canvas作为自定义组件，保存画布提示canvasToTempFilePath: fail canvas is empty？

在页面里面直接写是OK的，封装到组件内部就不能用

const that = this

setTimeout(()=>{

Taro.canvasToTempFilePath({

canvasId: 'measure-angle',

success(result) {

console.log(111, result)

},

fail(result) {

console.log(222, result)

}

}, that)

}, 500)