ios - 径向 CAGradientLayer 在运行时无法正确渲染

Question

我正在尝试快速创建一个径向 CAGradientLayer。普通 CAGradientLayers(默认类型 axial)没有问题。它们在模拟器中的快照和运行时呈现良好。

当我拍摄我创建的 UIView 的快照时(仅针对 CAGradientLayer)，它显示了一个漂亮的径向 CAGradientLayer。但是，当我使用这个 View 并在运行时在模拟器中查看它时，它看起来很糟糕。我不知道我做错了什么。

这是 CAGradientLayer 的代码:

import Foundation

class ChannelGradientView: UIView {

  // MARK: - Init

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

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

  // MARK: - Setup

  private func setup() {
    backgroundColor = .clear
    setupGradient()
  }

  private func setupGradient() {
    let gradient = layer as? CAGradientLayer
    gradient?.type = .radial

    gradient?.colors = [
      UIColor.black.withAlphaComponent(0.8),
      UIColor.black.withAlphaComponent(0)
    ].map { $0.cgColor }

    let blackPoint = CGPoint(x: 1, y: 0)
    let clearPoint = CGPoint(x: 1, y: 1)

    // startpoint is center (for example black here)
    gradient?.startPoint = blackPoint
    gradient?.endPoint = clearPoint
  }

  // MARK: - Layer

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

这是我进行快照单元测试时的样子(使用 Nimble Snapshot 库):

这是它在模拟器上运行时的样子:

有人知道我做错了什么吗？

编辑:在实际设备上运行时，它甚至不显示任何渐变层，即使是糟糕的层也不行。这是一个透明的盒子。

Answer 1

不确定您在模拟器和设备上看到的内容之间还发生了什么，但是...

如果我按原样使用您的代码，我会得到这个(红色边框显示框架):

如果我改变你的clearPoint:

//let clearPoint = CGPoint(x: 1, y: 1)
let clearPoint = CGPoint(x: 0, y: 1)    // bottom-left-corner

我明白了:

模拟器和设备上的外观相同

---

编辑

一些额外的说明...

径向渐变不像.axial(线性)渐变那样使用.startPoint 和.endPoint。

用.radial绘制一个渐变*椭圆，以.startPoint为中心，startPoint的区别.x 和 endPoint.x 乘以 2 作为其宽度，startPoint.y 和 endPoint.y 之间的差异乘以 2 作为它的高度。

因此，要获得所需的从右上角到左下角的径向渐变，您需要将 .startPoint 设置为 1,0 和 。 endPoint 到导致大小为 2 x 2 的渐变椭圆的值:

startPoint = 1,0

endPoint = 0,1

    width:  abs(1 - 0) * 2 = 2
    height: abs(0 - 1) * 2 = 2

请注意，您可以通过以下方式获得相同的结果:

startPoint = 1,0

endPoint = 2,-1

    width:  abs(1 - 2) * 2 = 2
    height: abs(0 - (-1)) * 2 = 2

.radial 渐变 不 喜欢的是将其宽度或高度设置为零，这正是我们得到的:

startPoint = 1,0

endPoint = 1,1

    width:  abs(1 - 1) * 2 = 0   // problem!!!!
    height: abs(0 - 1) * 2 = 2

如我们所见，结果是奇怪的线条图案。

这里有一些实际的例子来演示。

轴向/线性右上到左下:

径向居中且宽度= View 宽度/高度= View 高度:

径向居中且宽度=二分之一 View 宽度/高度= View 高度:

径向居中和宽度 = 二分之一 View 宽度/高度 = View 高度...完全相同的结果，但请注意 .endPoint.x 值为 0.75而不是 0.25:

现在我们改变 .startPoint.x = 0.75，但是我们保留 .endPoint.x = 0.25，所以椭圆的中心移动到 3/4ths View 的宽度，但椭圆的宽度变得等于 View 的宽度... abs(0.75 - 0.25) * 2 == 1.0:

更改 .endPoint.x = 0.5 并且宽度返回到 View 宽度的 1/2... abs(0.75 - 0.5) * 2 = 0.5 :

最后是从右上角到左下角的径向渐变:

这是我用来生成这些图像的代码。它有一个“梯度定义”的数据 block ......您可以添加/更改这些定义以试验差异。

//
//  GradTestViewController.swift
//
//  Created by Don Mag on 9/12/19.
//

import UIKit

class TestGradientView: UIView {

    // MARK: - Init
    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }

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

    // MARK: - Setup
    private func setup() {

        // just set the background to clear and the
        //  gradient colors to blue -> green
        backgroundColor = .clear

        if let gradient = layer as? CAGradientLayer {
            gradient.colors = [
                UIColor.blue,
                UIColor.green,
                ].map { $0.cgColor }
        }

    }

    // MARK: - Layer
    override public class var layerClass: Swift.AnyClass {
        return CAGradientLayer.self
    }

}

struct GradDef {
    var gradType: CAGradientLayerType = .axial
    var startPoint: CGPoint = CGPoint(x: 1.0, y: 0.0)
    var endPoint: CGPoint = CGPoint(x: 0.0, y: 1.0)
}

class GradTestViewController: UIViewController {

    var theButton: UIButton = {
        let v = UIButton()
        v.setTitle("Tap", for: .normal)
        v.setTitleColor(.white, for: .normal)
        v.setTitleColor(.lightGray, for: .highlighted)
        v.backgroundColor = .red
        return v
    }()

    var counterLabel: UILabel = {
        let v = UILabel()
        return v
    }()

    var descLabel: UILabel = {
        let v = UILabel()
        v.numberOfLines = 0
        return v
    }()

    var gradContainerView: UIView = {
        let v = UIView()
        return v
    }()

    var gradView: TestGradientView = {
        let v = TestGradientView()
        return v
    }()

    var tlLabel: UILabel = {
        let v = UILabel()
        v.text = "0,0"
        return v
    }()

    var trLabel: UILabel = {
        let v = UILabel()
        v.text = "1,0"
        return v
    }()

    var blLabel: UILabel = {
        let v = UILabel()
        v.text = "0,1"
        return v
    }()

    var brLabel: UILabel = {
        let v = UILabel()
        v.text = "1,1"
        return v
    }()

    var theStackView: UIStackView = {
        let v = UIStackView()
        v.axis = .vertical
        v.alignment = .center
        v.distribution = .fill
        v.spacing = 20.0
        return v
    }()

    var gradDefs: [GradDef] = [
        GradDef(gradType: .axial,  startPoint: CGPoint(x: 1.0,  y: 0.0), endPoint: CGPoint(x: 0.0,  y: 1.0)),
        GradDef(gradType: .radial, startPoint: CGPoint(x: 0.5,  y: 0.5), endPoint: CGPoint(x: 0.0,  y: 0.0)),
        GradDef(gradType: .radial, startPoint: CGPoint(x: 0.5,  y: 0.5), endPoint: CGPoint(x: 0.25, y: 0.0)),
        GradDef(gradType: .radial, startPoint: CGPoint(x: 0.5,  y: 0.5), endPoint: CGPoint(x: 0.75, y: 0.0)),
        GradDef(gradType: .radial, startPoint: CGPoint(x: 0.75, y: 0.5), endPoint: CGPoint(x: 0.25, y: 0.0)),
        GradDef(gradType: .radial, startPoint: CGPoint(x: 0.75, y: 0.5), endPoint: CGPoint(x: 1.0,  y: 0.0)),
        GradDef(gradType: .radial, startPoint: CGPoint(x: 1.0,  y: 0.0), endPoint: CGPoint(x: 0.0,  y: 1.0)),
    ]

    var idx: Int = 0

    override func viewDidLoad() {
        super.viewDidLoad()

        [theButton, counterLabel, gradContainerView, gradView, tlLabel, trLabel, blLabel, brLabel, descLabel, theStackView].forEach {
            $0.translatesAutoresizingMaskIntoConstraints = false
        }

        [theButton, counterLabel, gradContainerView, descLabel].forEach {
            theStackView.addArrangedSubview($0)
        }

        [gradView, tlLabel, trLabel, blLabel, brLabel].forEach {
            gradContainerView.addSubview($0)
        }

        [counterLabel, tlLabel, trLabel, blLabel, brLabel, descLabel].forEach {
            $0.font = UIFont.monospacedDigitSystemFont(ofSize: 14.0, weight: .regular)
        }

        NSLayoutConstraint.activate([

            gradView.widthAnchor.constraint(equalToConstant: 120),
            gradView.heightAnchor.constraint(equalTo: gradView.widthAnchor),
            gradView.centerXAnchor.constraint(equalTo: gradContainerView.centerXAnchor),
            gradView.centerYAnchor.constraint(equalTo: gradContainerView.centerYAnchor),

            tlLabel.centerXAnchor.constraint(equalTo: gradView.leadingAnchor, constant: 0.0),
            blLabel.centerXAnchor.constraint(equalTo: gradView.leadingAnchor, constant: 0.0),
            trLabel.centerXAnchor.constraint(equalTo: gradView.trailingAnchor, constant: 0.0),
            brLabel.centerXAnchor.constraint(equalTo: gradView.trailingAnchor, constant: 0.0),

            tlLabel.bottomAnchor.constraint(equalTo: gradView.topAnchor, constant: -2.0),
            trLabel.bottomAnchor.constraint(equalTo: gradView.topAnchor, constant: -2.0),

            blLabel.topAnchor.constraint(equalTo: gradView.bottomAnchor, constant: 2.0),
            brLabel.topAnchor.constraint(equalTo: gradView.bottomAnchor, constant: 2.0),

            tlLabel.topAnchor.constraint(equalTo: gradContainerView.topAnchor, constant: 4.0),
            tlLabel.leadingAnchor.constraint(equalTo: gradContainerView.leadingAnchor, constant: 4.0),

            brLabel.trailingAnchor.constraint(equalTo: gradContainerView.trailingAnchor, constant: -4.0),
            brLabel.bottomAnchor.constraint(equalTo: gradContainerView.bottomAnchor, constant: -4.0),

            ])

        view.addSubview(theStackView)

        NSLayoutConstraint.activate([

            theStackView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 40.0),
            theStackView.centerXAnchor.constraint(equalTo: view.centerXAnchor),

            theButton.widthAnchor.constraint(equalToConstant: 160.0),
            descLabel.widthAnchor.constraint(equalToConstant: 240.0),

            ])

        theButton.addTarget(self, action: #selector(didTap(_:)), for: .touchUpInside)

        idx = -1
        didTap(nil)
    }

    @IBAction func didTap(_ sender: Any?) {

        guard let gLayer = gradView.layer as? CAGradientLayer else {
            fatalError("Could not get the gradient layer!")
        }

        idx += 1

        if idx >= gradDefs.count {
            idx = 0
        }

        let gDef = gradDefs[idx]

        gLayer.type = gDef.gradType
        gLayer.startPoint = gDef.startPoint
        gLayer.endPoint = gDef.endPoint

        var s = ""
        s += "Gradient Type: " + (gDef.gradType == CAGradientLayerType.axial ? "Axial" : "Radial")
        s += "\n\n"
        s += "Start Point: \(gDef.startPoint)"
        s += "\n"
        s += "End Point:   \(gDef.endPoint)"

        if gDef.gradType == CAGradientLayerType.radial {
            let w = abs(gDef.startPoint.x - gDef.endPoint.x) * 2
            let h = abs(gDef.startPoint.y - gDef.endPoint.y) * 2
            s += "\n\n"
            s += "\t" + "Radial Width:"
            s += "\n"
            s += "\t\t" + "abs(\(gDef.startPoint.x) - \(gDef.endPoint.x)) * 2 == \(w)"
            s += "\n\n"
            s += "\t" + "Radial Height:"
            s += "\n"
            s += "\t\t" + "abs(\(gDef.startPoint.y) - \(gDef.endPoint.y)) * 2 == \(h)"
        }

        s += "\n"

        descLabel.text = s

        counterLabel.text = "Variation \(idx + 1) of \(gradDefs.count)"

    }

}

一切都在代码中完成 - 没有 @IBOutlets 或 @IBActions，所以只需创建一个新的 View Controller 并将其自定义类分配给 GradTestViewController .