Transitions

So far we have explored how to create components and draw elements on the screen. But everything has been rather static so far.

We did learn how to reactively changes values and trigger rerenders based on that. So if you try the example below, you'll see that indeed our golden element changes position. But it just jumps from one place to another:

export default Blits.Component('Gold', {
  template: `
    <Element color="gold" w="100" h="100" :x="$x" :y="$y" />
  `,
  state() {
    return {
      x: 0,
      y: 0,
    }
  },
  input: {
    enter() {
      this.x = this.x + 100
      this.y = this.y + 50
    }
  },

Using transitions we really make our App come alive.

Applying a transition

Blits offers an easy and intuitive way to apply transitions. All you need to do is add the .transition modifier to a reactive attribute, and now whenever you change the value referenced in the attribute, it will automatically smooth into the new value.

export default Blits.Component('Gold', {
  template: `
    <Element color="gold" w="100" h="100" :x.transition="$x" :y.transition="$y" />
  `,
  state() {
    return {
      x: 0,
      y: 0,
    }
  },
  input: {
    enter() {
      this.x = this.x + 100
      this.y = this.y + 50
    }
  },

If we try out the modified example above, you'll notice how much difference adding a simple transition makes.

When the .transition-modifier is added to a reactive attribute, a default ease-in transition with a duration of 300ms is applied

Customizing transitions

While the default transition will look pretty good out of the box and is great for quickly improving the look of your App, you may want to customize specific transitions.

Blits gives you full control over your transitions, with a simple to use format, right inside your template.

Instead of referencing your component's state variable directly, you can also supply an inline object literal`` in the attribute with a .transition`-modifier. This will allow you to pass a number of options to change the way the transition will work.

The transition object must have a value key, which references the state variable (or prop or computed property) that triggers the transition.

Furthermore, you can specify:

• duration - the duration of the transition in ms
• delay - the time in ms after which the transition should initiate
• easing - the easing function of the transition. Can be one of the default easing options listed below, or a custom cubic-bezier definition (i.e. cubic-bezier(0,1.35,.99,-0.07)).

<Element color="gold" w="100" h="100"
  :x.transition="{value: $x, duration: 300, easing: 'ease-in-back', delay: 400}"
  :y.transition="{value: $x, duration: 300, easing: 'cubic-bezier(1,-0.64,0.39,1.44)'}">
</Element>

Besides a reference to the value, you can also use dynamic values for the other keys in the transition object. This way you'll be able to dynamically control the delay, duration or easing function.

<Element color="gold" w="100" h="100"
  :x.transition="{value: $x, duration: $dynamicDuration, delay: $dynamicDelay}">
</Element>

Available easing functions

• ease-in
• ease-out
• ease-in-out
• ease-in-sine
• ease-out-sine
• ease-in-out-sine
• ease-in-cubic
• ease-out-cubic
• ease-in-out-cubic
• ease-in-circ
• ease-out-circ
• ease-in-out-circ
• ease-in-back
• ease-out-back
• ease-in-out-back

Listening to transition events

Sometimes you may want to perform an action when a transition ends or when it starts.

You can easily hook into these transition events by adding a start and end key to the transition object. Both values should be a function (or a reference to a method on your component).

The start function will be called when the transition actually starts (after a possible specified delay) and the end function is called as soon as the transition is finished.

export default Blits.Component('Gold', {
  template: `
    <Element color="gold" w="100" h="100"
      :x.transition="{value: $x, start: $transitionBegin, end: $transitionEnd}"
    />
  `,
  ///
  methods: {
    transitionBegin() {
      //
    },
    transitionEnd() {
      //
    },
  }

It is also possible to keep track of the entire progress of a transition. Every frametick during a transition (ideally once every 1.6ms), the renderer reports the progress of the transition. You can hook into this event by specifying a progress key on the transition configuration object, with a function to excute.

This function is executed every frametick, and receives the current progress and the previous progress as its arguments. The progress is indicated as a value between 0 and 1, where 0 means start and 1 means finished.

export default Blits.Component('Gold', {
  template: `
    <Element color="gold" w="100" h="100"
      :x.transition="{value: $x, progress: $transitionProgress}"
    />
  `,
  ///
  methods: {
    transitionprogress(progress, previousProgress) {
      if(progress >= 0.5 && previousProgress < 0.5) {
        // halfway through the transition
      }
    },
  }