1. Using computed in mobx does not work

class ODOM {
  constructor(props) {
    console.log("how many times")
  }
  @observable speed = 0
  @action change(obj) {
    console.log(obj)
    Object.keys(obj).forEach(item => {
      this[item] = obj[item]
    })
  }
  @computed get velocity() {
    console.log("entry")
    return this.speed*60*60/1000
  }
}
const model = new ODOM()
let total = 0
setInterval(() => {
  model.change({
    speed: PPtotal
  })
}, 3000)
export default model

2.computed is triggered only once
3. Do not see what is wrong with the code, solve