGPoint(x: 1, y: 1)
        updateColors()
    }

    private func updateColors() {
        gradientLayer.colors = [color1.cgColor, color2.cgColor]
    }

}

E.g.

Note, unrelated to the immediate issue:

• If you’re going to add the gradient as a sublayer, you want to update this sublayer’s frame in layoutSubviews so that as the view's bounds changes, so does the frame of the gradientLayer. But, better than that, override the layerClass of the view, and it will not only instantiate the CAGradientLayer for you, but you also enjoy dynamic adjustments of the gradient as the view’s size changes, notably handling animated changes more gracefully.

• Likewise, I set color1 and color2 such that they'll trigger an updating of the gradient, so that any changes in colors will be immediately reflected in the view.

• I made this @IBDesignable, so that if I drop this in its own framework, and then add the GradientView in IB, I'll see the effect rendered in IB.

For Swift 2 implementation, see previous revision of this answer.

回答2:

It looks like you forgot to set the startPoint on your CAGradientLayer(). The code below is the code you've provide, plus my addition.

import UIKit

class GradientView: UIView {

    let gradientLayer = CAGradientLayer()

    override func awakeFromNib() {
        // 1
        self.backgroundColor = ColorPalette.White

        // 2
        gradientLayer.frame = self.bounds

        // 3
        let color1 = ColorPalette.GrdTop.CGColor as CGColorRef
        let color2 = ColorPalette.GrdBottom.CGColor as CGColorRef
        gradientLayer.colors = [color1, color2]

        //** This code should do the trick... **//
        gradientLayer.startPoint = CGPointMake(0.0, 0.5)

        // 4
        gradientLayer.locations = [0.0, 1.0]

        // 5
        self.layer.addSublayer(gradientLayer)
    }
}

回答3:

Gradient Start & End Points for Any Angle

Swift 4.2, Xcode 10.0

Given any angle, my code will set the respective start point and end point of a gradient layer.

If an angle greater than 360° is inputted, it will use the remainder when divided by 360.

• An input of 415° would yield the same result as an input of 55°

If an angle less than 0° is inputted, it will reverse the clockwise direction of rotation

• An input of -15° would yield the same result as an input of 345°

---

Code:

public extension CAGradientLayer {

    /// Sets the start and end points on a gradient layer for a given angle.
    ///
    /// - Important:
    /// *0°* is a horizontal gradient from left to right.
    ///
    /// With a positive input, the rotational direction is clockwise.
    ///
    ///    * An input of *400°* will have the same output as an input of *40°*
    ///
    /// With a negative input, the rotational direction is clockwise.
    ///
    ///    * An input of *-15°* will have the same output as *345°*
    ///
    /// - Parameters:
    ///     - angle: The angle of the gradient.
    ///                  
    public func calculatePoints(for angle: CGFloat) {


        var ang = (-angle).truncatingRemainder(dividingBy: 360)

        if ang < 0 { ang = 360 + ang }

        let n: CGFloat = 0.5

        switch ang {

        case 0...45, 315...360:
            let a = CGPoint(x: 0, y: n * tanx(ang) + n)
            let b = CGPoint(x: 1, y: n * tanx(-ang) + n)
            startPoint = a
            endPoint = b

        case 45...135:
            let a = CGPoint(x: n * tanx(ang - 90) + n, y: 1)
            let b = CGPoint(x: n * tanx(-ang - 90) + n, y: 0)
            startPoint = a
            endPoint = b

        case 135...225:
            let a = CGPoint(x: 1, y: n * tanx(-ang) + n)
            let b = CGPoint(x: 0, y: n * tanx(ang) + n)
           startPoint = a
            endPoint = b

        case 225...315:
            let a = CGPoint(x: n * tanx(-ang - 90) + n, y: 0)
            let b = CGPoint(x: n * tanx(ang - 90) + n, y: 1)
            startPoint = a
            endPoint = b

        default:
            let a = CGPoint(x: 0, y: n)
            let b = CGPoint(x: 1, y: n)
            startPoint = a
            endPoint = b

        }
    }

    /// Private function to aid with the math when calculating the gradient angle
    private func tanx(_