当渐变重新启动时，如何在快速使用图像时修复文本上的渐变

Question

我正在尝试在文本上创建渐变，我使用 UIGraphics 来使用渐变图像来创建它。我遇到的问题是渐变正在重新启动。有谁知道我如何缩放渐变以拉伸到文本？

文本位于线框上，将被更改几次。有时它会是完美的，但有时它不是。

渐变应该从黄色变为蓝色，但它会重新启动，见下图：

import UIKit

func colourTextWithGrad(label: UILabel) {
    UIGraphicsBeginImageContext(label.frame.size)
    UIImage(named: "testt.png")?.drawInRect(label.bounds)
    let myGradient: UIImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    label.textColor = UIColor(patternImage: myGradient)

}

Answer 1

每次标签大小发生变化时，您都必须重新绘制图像

这是因为有图案的UIColor 只会被平铺。来自the documentation：

在绘图期间，图案颜色的图像会根据需要平铺以覆盖给定区域。

因此，当标签的边界发生变化时，您需要自己更改图像大小——因为图案图像不支持拉伸。为此，您可以继承UILabel，并覆盖layoutSubviews 方法。这样的事情应该可以达到预期的效果：

class GradientLabel: UILabel {
    
    let gradientImage = UIImage(named:"gradient.png")
    
    override func layoutSubviews() {
        
        guard let grad = gradientImage else { // skip re-drawing gradient if it doesn't exist
            return
        }
        
        // redraw your gradient image
        UIGraphicsBeginImageContext(frame.size)
        grad.drawInRect(bounds)
        let myGradient = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        // update text color
        textColor = UIColor(patternImage: myGradient)
    }
}

---

虽然值得注意的是，我总是更喜欢自己绘制渐变 - 因为您可以拥有更大的灵活性（假设您想稍后添加另一种颜色）。此外，当您以不同的尺寸重新绘制图像时，图像的质量可能会下降（尽管由于渐变的性质，这应该是相当小的）。

您可以通过覆盖 UILabel 子类的 drawRect 来相当简单地绘制自己的渐变。例如：

override func drawRect(rect: CGRect) {
    
    // begin new image context to let the superclass draw the text in (so we can use it as a mask)
    UIGraphicsBeginImageContextWithOptions(bounds.size, false, 0.0)
    do {
        // get your image context
        let ctx = UIGraphicsGetCurrentContext()
        
        // flip context
        CGContextScaleCTM(ctx, 1, -1)
        CGContextTranslateCTM(ctx, 0, -bounds.size.height)
        
        // get the superclass to draw text
        super.drawRect(rect)
    }
    
    // get image and end context
    let img = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    
    // get drawRect context
    let ctx = UIGraphicsGetCurrentContext()
    
    // clip context to image
    CGContextClipToMask(ctx, bounds, img.CGImage)
    
    // define your colors and locations
    let colors = [UIColor.orangeColor().CGColor, UIColor.redColor().CGColor, UIColor.purpleColor().CGColor, UIColor.blueColor().CGColor]
    let locs:[CGFloat] = [0.0, 0.3, 0.6, 1.0]
    
    // create your gradient
    let grad = CGGradientCreateWithColors(CGColorSpaceCreateDeviceRGB(), colors, locs)
    
    // draw gradient
    CGContextDrawLinearGradient(ctx, grad, CGPoint(x: 0, y:bounds.size.height*0.5), CGPoint(x:bounds.size.width, y:bounds.size.height*0.5), CGGradientDrawingOptions(rawValue: 0))

}

输出：

---

Swift 4 & 作为子类

class GradientLabel: UILabel {
    
    // MARK: - Colors to create gradient from
    @IBInspectable open var gradientFrom: UIColor?
    @IBInspectable open var gradientTo: UIColor?
    
    override func draw(_ rect: CGRect) {
        // begin new image context to let the superclass draw the text in (so we can use it as a mask)
        UIGraphicsBeginImageContextWithOptions(bounds.size, false, 0.0)
        do {
            // get your image context
            guard let ctx = UIGraphicsGetCurrentContext() else { super.draw(rect); return }
            // flip context
            ctx.scaleBy(x: 1, y: -1)
            ctx.translateBy(x: 0, y: -bounds.size.height)
            // get the superclass to draw text
            super.draw(rect)
        }
        // get image and end context
        guard let img = UIGraphicsGetImageFromCurrentImageContext(), img.cgImage != nil else { return }
        UIGraphicsEndImageContext()
        // get drawRect context
        guard let ctx = UIGraphicsGetCurrentContext() else { return }
        // clip context to image
        ctx.clip(to: bounds, mask: img.cgImage!)
        // define your colors and locations
        let colors: [CGColor] = [UIColor.orange.cgColor, UIColor.red.cgColor, UIColor.purple.cgColor, UIColor.blue.cgColor]
        let locs: [CGFloat] = [0.0, 0.3, 0.6, 1.0]
        // create your gradient
        guard let grad = CGGradient(colorsSpace: CGColorSpaceCreateDeviceRGB(), colors: colors as CFArray, locations: locs) else { return }
        // draw gradient
        ctx.drawLinearGradient(grad, start: CGPoint(x: 0, y: bounds.size.height*0.5), end: CGPoint(x:bounds.size.width, y: bounds.size.height*0.5), options: CGGradientDrawingOptions(rawValue: 0))
    }
}