WeChat small program canvas canvas

Base library 1.0.0 starts to support, and low versions need to be compatible.

Canvas. 2 .9.0 supports a new set of Canvas 2D interfaces (type properties required) and same-layer rendering, with the original interface no longer maintained. Related api: Get canvas instances.

Bug & Tip

1. tip: Canvas label default width 300px, height 150px
2. tip: Canvas-id on the same page is not repeatable, and if you use a canvas-id that has already appeared, the canvas label's corresponding canvas will be hidden and no longer working properly
3. TIP: Please pay attention Native components use restrictions
4. TIP: The GPU hardware acceleration is turned off in the developer tool, and "Hardware Acceleration" improves WebGL rendering performance in the settings of the developer tool.
5. Tip: WebGL supports the canvas for transparent background by getContext ('WebGL', {alpha: true})
6. Tip: Canvas 2D (New Interface) Need to explicitly set the canvas wide (default 300x150)
7. BUG: Avoid setting too much as high, there will be Crash issues under Android

Canvas 2D sample code

Preview effect in the developer tool

  <!-- canvas.wxml -->
  <canvas type="2d" id="myCanvas"></canvas>

// canvas.js
Page({
  onReady() {
    const query = wx.createSelectorQuery()
    query.select('#myCanvas')
      .fields({ node: true, size: true })
      .exec((res) => {
        const canvas = res[0].node
        const ctx = canvas.getContext('2d')

        const dpr = wx.getSystemInfoSync().pixelRatio
        canvas.width = res[0].width * dpr
        canvas.height = res[0].height * dpr
        ctx.scale(dpr, dpr)

        ctx.fillRect(0, 0, 100, 100)
      })
  }
})

WebGL sample code

Preview the effect in the developer tool

  <!-- canvas.wxml -->
  <canvas type="webgl" id="myCanvas"></canvas>

// canvas.js
Page({
  onReady() {
    const query = wx.createSelectorQuery()
    query.select('#myCanvas').node().exec((res) => {
      const canvas = res[0].node
      const gl = canvas.getContext('webgl')
      gl.clearColor(1, 0, 1, 1)
      gl.clear(gl.COLOR_BUFFER_BIT)
    })
  }
})

Sample code (old interface)

Preview the effect in the developer tool Download

<!-- 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>

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()
  }
})