uni.createCanvasContext is not a function

根据官方的示例 https://uniapp.dcloud.io/component/canvas
报错 uni.createCanvasContext is not a function

<template>  
    <view>  
        <canvas style="width: 300px; height: 200px;" canvas-id="firstCanvas"></canvas>  
        <canvas style="width: 400px; height: 500px;" canvas-id="secondCanvas"></canvas>  
        <canvas style="width: 400px; height: 500px;" canvas-id="secondCanvas" @error="canvasIdErrorCallback"></canvas>  
    </view>  
</template>  
<script>  
export default {  
    onReady: function (e) {  
        var context = uni.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()  
    },  
    methods: {  
        canvasIdErrorCallback: function (e) {  
            console.error(e.detail.errMsg)  
        }  
    }  
}  
</script>