我在assets中存储了各主题对应的placeholder文件，想通过eventbus和全局mixin动态更换placeholder图片，虽然app.vue中的placeHolderNothing改变了,但是img的src并没有值,为什么？

<img :src="imgUrl||placeHolderNothing"/>

App.vue

created () {
      bus.$on('changeTheme', (themeId) => {
        themeId = +themeId
        let theme = 'purple'
        switch (themeId) {
          case 1:
            theme = 'purple'
            break
          case 2:
            theme = 'red'
            break
          case 3:
            theme = 'blue'
            break
          default:
            theme = 'purple'
        }
        this.placeHolderNothing = require(`@/assets/icon_nothing_${theme}.svg`)
      })

      Vue.mixin({
        data () {
          return {
            placeHolderNothing: this.placeHolderNothing
          }
        }
      })
      
      

切换主题时

    bus.$emit('changeTheme', this.themeId)