关于微信小程序canvas的使用

canvas简单理解成一张可以自定义大小(有上限)的画布，在画布上可以写字，画图形，放置图片。一般可以用来动态生成截图，方便用户保存和转发。

page.wxml

页面canvas被隐藏了

<!--画出试卷内容，通过css（position:fixed; left:100%;）隐藏canvas-->
<!--A4是2480*3508象素 210*297毫米-->
<canvas
  wx:if="{{page==4}}"
  type="2d"
  id="canvas"
  style="width:750rpx; height:1050rpx; position:fixed; left:100%;"
></canvas>

page.js

通过 onCanvas 事件开始绘制画布内容

每绘制完成一页，通过 wx.canvasToTempFilePath 保存图片到本地

全部绘制完成，通过 wx.previewImage 进行图片预览

  // 初始化canvas
  onCanvas: function (e) {
    // 绘制完成直接显示
    if(this.data.drawFinish) {
      this.previewPapers()
      return
    }
    wx.showLoading({
      title: '试卷转图片中',
    })
    const query = wx.createSelectorQuery()
    query.select('#canvas')
      .fields({ node: true, size: true })
      .exec((res) => {
        const canvas = res[0].node
        const ctx = canvas.getContext('2d')
        const dpr = wx.getSystemInfoSync().pixelRatio
        canvas.width = res[0].width * dpr
        canvas.height = res[0].height * dpr
        console.log('canvas.width, canvas.height', canvas.width, canvas.height)
        ctx.scale(dpr, dpr)
        // 绘制记忆卷
        this.drawMemoryPaperImage(canvas, ctx)
      })
  },
  // 绘制记忆卷
  drawMemoryPaperImage(canvas, ctx) {
    // 获取记忆卷数据
    let memoryList = this.data.memoryList
    // 绘制一页记忆卷(绘制第一页)
    let oneMemoryPaper = memoryList[0]
    let currentIndex = 0 // 0-表示绘制保存第一页数据
    let totalPages = memoryList.length
    this.drawOneMemoryPaperImage(canvas, ctx, oneMemoryPaper, currentIndex, totalPages)
  },
  // 绘制一页记忆卷
  drawOneMemoryPaperImage(canvas, ctx, oneMemoryPaper, currentIndex, totalPages) {
    let maxWidth = 750    
    let pg = oneMemoryPaper
    // 绘制前清空canvas
    ctx.clearRect(0, 0, canvas.width, canvas.height)
    // 绘制标题
    ctx.font = '18px sans-serif'
    ctx.textAlign = 'center';
    ctx.fillText('快速数字记忆卷', 375, 70, maxWidth )
    for(let j=0; j<pg.length; j++) {
      let row = pg[j]
      for(let k=0; k<row.length; k++) {
        let text = pg[j][k]
        let x = k * 17 + 40
        let y = j * 35 + 150
        // 绘制数字
        ctx.font = '14px sans-serif'
        ctx.textAlign = 'left';
        ctx.fillText(text, x, y, maxWidth )
        // 绘制行号
        if(k==39) {
          let rowNo = 'Row ' + (j<9?'0'+(j+1):(j+1))           
          ctx.fillText(rowNo, x+25, y, maxWidth )
        }
      }
    }
    // 绘制页码
    ctx.font = '18px sans-serif'
    ctx.textAlign = 'center';
    ctx.fillText('第 '+(currentIndex+1)+' 页', 375, 1050, maxWidth )
    // 保存试卷到小程序临时目录
    let paper = 'paper.memoryPaper['+currentIndex+']'
    this.saveToTempFilePath(canvas, ctx, paper, currentIndex, totalPages)
  },
  
  // 保存绘制的试卷到小程序临时目录
  saveToTempFilePath(canvas, ctx, paper, currentIndex, totalPages) {
    var that = this
    wx.canvasToTempFilePath({
      canvas: canvas,
      success(res) {
        that.setData({ 
          [paper]: res.tempFilePath,
        })
        // 绘制下一页数据
        let nextIndex = currentIndex + 1
        if(nextIndex < totalPages) {
          that.drawNextOneMemoryPaperImage(canvas, ctx, nextIndex, totalPages)
        }else {
          // 记忆卷全部绘制完成
          that.setData({ drawFinish:true })
          wx.hideLoading({
            success: (res) => {
              console.log('记忆卷全部绘制完成')
            },
          })
          that.previewPapers()
        }
      },
      fail(err) {
        console.log(err)
      }
    })
  },
  // 绘制下一页数据
  drawNextOneMemoryPaperImage(canvas, ctx, nextIndex, totalPages) {
    // 获取记忆卷数据
    let memoryList = this.data.memoryList
    let currentIndex = nextIndex
    let oneMemoryPaper = memoryList[nextIndex]
    this.drawOneMemoryPaperImage(canvas, ctx, oneMemoryPaper, currentIndex, totalPages)    
  },
  // 预览试卷
  previewPapers() {
    let paper = this.data.paper
    wx.previewImage({
      current: paper.memoryPaper[0],
      urls: paper.memoryPaper
    })
  },