canvas

画布。。

注：

1. canvas标签默认宽度300px、高度225px

2. 统一页面中的canvas-id不可重复，，，，若是使用一个已经泛起过的canvas-id，，，，该canvas标签对应的画布将被隐藏并不再正常事情

示例代码：

<!-- 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>
<!-- 由于canvas-id与前一个canvas重复，，，，该canvas不会显示，，，，并会发送一个过失事务到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.createCanvasContext('firstCanvas')

    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()
    context.draw()
  }
})

相关api：wx.createCanvasContext

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