Switch the monitored variable's value in VueJS

Question

Switch the monitored variable's value in VueJS

` `

I have a data variable that is of boolean type, and I have included this in a watch hook. When the value changes to true, I execute certain tasks within the watch function and everything works perfectly.

` `

watch: {`
`   boolVar: function (val) {`
`     if(val === true){`
`       //performing some tasks`
`     }`
`   }`
`}`
`

` `

Later, I needed to add additional functionality where I had to toggle the variable if it was already set to true in order to re-trigger the watcher. If at a certain point, boolVar is already true and I need the watcher to run again, what would be the best approach?

` `

Initially, I tried toggling the boolVar like this:

` `

//inside another function`
`if(this.boolVar === true){`
`  this.boolVar = false;`
`  this.boolVar = true;`
`}`
`

` `

However, this approach did not work as expected. After some experimentation, I discovered that there was a difference in the time delay at which the watcher was listening, causing the code to not function as intended.

` `

I then implemented a different method using the sleep function and promises to introduce a 1 millisecond delay so that the watcher could listen properly, and it worked according to my requirements:

` `

//inside another function`
`//making the function async to use await`

`if(this.boolVar === true){`
`  this.boolVar = false;`
`  await this.sleep(1);`
`  this.boolVar = true;`
`}`
`

` `

//sleep function`
`sleep(ms) {`
`      return new Promise((resolve) => setTimeout(resolve, ms));`
`},`
`

` `

Is there a better approach to handle this scenario, where you need to toggle a variable being watched and re-run tasks? While my current workaround is functional, I believe the watcher should inherently detect when the same variable is toggled (which it currently does not).

` `

javascript vue.js vuejs2 watch

Answer 1

Answer №1

If you're looking to optimize your code, consider using Vue.$nextTick.

if(this.boolVar === true){
  this.boolVar = false;
  this.$nextTick(() => {
    this.boolVar = true;
  })
}

Alternatively, if you prefer not to use $nextTick, you can implement it this way:

  watch: {
    boolVar: {
      handler(val) {
        console.log(val)
      },
      deep: true
    }
  }

Another approach could be creating a specific function, like doSomeWork(), and calling it within your watcher instead of toggling the boolean. This might provide a more logical structure for your code:

watch: {
   boolVar: function (val) {
     if(val === true){
       this.doingSomeTasks()
     }
   }
}

Additionally:

if(this.boolVar === true){
  this.doingSomeTasks()
}

Answer 2

If you're looking to optimize your code, consider using Vue.$nextTick.

if(this.boolVar === true){
  this.boolVar = false;
  this.$nextTick(() => {
    this.boolVar = true;
  })
}

Alternatively, if you prefer not to use $nextTick, you can implement it this way:

  watch: {
    boolVar: {
      handler(val) {
        console.log(val)
      },
      deep: true
    }
  }

Another approach could be creating a specific function, like doSomeWork(), and calling it within your watcher instead of toggling the boolean. This might provide a more logical structure for your code:

watch: {
   boolVar: function (val) {
     if(val === true){
       this.doingSomeTasks()
     }
   }
}

Additionally:

if(this.boolVar === true){
  this.doingSomeTasks()
}

Answer 3

Answer №2

Your code isn't functioning as expected because you assume that all watch handlers run synchronously, but they don't. Vue utilizes an Async Update Queue

When a data change is detected, Vue creates a queue and stores all the data changes that occur in the same event loop. If the same watcher is triggered multiple times, it only gets added to the queue once. This de-duplication process helps prevent unnecessary calculations and DOM manipulations. Subsequently, in the next event loop “tick”, Vue flushes the queue and performs the necessary (de-duped) tasks.

So, when you execute

this.boolVar = false; this.boolVar = true;

, both changes are merely being "registered" by Vue. After all your code finishes executing, the "next event loop" begins - Vue checks the queue for changes, finds the registered watcher, and compares the old value (from the last handler execution) to the new value. In your case, both are true, so Vue sees no actual change and doesn't execute the handler.

The proper technical solution is to delay the second change (this.boolVar = true;) to a later "tick", so that Vue recognizes the false 1st and triggers the watcher in the next tick (from false to true).

However, good code not only works but also clearly communicates the programmer's intent and is easily understandable. The code

this.boolVar = false; this.boolVar = true;

violates this principle significantly.

If you need to perform someTasks when your variable transitions from false to true, that's fine. But you also want to run someTasks in other scenarios where the variable is already true. It's obvious that someTasks should not reside in the watcher, but should be extracted to a standalone method as @Julien suggested...

Update ...just to be complete

What you're seeking is achievable in Vue 3 with the new flush option of the watch - simply use the value

sync</code, and your watcher will be executed synchronously whenever the value changes. Check out the demo below...</p>
<p><div>
<div>
<pre class="lang-js"><code>const app = Vue.createApp({
  data() {
    return {
      value: true,
      flushMode: 'sync',
      unwatch: null
    }
  },
  methods: {
    toggle() {
      this.value = false
      this.value = true
    }
  },
  watch: {
    flushMode: {
      handler: function() {
        this.unwatch && this.unwatch()
        this.unwatch = this.$watch('value', 
          function(newValue) {
            console.log(`New value = ${newValue}`)
          }, 
          { flush: this.flushMode }
        )
      },
      immediate: true    
    }
  },
  updated() {
    console.log('Component updated')
  }
})

app.mount('#app')

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/3.1.1/vue.global.prod.min.js" integrity="sha512-vRIbP8hGYYKnayuLhVRwQXTxvAJ/wUm4vqIEzIBKLfDcM2ArSENuojG1jEIactQ7F/0unOe9VQ+U9KagTEyQvA==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<div id="app">
  <div>
    Variable: {{ value }}
  </div>
  <button @click="toggle">Toggle</button>
  <div>
    <label for="pre">Flush mode:</lael>
    <input type="radio" id="pre" value="pre" v-model="flushMode" /> pre (default)
    <input type="radio" id="sync" value="sync" v-model="flushMode" /> sync
  </div>
</div>

Answer 4

Your code isn't functioning as expected because you assume that all watch handlers run synchronously, but they don't. Vue utilizes an Async Update Queue

When a data change is detected, Vue creates a queue and stores all the data changes that occur in the same event loop. If the same watcher is triggered multiple times, it only gets added to the queue once. This de-duplication process helps prevent unnecessary calculations and DOM manipulations. Subsequently, in the next event loop “tick”, Vue flushes the queue and performs the necessary (de-duped) tasks.

So, when you execute

this.boolVar = false; this.boolVar = true;

, both changes are merely being "registered" by Vue. After all your code finishes executing, the "next event loop" begins - Vue checks the queue for changes, finds the registered watcher, and compares the old value (from the last handler execution) to the new value. In your case, both are true, so Vue sees no actual change and doesn't execute the handler.

The proper technical solution is to delay the second change (this.boolVar = true;) to a later "tick", so that Vue recognizes the false 1st and triggers the watcher in the next tick (from false to true).

However, good code not only works but also clearly communicates the programmer's intent and is easily understandable. The code

this.boolVar = false; this.boolVar = true;

violates this principle significantly.

If you need to perform someTasks when your variable transitions from false to true, that's fine. But you also want to run someTasks in other scenarios where the variable is already true. It's obvious that someTasks should not reside in the watcher, but should be extracted to a standalone method as @Julien suggested...

Update ...just to be complete

What you're seeking is achievable in Vue 3 with the new flush option of the watch - simply use the value

sync</code, and your watcher will be executed synchronously whenever the value changes. Check out the demo below...</p>
<p><div>
<div>
<pre class="lang-js"><code>const app = Vue.createApp({
  data() {
    return {
      value: true,
      flushMode: 'sync',
      unwatch: null
    }
  },
  methods: {
    toggle() {
      this.value = false
      this.value = true
    }
  },
  watch: {
    flushMode: {
      handler: function() {
        this.unwatch && this.unwatch()
        this.unwatch = this.$watch('value', 
          function(newValue) {
            console.log(`New value = ${newValue}`)
          }, 
          { flush: this.flushMode }
        )
      },
      immediate: true    
    }
  },
  updated() {
    console.log('Component updated')
  }
})

app.mount('#app')

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/3.1.1/vue.global.prod.min.js" integrity="sha512-vRIbP8hGYYKnayuLhVRwQXTxvAJ/wUm4vqIEzIBKLfDcM2ArSENuojG1jEIactQ7F/0unOe9VQ+U9KagTEyQvA==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<div id="app">
  <div>
    Variable: {{ value }}
  </div>
  <button @click="toggle">Toggle</button>
  <div>
    <label for="pre">Flush mode:</lael>
    <input type="radio" id="pre" value="pre" v-model="flushMode" /> pre (default)
    <input type="radio" id="sync" value="sync" v-model="flushMode" /> sync
  </div>
</div>

Switch the monitored variable's value in VueJS

Answer №1

Answer №2

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