let that = this
const query = wx.createSelectorQuery()
query.select('#myCanvas').fields({
	node: true,
	size: true
}).exec((res) => {			
	const canvas = res[0].node
	const ctx = canvas.getContext('2d')
	const headerImg = canvas.createImage();
	const backImg = canvas.createImage();
	headerImg.src = that.filePath;
	backImg.src = '../../static/wish.png';
	backImg.onload = () => {
		ctx.drawImage(backImg, 0, 0, 150, 100)
	}	
	headerImg.onload = () => {
		ctx.drawImage(headerImg, 0, 0, 150, 100)
	}											
})

// 创建离屏 2D canvas 实例
const canvas = wx.createOffscreenCanvas({
type: '2d',
width: 300,
height: 150
})
// 获取 context。注意这里必须要与创建时的 type 一致
const context = canvas.getContext('2d')
// 创建一个图片
const image = canvas.createImage()
// 等待图片加载

image.src = '../../static/wish.png' // 要加载的图片 url
image.onload = () => {
// 把图片画到离屏 canvas 上
context.clearRect(0, 0, 300, 150)
context.drawImage(image, 0, 0, 300, 150)

// 获取画完后的数据
const imgData = context.getImageData(0, 0, 300, 150)
console.log(imgData)
// 图片的base64格式
console.log(context.canvas.toDataURL())
that.src = context.canvas.toDataURL()
}