只是type="2d"的画布

一、把type、id、canvas-id通通加上，选择器加上in(app.proxy)，app.proxy相当于二代的this，记住！它全部都要。

1. TypeError: Cannot read property ‘node’ of null
2. Cannot read property ‘getContext’ of null

<canvas type="2d" id="myCanvas" canvas-id="myCanvas"></canvas>
const app = getCurrentInstance()
const query = uni.createSelectorQuery().in(app.proxy)
query.select('#myCanvas').fields({
			node: true,
			size: true
}).exec((res) => {
			const canvas = res[0].node
			const ctx = res[0].node.getContext('2d')
            ...
            // outCanvas.value = canvas
            // outCtx.value = ctx , 给外部响应式变量赋值的，请往下看
 })

二、尽管绘制文字等操作可以使用外部响应式变量outCtx.value.fillText()方法，但绘制图片ctx.drawImage()时，仍要获取画布实例和上下文对象，请交给里面的ctx干活。

3.TypeError: Cannot read property ‘createImage’ of undefined
4.TypeError: Cannot read property ‘nodeId’ of undefined

const drawImage = function(img, x, y, w, h) {
		return new Promise((resolve, reject) => {
			const query = uni.createSelectorQuery().in(app.proxy)
			query.select('#myCanvas').fields({
				node: true,
				size: true
			}).exec((res) => {
				const canvas = res[0].node
				const ctx = res[0].node.getContext('2d')
				const image = canvas.createImage()
				image.onload = () => {
					ctx.drawImage(image, x, y, w, h)
					resolve(true)
				}
				image.onerror = (err) => {
					resolve(false)
				}
				image.src = img
			})
		})
	}

三、你仍然不信邪，然后你尝试了以下方法，发现可以在开发者工具上显示图片，即将大功告成，但是我遇到了真机绘图静默失败的情况。要不，您再试试看。

const image = unref(outCanvas).createImage()
image.onload = ()=>{ ctx.value.drawImage(image,x,y,w,h)}
image.srcset = img 

四、准备变成一张漂亮的海报时，记住当初的山盟海誓。回过头来，还是里面的ctx靠谱。

5. canvasToTempFilePath: fail canvas is empty

wx.canvasToTempFilePath(Object object, Object this)

const query = uni.createSelectorQuery().in(app.proxy)
query.select('#myCanvas').fields({
			node: true,
			size: true
		}).exec((res) => {
			const canvas = res[0].node
			const ctx = res[0].node.getContext('2d')
                        uni.canvasToTempFilePath({
				canvas: canvas,
				fileType: 'png',
				destWidth: 500,
				destHeight: 500,
				quality: 1,
				success: (res) => {
					console.log(res)
					previewImgSrc.value = res.tempFilePath
					// uni.previewImage({
					// 	urls: [res.tempFilePath]
					// })
				},
				fail: (err) => {
					console.log(err)
				}
			}, app.proxy)
               })