一、Canvas 画布

canvas 组件提供了绘制界面，可以在之上进行任意绘制

功能描述
Canvas 画布。2.9.0 起支持一套新 Canvas 2D 接口（需指定 type 属性），同时支持同层渲染，原有接口不再维护。

二、Canvas 和Canvas 2D 区别

Canvas 2D 需指定 type 属性
Canvas 属性 canvas-id ,Canvas 2D改成 id
Canvas 标签默认宽度300px、高度150px
Canvas 2D（新接口）需要显式设置画布宽高，默认：300*150，最大：1365*1365

<canvas class="pie-chart" canvas-id="chartOrg"></canvas>
<canvas class="pie-chart" type="2d" id="chart2d"></canvas>

API的区别

Canvas	新版 Canvas 2D
canvasContext.draw()	canvasContext接口没有 draw 方法
canvasContext.setFillStyle(“#000000”)	canvasContext.fillStyle = “#000000”
canvasContext.setFontSize(16)	canvasContext.font = ‘16px PingFang SC’
canvasContext.setTextAlign(‘center’)	canvasContext.textAlign = “center”

提醒：上面API的区别是下面示例用到的，还有其他API的变化，感兴趣的读者自行查阅。

三、示例用到的uni API

3.1 uni.createCanvasContext(canvasId, componentInstance)

3.1.1 定义

创建 canvas 绘图上下文（指定 canvasId）。在自定义组件下，第二个参数传入组件实例this，以操作组件内组件

提醒: 需要指定 canvasId，该绘图上下文只作用于对应的

参数

参数	类型	说明
canvasId	String	画布标识，传入定义在的 canvas-id或id（支付宝小程序是id、其他平台是canvas-id）
componentInstance	Object	自定义组件实例 this ，表示在这个自定义组件下查找拥有 canvas-id 的，如果省略，则不在任何自定义组件内查找

返回值：CanvasContext

3.2 uni.getSystemInfoSync()

获取系统信息的同步接口。调用参数和返回值和调用 uni.getSystemInfo() 一样。

3.3 uni.createSelectorQuery()

提醒：

使用 uni.createSelectorQuery() 需要在生命周期 mounted 后进行调用。
默认需要使用到 selectorQuery.in 方法。

selectorQuery.in(component)
将选择器的选取范围更改为自定义组件 component 内，返回一个 SelectorQuery 对象实例。（初始时，选择器仅选取页面范围的节点，不会选取任何自定义组件中的节点）。

示例代码

const query = uni.createSelectorQuery().in(this);
query.select("#id").boundingClientRect((data) => {console.log("得到布局位置信息" + JSON.stringify(data));console.log("节点离页面顶部的距离为" + data.top);}).exec();

注意： 支付宝小程序不支持 in(component)，使用无效果

四、微信小程序使用Canvas 和Canvas 2D绘制图形示例

示例效果图

testCanvas.vue代码

<template><view class="view-root-wrap"><view class="view-content-wrap"><view class="view-content"><text>使用canvas绘制</text><text>使用canvas 2D 绘制</text></view><view class="view-content"><canvas class="pie-chart" canvas-id="chartOrg"></canvas><canvas class="pie-chart"  type="2d" id="chart2d"></canvas></view></view></view>
</template>
<script>
export default {data() {return {isReady: false,}},onReady() {this.isReady = true// this.drawChartOrg()// this.startDrawChart2D()},onLoad(options) {this.drawChartOrg()this.startDrawChart2D()},methods: {// 绘制饼图drawChartOrg() {console.log(`执行了 drawChartOrg 方法  this.isReady = ${this.isReady}`)const canvasContext = uni.createCanvasContext('chartOrg', this)const centerX = 70 // 圆心x坐标const centerY = 70 // 圆心y坐标const innerRadius = 46;// 内圆半径const outerRadius = 70; // 外圆半径let chartData = [{ value: 3, color: '#7539f5', name: 'Fortran' },{ value: 4, color: '#91041c', name: 'Delphi/Object Pascal' },{ value: 4.5, color: '#0582a9', name: 'Visual Basic' },{ value: 5.5, color: '#39c8f5', name: 'Go' },{ value: 6, color: '#f2d407', name: 'JavaScript' },{ value: 7, color: '#480479', name: 'C#' },{ value: 9, color: '#f29707', name: 'Java' },{ value: 11, color: '#5620f3', name: 'C' },{ value: 15, color: '#0676c2', name: 'C++' },{ value: 35, color: '#339AF0', name: 'Python' },];const totalPercent = 100let startAngle = 1.5 * Math.PI // 起始角度// 绘制外圆chartData.forEach(item => {const sliceAngle = (2 * Math.PI * item.value) / totalPercentconst endAngle = startAngle + sliceAnglecanvasContext.beginPath()canvasContext.moveTo(centerX, centerY)canvasContext.arc(centerX, centerY, outerRadius, startAngle, endAngle)canvasContext.closePath()canvasContext.setFillStyle(item.color)canvasContext.fill()startAngle = endAngle})// 绘制内圆canvasContext.beginPath()canvasContext.arc(centerX, centerY, innerRadius, 0, 2 * Math.PI)canvasContext.setFillStyle('#FFFFFF')canvasContext.fill()// 添加中间文字canvasContext.setFontSize(16)canvasContext.setFillStyle('#000000')canvasContext.setTextAlign('center')canvasContext.fillText("编程语言", centerX, centerY + 8)canvasContext.draw()},startDrawChart2D() {console.log(`执行了 startDrawChart 方法  this.isReady = ${this.isReady}`)let delayTime = 500;if (this.isReady) {delayTime = 150}setTimeout(() => {this.drawChart2D()}, delayTime);},// 使用 Canvas2D 绘制饼图drawChart2D() {const query = uni.createSelectorQuery().in(this)query.select('#chart2d').fields({ node: true, size: true }).exec((res) => {try {let { node, width, height } = res[0];const canvas = nodeconst canvasContext = canvas.getContext('2d')const dpr = uni.getSystemInfoSync().devicePixelRatioconst tempWidth = width * dprcanvas.width = tempWidthcanvas.height = tempWidthcanvasContext.scale(dpr, dpr)const radius = width / 2const centerX = radius  // 圆心x坐标const centerY = radius // 圆心y坐标const outerRadius = radius; // 外圆半径const innerRadius = outerRadius * 0.65; // 内圆半径let chartData = [{ value: 3, color: '#7539f5', name: 'Fortran' },{ value: 4, color: '#91041c', name: 'Delphi/Object Pascal' },{ value: 4.5, color: '#0582a9', name: 'Visual Basic' },{ value: 5.5, color: '#39c8f5', name: 'Go' },{ value: 6, color: '#f2d407', name: 'JavaScript' },{ value: 7, color: '#480479', name: 'C#' },{ value: 9, color: '#f29707', name: 'Java' },{ value: 11, color: '#5620f3', name: 'C' },{ value: 15, color: '#0676c2', name: 'C++' },{ value: 35, color: '#339AF0', name: 'Python' },];const totalPercent = 100let startAngle = 1.5 * Math.PI // 起始角度// 绘制外圆chartData.forEach(item => {const sliceAngle = (2 * Math.PI * item.value) / totalPercentconst endAngle = startAngle + sliceAnglecanvasContext.beginPath()canvasContext.moveTo(centerX, centerY)canvasContext.arc(centerX, centerY, outerRadius, startAngle, endAngle)canvasContext.closePath()canvasContext.fillStyle = item.colorcanvasContext.fill()startAngle = endAngle})// 绘制内圆canvasContext.beginPath()canvasContext.arc(centerX, centerY, innerRadius, 0, 2 * Math.PI)canvasContext.fillStyle = "#FFFFFF"canvasContext.fill()// 添加中间文字canvasContext.font = '16px PingFang SC'canvasContext.fillStyle = "#000000"canvasContext.textAlign = "center"canvasContext.fillText("编程语言", centerX, centerY + 8)} catch (error) {console.log("drawChart2D 绘制发生异常: " + error)}})},}
}
</script>
<style scoped>
.view-root-wrap {padding: 12px;
}.view-content-wrap {display: flex;flex-direction: column;justify-content: center;background-color: white;padding: 28rpx;border-radius: 16rpx;
}.view-content {display: flex;flex-direction: row;justify-content: space-around;margin-top: 32rpx;
}.pie-chart {width: 140px;height: 140px;
}
</style>