Gween (go-between) is a small library to perform tweening in Go. It has a minimal interface, and it comes with several easing functions.

package gween

import (
  "github.com/tanema/gween/ease"
  "github.com/tanema/gween/gween"
)

// increase the value from 0 to 5 in 10 seconds
var tweenLinear = gween.New(0, 5, 10, ease.Linear)
current, isFinished := tweenLinear.Update(dt)

// make some text fall from the top of the screen, bouncing on y=300, in 4 seconds
var tweenLabel = gween.new(0, 300, 4, ease.OutBounce)
label.Y, _ = tweenLabel.Update(dt)

// fade background from white to black and foregrond from black to red in 2 seconds
currentBGColor = [4]float32{255, 255, 255, 255}
currentColor = [4]float32{0, 0, 0, 0}
var tweenBackground = gween.new(255, 0, 2, ease.Linear)
var tweenRed = gween.new(255, 0, 2, ease.Linear)
currentBG, _ := tweenBackground.Update(dt)
currentBGColor = [4]float32{currentBG, currentBG, currentBG, currentBG}
currentColor[0], _ = tweenRed.Update(dt)

t := gween.New(begin, end, duration, easingFunction)

Creates a new tween.

• begin is the start value
• end is the ending value
• duration means how much the change will take until it's finished. It must be a positive number.
• easingFunction can be either a function or a function name (see the easing section below).

This function only creates and returns the tween. It must be captured in a variable and updated via t.Update(dt) in order for the changes to take place.

currentValue, isFinished := t.Update(dt)

Gradually changes the currentValue toward the end value as time passes.

• t is a tween returned by gween.New
• dt is the difference in time. It will be added to the internal time counter of the tween. The current value at the current value will be returned using selected easing function.
• currentValue is the current eased value for the current time.
• isFinised is true if the tween has reached its limit (its internal clock is >= duration). It is false otherwise.

When the tween is complete, the currentValue will be equal to the end value. The way they change over time will depend on the chosen easing function.

If dt is positive, the easing will be applied until the internal clock equals duration, at which point the easing will stop. If it is negative, the easing will play "backwards", until it reaches the initial value.

currentValue, isFinished := t.Set(clock)

Moves a tween's internal clock to a particular moment.

• t is a tween returned by gween.New
• clock is a positive number or 0. It's the new value of the tween's internal clock.
• currentValue is the value of the tween at the time set.
• isFinished works like in t.Update; it's true if the tween has reached its end, and false otherwise.

Easing functions are functions that express how slow/fast the interpolation happens in tween.

Gween comes with 45 default easing functions already built-in (adapted from Enrique García Cota's easing library).

The easing functions can be found in the ease package.

They can be divided into several families:

• linear is the simplest easing function, straight from one value to the other.
• quad, cubic, quart, quint, expo, sine and circle are all "smooth" curves that will make transitions look natural.
• The back family starts by moving the interpolation slightly "backwards" before moving it forward.
• The bounce family simulates the motion of an object bouncing.
• The elastic family simulates inertia in the easing, like an elastic gum.

Each family (except linear) has 4 variants:

• In starts slow, and accelerates at the end
• Out starts fast, and decelerates at the end
• InOut starts and ends slow, but it's fast in the middle
• OutIn starts and ends fast, but it's slow in the middle

You are not limited to gween's easing functions; if you pass a function parameter in the easing, it will be used.

The passed function will need to suite the TweenFunc interface: func(t, b, c, d float32) float32

• t (time): starts in 0 and usually moves towards duration
• b (begin): initial value of the of the property being eased.
• c (change): ending value of the property - starting value of the property
• d (duration): total duration of the tween

And must return the new value after the interpolation occurs.

Here's an example using a custom easing.

labelTween := tween.new(0, 300, 4, func(t, b, c, d) float32 {
  return c*t/d   b // linear ease
})

Sequences can be used to execute tweens in sequence easily.

var numberSequence := gween.NewSequence(
  gween.New(0, 4, 10, ease.Linear),
  gween.New(4, 0, 10, ease.Linear),
)
current, tweenFinished, sequenceFinished := numberSequence.Update(dt) 

// current will rise from 0 to 4, and then back to 0 over a period of 
// 20 seconds

// tweenFinished will be true when any tween finishes

// sequenceFinished will be true when the entire sequence of Tweens finishes

The easing functions have been translated from Enrique García Cota's project in

https://github.com/kikito/tween.lua

See the LICENSE file for details.