如何以梯形（或圆形）绘制文本？

Question

假设我有一些文本：“梯形中的一些属性文本。”

我有 NSAttributedString 扩展，它返回带有属性文本的 UIImage：

extension NSAttributedString {
    func asImage() -> UIImage? {
        defer {
            UIGraphicsEndImageContext()
        }
        let size = boundingRect(with: CGSize.zero, options: [.usesLineFragmentOrigin, .truncatesLastVisibleLine], context: nil).size
        UIGraphicsBeginImageContext(size)
        draw(at: CGPoint.zero)
        return UIGraphicsGetImageFromCurrentImageContext()
    }
}

但是由于使用了boundingRect，这个函数在一行中返回了我的文本：

------------------------------------
|Some attributed text in trapezoid.|
------------------------------------

如果我要使用自定义矩形来绘制文本，那将无济于事...

UIGraphicsBeginImageContext(CGRect(x: 0, y: 0, width: 100, height: 30))
draw(at: CGPoint.zero)

...因为文本将在矩形中：

--------------
|Some attribu|
|ted text in |
|trapezoid.  |
--------------

我需要的是在具有已知角位置的梯形中（或在具有已知半径的圆中）绘制文本。所以每一行新的文本都应该以一个小的偏移量开始，见例子：

所以我想看到类似的东西：

---------------
\Some attribut/
 \ed text in /
  \trapezoid/
   ---------

我怎样才能达到这个结果？

Answer 1

您必须在此处下拉到 CoreText 级别。好消息是，您将能够以您想要的任何形状绘制文本！

extension NSAttributedString {
    public func draw(in path: CGPath) {
        let context = UIGraphicsGetCurrentContext()!

        let transform = CGAffineTransform(scaleX: +1, y: -1)

        let flippedPath = CGMutablePath()
        flippedPath.addPath(path, transform: transform)

        let range = CFRange(location: 0, length: 0)
        let framesetter = CTFramesetterCreateWithAttributedString(self)
        let frame = CTFramesetterCreateFrame(framesetter, range, flippedPath, nil)

        context.saveGState()

        // Debug: fill path.
        context.setFillColor(red: 1.0, green: 0.0, blue: 0.0, alpha: 0.5)
        context.beginPath()
        context.addPath(path)
        context.fillPath()

        context.concatenate(transform)

        CTFrameDraw(frame, context)

        context.restoreGState()
    }
}

你可以像这样使用它：

let string = NSAttributedString(string: "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Lorem ipsum dolor sit amet, consectetur adipiscing elit.")

let bounds = CGRect(x: 0, y: 0, width: 120, height: 120)

let path = CGMutablePath()
path.move(to: CGPoint(x: 10, y: 10))
path.addLine(to: CGPoint(x: 110, y: 10))
path.addLine(to: CGPoint(x: 90, y: 110))
path.addLine(to: CGPoint(x: 30, y: 110))
path.closeSubpath()

UIGraphicsBeginImageContextWithOptions(bounds.integral.size, true, 0)
defer { UIGraphicsEndImageContext() }

let context = UIGraphicsGetCurrentContext()!
context.setFillColor(UIColor.white.cgColor)
context.fill(.infinite)

string.draw(in: path)

let image = UIGraphicsGetImageFromCurrentImageContext()!

---

坏消息是，这个解决方案最后不会给你一个省略号。如果你真的想拥有它，你可能需要对 framesetter 给你的最后一行做一些调整。

Answer 2

与Christian Schnorr answer 一起为我的目的：

import PlaygroundSupport
import UIKit

extension NSAttributedString {

    /// Draws attributed string in selected CGPath.
    func draw(in path: CGPath) {
        guard let context = UIGraphicsGetCurrentContext() else { return }
        let transform = CGAffineTransform(scaleX: 1.0, y: -1.0)
        let flippedPath = CGMutablePath()
        flippedPath.addPath(path, transform: transform)
        let range = CFRange(location: 0, length: 0)
        let framesetter = CTFramesetterCreateWithAttributedString(self)
        let frame = CTFramesetterCreateFrame(framesetter, range, flippedPath, nil)
        context.saveGState()
        context.concatenate(transform)
        CTFrameDraw(frame, context)
        context.restoreGState()
    }

    /// Renders attributed string.
    ///
    /// - Parameters:
    ///   - size: A 'CGSize' for rendering string in trapezoid.
    ///   - degree: A `CGFloat`, representing trapezoid angles in degrees.
    /// - Returns: An optional `UIImage` with rendered string.
    func asTrapezoidImage(size: CGSize, degree: CGFloat) -> UIImage? {
        UIGraphicsBeginImageContextWithOptions(size, false, 0)
        defer { UIGraphicsEndImageContext() }
        draw(in: size.trapezoidPath(degree))
        return UIGraphicsGetImageFromCurrentImageContext()
    }
}

extension CGSize {

    /// Converts CGSize into trapezoid CGPath.
    ///
    /// - Parameter degree: A `CGFloat`, representing trapezoid angles in degrees.
    /// - Returns: A `CGPath` with trapezoid.
    func trapezoidPath(_ degree: CGFloat) -> CGPath {
        var offset = height * tan(CGFloat.pi * degree / 180.0)
        offset = max(0, min(width / 2.0, offset))
        let path = CGMutablePath()
        path.move(to: CGPoint.zero)
        path.addLine(to: CGPoint(x: width, y: 0.0))
        path.addLine(to: CGPoint(x: width - offset, y: height))
        path.addLine(to: CGPoint(x: offset, y: height))
        path.closeSubpath()
        return path
    }
}

使用：

let string = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Lorem ipsum dolor sit amet, consectetur adipiscing elit."
let attributes: [NSAttributedStringKey: Any] = [
    .foregroundColor: UIColor.blue,
    .backgroundColor: UIColor.white,
    .font: UIFont.systemFont(ofSize: 24.0)
]
let attrString = NSAttributedString(string: string, attributes: attributes)
let size = CGSize(width: 400.0, height: 120.0)
let image = attrString.asTrapezoidImage(size: size, degree: 12.0)
let imageView = UIImageView(image: image)
imageView.frame = CGRect(origin: CGPoint.zero, size: size)
PlaygroundPage.current.needsIndefiniteExecution = false
PlaygroundPage.current.liveView = imageView

注意允许的度数从 0° 到 90°