Tweens

A Tween can be used to interpolate some state over time.

A Tween is created by instancing the Scar::Tween class and managed by a Scar::App. To start the Tween, use app#tween(t : Tween) method. The constructor works like this:

def initialize(
    duration : Number,
    @ease : Easing::EasingDefinition,
    @on_update : Proc(Tween, Nil) = ->(t : Tween) {},
    @on_completed : Proc(Tween, Nil) = ->(t : Tween) {}
    )

• duration denotes how long the tweening should last (in seconds)

• ease defines how the fraction is calculated (see below)

• on_update is called every frame

• on_completed is called before the Tween is deleted

In on_update you should define what the Tween should modify. For example, if you want to tween a screen transition, you would modify the alpha value in on_update.

In on_completed you could for example start another tween to fade the alpha value back.

There are some instance methods to query or modify the state of a Tween:

• linear_fraction returns the linear progression

• fraction returns the fraction calculated by @ease

• completed? returns if the duration has passed

• reset resets the elapsed time to 0 (restarts the Tween)

• abort cancels execution WITHOUT calling on_completed

• getter and setter for @paused to pause/unpause the Tween

EasingDefinition

The abstract struct Scar::Easing::EasingDefinition exists to define how a Tween calculates it's fraction. You can either inherit this struct, use Scar::Easing::EaseWithFunction which can be initialized with a calculate function def initialize(@fn : Proc(Float32, Float32)); end or use the Scar::Easing#simple_easing_function(name, fn) macro where you can provide fn as a stringified mathematical expression like simple_easing_function(:EaseInQuad, "lf ** 2").

There are some builtin easing definitions, please refer to the reference for further information.

Example

app.tween(Tween.new 
    JUMP_TIME,
    Easing::EaseOutQuad.new, 
    ->(t : Tween) { tr.pos.y = t.fraction * JUMP_HEIGHT; nil },
    ->(t : Tween) { nil })