Teach you step by step how to use canvas to draw a weather line chart in the WeChat applet (with code)

Component({ externalClasses: ['line-class'], properties: { width: String, height: String, data: Array, }, observers: { width() { // 这里监听 width 变化重绘 canvas // 动态传入 width 好像只能这样了.. const query = this.createSelectorQuery(); query .select('#line') .fields({ node: true, size: true }) .exec(res => { const canvas = res[0].node; const ctx = canvas.getContext('2d'); const width = res[0].width; // 画布宽度 const height = res[0].height; // 画布高度 console.log(`宽度: ${width}, 高度: ${height}`); const dpr = wx.getSystemInfoSync().pixelRatio; canvas.width = width * dpr; canvas.height = height * dpr; ctx.scale(dpr, dpr); // 开始绘图 this.drawLine(ctx, width, height, this.data.data); }); }, }, methods: { drawLine(ctx, width, height, data) { const Max = Math.max(...data); const Min = Math.min(...data); // 把 canvas 的宽度, 高度按一定规则平分 const startX = width / (data.length * 2), // 起始点的横坐标 X baseY = height * 0.9, // 基线纵坐标 Y diffX = width / data.length, diffY = (height * 0.7) / (Max - Min); // 高度预留 0.2 写温度 ctx.beginPath(); ctx.textAlign = 'center'; ctx.font = '13px Microsoft YaHei'; ctx.lineWidth = 2; ctx.strokeStyle = '#ABDCFF'; // 画折线图的线 data.forEach((item, index) => { const x = startX + diffX * index, y = baseY - (item - Min) * diffY; ctx.fillText(`${item}°`, x, y - 10); ctx.lineTo(x, y); }); ctx.stroke(); // 画折线图背景 ctx.lineTo(startX + (data.length - 1) * diffX, baseY); // 基线终点 ctx.lineTo(startX, baseY); // 基线起点 const lingrad = ctx.createLinearGradient(0, 0, 0, height * 0.7); lingrad.addColorStop(0, 'rgba(255,255,255,0.9)'); lingrad.addColorStop(1, 'rgba(171,220,255,0)'); ctx.fillStyle = lingrad; ctx.fill(); // 画折线图上的小圆点 ctx.beginPath(); data.forEach((item, index) => { const x = startX + diffX * index, y = baseY - (item - Min) * diffY; ctx.moveTo(x, y); ctx.arc(x, y, 3, 0, 2 * Math.PI); }); ctx.fillStyle = '#0396FF'; ctx.fill(); }, }, });

// 获取 scroll-view 的总宽度 wx.createSelectorQuery() .select('.hourly') .boundingClientRect(rect => { this.setData({ scrollWidth: rect.right - rect.left, }); }) .exec();

小时概述   {{item}}   

.scroll { position: relative; height: 150px; width: 100%; } .hourly { display: flex; height: 150px; position: absolute; top: 0; } .hourly > view { min-width: 3.5em; text-align: center; } .line { // 折线图绝对定位到底部 position: absolute; bottom: 0; }

class Point { constructor(x, y) { this.x = x; this.y = y; } }

/** * 计算当前点的贝塞尔曲线控制点 * @param {Point} previousPoint: 前一个点 * @param {Point} currentPoint: 当前点 * @param {Point} nextPoint1: 下一个点 * @param {Point} nextPoint2: 下下个点 * @param {Number} scale: 系数 */ calcBezierControlPoints( previousPoint, currentPoint, nextPoint1, nextPoint2, scale = 0.25 ) { let x = currentPoint.x + scale * (nextPoint1.x - previousPoint.x); let y = currentPoint.y + scale * (nextPoint1.y - previousPoint.y); const controlPointA = new Point(x, y); // 控制点 A x = nextPoint1.x - scale * (nextPoint2.x - currentPoint.x); y = nextPoint1.y - scale * (nextPoint2.y - currentPoint.y); const controlPointB = new Point(x, y); // 控制点 B return { controlPointA, controlPointB }; }

/** * 绘制贝塞尔曲线 * ctx.bezierCurveTo(控制点1, 控制点2, 当前点); */ drawBezierLine(ctx, data, options) { const { startX, diffX, baseY, diffY, Min } = options; ctx.beginPath(); // 先移动到第一个点 ctx.moveTo(startX, baseY - (data[0] - Min) * diffY); data.forEach((e, i) => { let curPoint, prePoint, nextPoint1, nextPoint2, x, y; // 当前点 x = startX + diffX * i; y = baseY - (e - Min) * diffY; curPoint = new Point(x, y); // 前一个点 x = startX + diffX * (i - 1); y = baseY - (data[i - 1] - Min) * diffY; prePoint = new Point(x, y); // 下一个点 x = startX + diffX * (i + 1); y = baseY - (data[i + 1] - Min) * diffY; nextPoint1 = new Point(x, y); // 下下个点 x = startX + diffX * (i + 2); y = baseY - (data[i + 2] - Min) * diffY; nextPoint2 = new Point(x, y); if (i === 0) { // 如果是第一个点, 则前一个点用当前点代替 prePoint = curPoint; } else if (i === data.length - 2) { // 如果是倒数第二个点, 则下下个点用下一个点代替 nextPoint2 = nextPoint1; } else if (i === data.length - 1) { // 最后一个点直接退出 return; } const { controlPointA, controlPointB } = this.calcBezierControlPoints( prePoint, curPoint, nextPoint1, nextPoint2 ); ctx.bezierCurveTo( controlPointA.x, controlPointA.y, controlPointB.x, controlPointB.y, nextPoint1.x, nextPoint1.y ); }); ctx.stroke(); },