<!-- canvas.wxml -->
<canvas style="width: 300px; height: 200px;" canvas-id="firstCanvas"></canvas>
<!-- 当使用绝对定位时，文档流后边的canvas的显示层级高于前边的canvas-->
<canvas style="width: 400px; height: 500px;" canvas-id="secondCanvas"></canvas>
<!-- 因为canvas-id与前一个canvas重复，该canvas不会显示，并会发送一个错误事件到AppService -->
<canvas style="width: 400px; height: 500px;" canvas-id="secondCanvas" binderror="canvasIdErrorCallback"></canvas>
// canvas.js

Page({
 canvasIdErrorCallback: function (e) {
  console.error(e.detail.errMsg);
 },
 onReady: function (e) {

  //使用wx.createContext获取绘图上下文context
  var context = wx.createContext();

  context.setStrokeStyle("#00ff00");
  context.setLineWidth(5);
  context.rect(0,0,200,200);
  context.stroke()
  context.setStrokeStyle ("#ff0000") ;
  context.setLineWidth (2)
  context.moveTo(160,100)
  context.arc(100,100,60,0,2*Math.PI,true);
  context.moveTo(140,100);
  context.arc(100,100,40,0,Math.PI,false);
  context.moveTo(85,80);
  context.arc(80,80,5,0,2*Math.PI,true);
  context.moveTo(125,80);
  context.arc(120,80,5,0,2*Math.PI,true);
  context.stroke();

  //调用wx.drawCanvas，通过canvasId指定在哪张画布上绘制，通过actions指定绘制行为
  wx.drawCanvas({
   canvasId: 'firstCanvas',
   actions: context.getActions() //获取绘图动作数组
  });
 }
});