關于微信小程序canvas的使用

canvas簡單理解成一張可以自定義大小(有上限)的畫布，在畫布上可以寫字，畫圖形，放置圖片。一般可以用來動態生成截圖，方便用戶保存和轉發。

page.wxml

頁面canvas被隱藏了

<!--畫出試卷內容，通過css（position:fixed; left:100%;）隱藏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

通過 onCanvas 事件開始繪制畫布內容

每繪制完成一頁，通過 wx.canvasToTempFilePath 保存圖片到本地

全部繪制完成，通過 wx.previewImage 進行圖片預覽

  // 初始化canvas
  onCanvas: function (e) {
    // 繪制完成直接顯示
    if(this.data.drawFinish) {
      this.previewPapers()
      return
    }
    wx.showLoading({
      title: '試卷轉圖片中',
    })
    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)
        // 繪制記憶卷
        this.drawMemoryPaperImage(canvas, ctx)
      })
  },
  // 繪制記憶卷
  drawMemoryPaperImage(canvas, ctx) {
    // 獲取記憶卷數據
    let memoryList = this.data.memoryList
    // 繪制一頁記憶卷(繪制第一頁)
    let oneMemoryPaper = memoryList[0]
    let currentIndex = 0 // 0-表示繪制保存第一頁數據
    let totalPages = memoryList.length
    this.drawOneMemoryPaperImage(canvas, ctx, oneMemoryPaper, currentIndex, totalPages)
  },
  // 繪制一頁記憶卷
  drawOneMemoryPaperImage(canvas, ctx, oneMemoryPaper, currentIndex, totalPages) {
    let maxWidth = 750    
    let pg = oneMemoryPaper
    // 繪制前清空canvas
    ctx.clearRect(0, 0, canvas.width, canvas.height)
    // 繪制標題
    ctx.font = '18px sans-serif'
    ctx.textAlign = 'center';
    ctx.fillText('快速數字記憶卷', 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
        // 繪制數字
        ctx.font = '14px sans-serif'
        ctx.textAlign = 'left';
        ctx.fillText(text, x, y, maxWidth )
        // 繪制行號
        if(k==39) {
          let rowNo = 'Row ' + (j<9?'0'+(j+1):(j+1))           
          ctx.fillText(rowNo, x+25, y, maxWidth )
        }
      }
    }
    // 繪制頁碼
    ctx.font = '18px sans-serif'
    ctx.textAlign = 'center';
    ctx.fillText('第 '+(currentIndex+1)+' 頁', 375, 1050, maxWidth )
    // 保存試卷到小程序臨時目錄
    let paper = 'paper.memoryPaper['+currentIndex+']'
    this.saveToTempFilePath(canvas, ctx, paper, currentIndex, totalPages)
  },
  
  // 保存繪制的試卷到小程序臨時目錄
  saveToTempFilePath(canvas, ctx, paper, currentIndex, totalPages) {
    var that = this
    wx.canvasToTempFilePath({
      canvas: canvas,
      success(res) {
        that.setData({ 
          [paper]: res.tempFilePath,
        })
        // 繪制下一頁數據
        let nextIndex = currentIndex + 1
        if(nextIndex < totalPages) {
          that.drawNextOneMemoryPaperImage(canvas, ctx, nextIndex, totalPages)
        }else {
          // 記憶卷全部繪制完成
          that.setData({ drawFinish:true })
          wx.hideLoading({
            success: (res) => {
              console.log('記憶卷全部繪制完成')
            },
          })
          that.previewPapers()
        }
      },
      fail(err) {
        console.log(err)
      }
    })
  },
  // 繪制下一頁數據
  drawNextOneMemoryPaperImage(canvas, ctx, nextIndex, totalPages) {
    // 獲取記憶卷數據
    let memoryList = this.data.memoryList
    let currentIndex = nextIndex
    let oneMemoryPaper = memoryList[nextIndex]
    this.drawOneMemoryPaperImage(canvas, ctx, oneMemoryPaper, currentIndex, totalPages)    
  },
  // 預覽試卷
  previewPapers() {
    let paper = this.data.paper
    wx.previewImage({
      current: paper.memoryPaper[0],
      urls: paper.memoryPaper
    })
  },