HTML5 Canvas实现自动换行和竖排文本

浅浅的花香味﹌ 2022-05-24 13:07 433阅读 0赞

Canvas的渣渣API不支持自动换行和竖排，找了一下没有满意的解决方案，只好自己写一个了。

## 方法1：根据文字尺寸计算在哪换行 ##

// 画个框用来调试
    const DEBUG = true
    
    // 画文本，支持多行、自动换行、竖排文字
    function drawText (ctx, text, x, y, width, height, hasStroke = false, isVertical = false) { 
      if (DEBUG) {
        let [oldLineWidth, oldStrokeStyle] = [ctx.lineWidth, ctx.strokeStyle];
        [ctx.lineWidth, ctx.strokeStyle] = [1, 'red']
        ctx.strokeRect(x, y, width, height);
        [ctx.lineWidth, ctx.strokeStyle] = [oldLineWidth, oldStrokeStyle]
      }
    
      if (!isVertical) {
        drawTextHorizontal(ctx, text, x, y, width, height, hasStroke)
      } else {
        drawTextVertical(ctx, text, x, y, width, height, hasStroke)
      }
    }
    
    // 画横排文本，垂直居中，可以垂直溢出
    function drawTextHorizontal (ctx, text, x, y, width, height, hasStroke = false) { 
      let oldBaseLine = ctx.textBaseline
      ctx.textBaseline = 'hanging'
      let lineHeight = parseInt(ctx.font) // ctx.font必须以'XXpx'开头
    
      // 计算每一行
      let lines = []
      let curLine = ''
      for (let char of text) {
        let nextLine = curLine + char
        if (char === '\n' || ctx.measureText(nextLine).width > width) {
          lines.push(curLine)
          curLine = char === '\n' ? '' : char
        } else {
          curLine = nextLine
        }
      }
      lines.push(curLine)
    
      // 逐行画文本
      let lineY = y + (height - lineHeight * lines.length) / 2
      for (let line of lines) {
        let lineX
        if (ctx.textAlign === 'center') {
          lineX = x + width / 2
        } else if (ctx.textAlign === 'right') {
          lineX = x + width
        } else {
          lineX = x
        }
        if (hasStroke) {
          ctx.strokeText(line, lineX, lineY, width)
        }
        ctx.fillText(line, lineX, lineY, width)
        lineY += lineHeight
      }
    
      ctx.textBaseline = oldBaseLine
    }
    
    // 画竖排文本，从右到左，水平居中，可以水平溢出
    function drawTextVertical (ctx, text, x, y, width, height, hasStroke = false) { 
      let [oldAlign, oldBaseLine] = [ctx.textAlign, ctx.textBaseline];
      [ctx.textAlign, ctx.textBaseline] = ['center', 'middle']
      let lineWidth = parseInt(ctx.font) // ctx.font必须以'XXpx'开头
    
      // 计算每个字符的尺寸信息
      let charInfo = []
      for (let char of text) {
        let cInfo = {
          char: char,
          needsRotation: needsRotation(char) // 中日韩文字不用旋转
        }
        if (cInfo.needsRotation) {
          [cInfo.width, cInfo.height] = [lineWidth, ctx.measureText(char).width]
        } else {
          [cInfo.width, cInfo.height] = [ctx.measureText(char).width, lineWidth]
        }
        charInfo.push(cInfo)
      }
    
      // 计算每一列
      let lineInfo = []
      let curLine = []
      let curLineHeight = 0
      for (let info of charInfo) {
        if (info.char === '\n' || curLineHeight + info.height > height) {
          lineInfo.push({
            charInfo: curLine,
            height: curLineHeight
          })
          curLine = info.char === '\n' ? [] : [info]
          curLineHeight = info.height
        } else {
          curLine.push(info)
          curLineHeight += info.height
        }
      }
      lineInfo.push({
        charInfo: curLine,
        height: curLineHeight
      })
    
      // 逐字画文本
      let lineX = x + (width + lineWidth * lineInfo.length) / 2 - lineWidth / 2 // 列中心的坐标
      for (let lInfo of lineInfo) {
        let charY // 字符顶端的坐标
        if (oldAlign === 'center') {
          charY = y + (height - lInfo.height) / 2
        } else if (oldAlign === 'right') { // 这里右对齐视为底端对齐，左对齐视为顶端对齐
          charY = y + height - lInfo.height
        } else {
          charY = y
        }
    
        // 画一列文本
        for (let cInfo of lInfo.charInfo) {
          ctx.translate(lineX, charY + cInfo.height / 2)
          if (cInfo.needsRotation) {
            ctx.rotate(90 * Math.PI / 180)
          }
          // 画一个字符
          if (hasStroke) {
            ctx.strokeText(cInfo.char, 0, 0)
          }
          ctx.fillText(cInfo.char, 0, 0)
          ctx.setTransform(1, 0, 0, 1, 0, 0)
          charY += cInfo.height
        }
        lineX -= lineWidth
      }
    
      [ctx.textAlign, ctx.textBaseline] = [oldAlign, oldBaseLine]
    }
    
    // 需要旋转的Unicode码范围，基本上是CJK文字
    const NO_ROTATION_RANGE = [
      [0x2E80, 0x2FEF],
      [0x3040, 0x9FFF],
      [0xAC00, 0xD7FF],
      [0xF900, 0xFAFF],
      [0x1D300, 0x1D35F],
      [0x20000, 0x2FA1F]
    ]
    
    function needsRotation (char) { 
      let codePoint = char.codePointAt(0)
      for (let [lowerBound, upperBound] of NO_ROTATION_RANGE) {
        if (lowerBound <= codePoint && codePoint <= upperBound) {
          return false
        }
      }
      return true
    }

### 效果 ###

![横排][70]

![竖排][70 1]

## 方法2：利用SVG图片的foreignObject嵌入CSS排版 ##

这个方法不知道为什么竖排文本每列只有一个字，所以我就不用了，CSS真是玄学…

async function drawText (ctx, text, x, y, width, height, hasStroke = false, isVertical = false) { 
      let div = document.createElement('div')
      div.innerText = text
      let parentSize = DEBUG ? 'calc(100% - 2px)' : '100%'
      div.style = `
        word-wrap: break-word;
        word-break: break-all;
    
        font: ${ctx.font};
    
        text-align: ${isVertical ? 'left' : ctx.textAlign};
        ${isVertical ? 'height' : 'width'}: ${parentSize};
        position: relative;
        ${isVertical ? `right: 50%; transform: translateX(50%);` : `top: 50%; transform: translateY(-50%);`}
        ${isVertical ? 'writing-mode: vertical-rl;' : ''}
        ${isVertical ? 'float: right;' : ''}
    
        color: ${ctx.fillStyle};
        ${hasStroke ? `text-shadow: 0 1px ${ctx.strokeStyle}, 1px 0 ${ctx.strokeStyle}, -1px 0 ${ctx.strokeStyle}, 0 -1px ${ctx.strokeStyle};` : ''}
    
        ${DEBUG ? 'border: 1px solid red;' : ''}
      `
      let bodyStyle = `
        margin: 0;
        overflow: hidden;
        width: ${parentSize};
        height: ${parentSize};
        ${DEBUG ? 'border: 1px solid blue;' : ''}
      `
      let svg = `
        <svg xmlns="http://www.w3.org/2000/svg">
          <foreignObject width="${width}px" height="${height}px"> <body xmlns="http://www.w3.org/1999/xhtml" style="${bodyStyle}"> ${div.outerHTML} </body> </foreignObject> </svg> ` if (DEBUG) { console.log(svg) } return new Promise((resolve, reject) => { let img = new window.Image() img.onload = () => { ctx.drawImage(img, x, y) resolve() } img.onerror = () => { reject(new Error(`Failed to load the image: ${img.src}`)) } img.src = `data:image/svg+xml;charset=utf-8,${svg}` }) }

### 效果 ###

这个我设置成了溢出部分隐藏

![横排][70 2]

![竖排][70 3]

[70]: /images/20220524/0ff559d473154d15acc14d2ef564d8b4.png
[70 1]: /images/20220524/612d629f40624b66bfa7406fb36925bf.png
[70 2]: /images/20220524/655a22e39dc549628880b5eb21205cd1.png
[70 3]: /images/20220524/d28f54bf8b004baba2c6d5e509878fa0.png