using { /Fortnite.com/Devices }
using { /Verse.org/Simulation }
using { /Verse.org/Simulation/Tags }
using { /Verse.org/Assets }
using { /UnrealEngine.com/Temporary/Diagnostics }

# CHANGE THE CAR'S COLOURS — Part 2 of "Scene Graph: Car Primitive".
#
# Players love a colour swap. The honest truth about Verse + materials in build
# 41.00: there is NO public per-parameter API. `material.SetColorProperty` /
# `SetFloatProperty` exist but are Epic-internal, and there is no
# `SetVectorParameter` / `material_parameter_collection` / `dynamic_material`
# anywhere in the public surface. What a creator CAN do is swap the WHOLE
# material on a placed prop with one public call:
#
#   creative_prop.SetMaterial(Material : material, ?Index : int)
#
# So the recipe is: author a few material assets (a red paint, a blue paint, a
# gold paint), expose them as `@editable ?material` slots, and cycle through them
# on a button press. Same car body, instant new paint job.
#
# Note the `?material` (optional) type: the `material` class is Epic-internal and
# cannot be constructed with `material{}`, so a concrete device can't give it a
# plain default. Declaring the slot as `?material = false` is the idiom — the
# editor fills it in, and Verse unwraps it with `Paint?`.

car_body_tag := class(tag) {}
car_paint_button_tag := class(tag) {}

car_material_colors_device := class(creative_device):

    # Three paint-job materials. Drop a Material asset into each slot in the
    # Details panel (any material works — these are whole-material swaps, not
    # colour tweaks).
    @editable
    PaintA : ?material = false
    @editable
    PaintB : ?material = false
    @editable
    PaintC : ?material = false

    # Which paint is showing now (0,1,2 -> A,B,C). `var` so we can advance it.
    var PaintIndex : int = 0

    OnBegin<override>()<suspends> : void =
        for (Obj : FindCreativeObjectsWithTag(car_paint_button_tag)):
            if (Button := button_device[Obj]):
                Button.InteractedWithEvent.Subscribe(OnPaintPressed)
                Print("CarColors: paint button wired.")

    OnPaintPressed(Agent : agent) : void =
        # Advance to the next paint, wrapping 2 -> 0. Mod can fail (decides), so
        # compute it in its own failure context, then assign.
        if (Next := Mod[PaintIndex + 1, 3]):
            set PaintIndex = Next
            # Pick the matching paint slot and repaint, only if that slot is
            # filled. Each branch unwraps its own optional (Paint?) in the if.
            if (PaintIndex = 0, Paint := PaintA?):
                ApplyPaint(Paint)
            else if (PaintIndex = 1, Paint := PaintB?):
                ApplyPaint(Paint)
            else if (Paint := PaintC?):
                ApplyPaint(Paint)

    # Repaint every tagged car body by swapping its whole material slot 0.
    ApplyPaint(Paint : material) : void =
        for (Obj : FindCreativeObjectsWithTag(car_body_tag)):
            if (Body := creative_prop[Obj]):
                Body.SetMaterial(Paint, ?Index := 0)
                Print("CarColors: repainted the car.")