获取 canvas 及 canvas context

// wxml type="2d" 是必须的  

// js
wx.createSelectorQuery()
     .select('#myCanvas').node().exec((res) => {
       this.myCanvas = res[0].node
       this.myCanvasCtx = this.myCanvas.getContext('2d')
     })

canvas 转 base64

    let ctxImageData = this.myCanvasCtx.createImageData(width, height);
    ctxImageData.data.set(data)
    this.myCanvasCtx.putImageData(ctxImageData, 0, 0)
    let base64 = this.myCanvas.toDataURL("image/png", 0.7);