Transitioning State

Vue’s transition system offers many simple ways to animate entering, leaving, and lists, but what about animating your data itself? For example:

• numbers and calculations
• colors displayed
• the positions of SVG nodes
• the sizes and other properties of elements

All of these are either already stored as raw numbers or can be converted into numbers. Once we do that, we can animate these state changes using 3rd-party libraries to tween state, in combination with Vue’s reactivity and component systems.

Animating State with Watchers

Watchers allow us to animate changes of any numerical property into another property. That may sound complicated in the abstract, so let’s dive into an example using Tween.js:

<script src="https://unpkg.com/[email protected]"></script>

<div id="animated-number-demo">
  <input v-model.number="number" type="number" step="20">
  <p>{{ animatedNumber }}</p>
</div>

new Vue({
  el: '#animated-number-demo',
  data: {
    number: 0,
    animatedNumber: 0
  },
  watch: {
    number: function(newValue, oldValue) {
      var vm = this
      var animationFrame
      function animate (time) {
        TWEEN.update(time)
        animationFrame = requestAnimationFrame(animate)
      }
      new TWEEN.Tween({ tweeningNumber: oldValue })
        .easing(TWEEN.Easing.Quadratic.Out)
        .to({ tweeningNumber: newValue }, 500)
        .onUpdate(function () {
          vm.animatedNumber = this.tweeningNumber.toFixed(0)
        })
        .onComplete(function () {
          cancelAnimationFrame(animationFrame)
        })
        .start()
      animationFrame = requestAnimationFrame(animate)
    }
  }
})

When you update the number, the change is animated below the input. This makes for a nice demo, but what about something that isn’t directly stored as a number, like any valid CSS color for example? Here’s how we could accomplish this with the addition of Color.js:

<script src="https://unpkg.com/[email protected]"></script>
<script src="https://unpkg.com/[email protected]/color.js"></script>

<div id="example-7">
  <input
    v-model="colorQuery"
    v-on:keyup.enter="updateColor"
    placeholder="Enter a color"
  >
  <button v-on:click="updateColor">Update</button>
  <p>Preview:</p>
  <span
    v-bind:style="{ backgroundColor: tweenedCSSColor }"
    class="example-7-color-preview"
  ></span>
  <p>{{ tweenedCSSColor }}</p>
</div>

var Color = net.brehaut.Color

new Vue({
  el: '#example-7',
  data: {
    colorQuery: '',
    color: {
      red: 0,
      green: 0,
      blue: 0,
      alpha: 1
    },
    tweenedColor: {}
  },
  created: function () {
    this.tweenedColor = Object.assign({}, this.color)
  },
  watch: {
    color: function () {
      var animationFrame
      function animate (time) {
        TWEEN.update(time)
        animationFrame = requestAnimationFrame(animate)
      }
      new TWEEN.Tween(this.tweenedColor)
        .to(this.color, 750)
        .onComplete(function () {
          cancelAnimationFrame(animationFrame)
        })
        .start()
      animationFrame = requestAnimationFrame(animate)
    }
  },
  computed: {
    tweenedCSSColor: function () {
      return new Color({
        red: this.tweenedColor.red,
        green: this.tweenedColor.green,
        blue: this.tweenedColor.blue,
        alpha: this.tweenedColor.alpha
      }).toCSS()
    }
  },
  methods: {
    updateColor: function () {
      this.color = new Color(this.colorQuery).toRGB()
      this.colorQuery = ''
    }
  }
})

.example-7-color-preview {
  display: inline-block;
  width: 50px;
  height: 50px;
}

Dynamic State Transitions

Just as with Vue’s transition components, the data backing state transitions can be updated in real time, which is especially useful for prototyping! Even using a simple SVG polygon, you can achieve many effects that would be difficult to conceive of until you’ve played with the variables a little.

See this fiddle for the complete code behind the above demo.

Organizing Transitions into Components

Managing many state transitions can quickly increase the complexity of a Vue instance or component. Fortunately, many animations can be extracted out into dedicated child components. Let’s do this with the animated integer from our earlier example:

<script src="https://unpkg.com/[email protected]"></script>

<div id="example-8">
  <input v-model.number="firstNumber" type="number" step="20"> +
  <input v-model.number="secondNumber" type="number" step="20"> =
  {{ result }}
  <p>
    <animated-integer v-bind:value="firstNumber"></animated-integer> +
    <animated-integer v-bind:value="secondNumber"></animated-integer> =
    <animated-integer v-bind:value="result"></animated-integer>
  </p>
</div>

// This complex tweening logic can now be reused between
// any integers we may wish to animate in our application.
// Components also offer a clean interface for configuring
// more dynamic transitions and complex transition
// strategies.
Vue.component('animated-integer', {
  template: '<span>{{ tweeningValue }}</span>',
  props: {
    value: {
      type: Number,
      required: true
    }
  },
  data: function () {
    return {
      tweeningValue: 0
    }
  },
  watch: {
    value: function (newValue, oldValue) {
      this.tween(oldValue, newValue)
    }
  },
  mounted: function () {
    this.tween(0, this.value)
  },
  methods: {
    tween: function (startValue, endValue) {
      var vm = this
      var animationFrame
      function animate (time) {
        TWEEN.update(time)
        animationFrame = requestAnimationFrame(animate)
      }
      new TWEEN.Tween({ tweeningValue: startValue })
        .to({ tweeningValue: endValue }, 500)
        .onUpdate(function () {
          vm.tweeningValue = this.tweeningValue.toFixed(0)
        })
        .onComplete(function () {
          cancelAnimationFrame(animationFrame)
        })
        .start()
      animationFrame = requestAnimationFrame(animate)
    }
  }
})

// All complexity has now been removed from the main Vue instance!
new Vue({
  el: '#example-8',
  data: {
    firstNumber: 20,
    secondNumber: 40
  },
  computed: {
    result: function () {
      return this.firstNumber + this.secondNumber
    }
  }
})

Within child components, we can use any combination of transition strategies that have been covered on this page, along with those offered by Vue’s built-in transition system. Together, there are very few limits to what can be accomplished.