/*
Crafter®.Graphics
Copyright (C) 2026 Catcrafts®
catcrafts.net

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License version 3.0 as published by the Free Software Foundation;

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA
*/
export module Crafter.Graphics:UIWidget;
import std;
import :UILength;
import :Types;        // for CrafterKeys

export namespace Crafter::UI {
    struct DrawList;        // forward decl (full def in :UIDrawList)

    // Threaded through layout. Holds anything every widget needs from the
    // surrounding scene at layout time (DPI scale, root surface size, …).
    struct LayoutContext {
        float scale = 1.0f;       // device scale (Window::scale)
        Size  surfaceSize{};      // root surface in device px
    };

    struct Widget {
        Length width_  = Length::Auto();
        Length height_ = Length::Auto();
        Edges padding_;
        Edges margin_;
        std::optional<Anchor> anchor_;

        // Layout output, filled by the engine.
        Rect computedRect{};
        Size desiredSize{};
        bool dirty = true;

        // Tree.
        Widget* parent = nullptr;
        std::vector<std::unique_ptr<Widget>> children_;

        Widget() = default;
        Widget(const Widget&) = delete;
        Widget& operator=(const Widget&) = delete;
        Widget(Widget&&) = default;
        Widget& operator=(Widget&&) = default;
        virtual ~Widget() = default;

        // Layout protocol — Measure returns the size this widget wants given
        // the available space; engine then calls Arrange with the final rect.
        virtual Size Measure(Size avail, const LayoutContext& ctx) = 0;
        virtual void Arrange(Rect rect, const LayoutContext& ctx) = 0;

        // Interaction protocol — return true if the event was handled and
        // should NOT bubble to the parent. Default: not handled.
        virtual bool OnMouseClick(float /*x*/, float /*y*/) { return false; }

        // Focus protocol. Widgets that opt in (e.g. InputField) return
        // true from IsFocusable; UIScene tracks the currently-focused
        // widget and routes keyboard events to it.
        virtual bool IsFocusable() const { return false; }
        virtual void OnFocus() {}
        virtual void OnBlur()  {}

        // Keyboard input. Both default to "not handled". OnTextInput
        // receives a UTF-8 substring (typically one codepoint per call).
        // OnKeyDown receives non-character keys (Backspace, arrows, …).
        virtual bool OnTextInput(std::string_view /*text*/)        { return false; }
        virtual bool OnKeyDown  (CrafterKeys      /*key*/)         { return false; }

        // Drawing protocol — emit GPU-bound draw items into `dl`. Default
        // implementation is "container behaviour": just descend into
        // children. Leaf widgets override to emit their own primitives;
        // containers that also draw (Button background, ScrollView clip
        // push/pop, TabView bar) override and explicitly recurse into
        // children where appropriate.
        //
        // The body just forwards to children, so the forward-declared
        // DrawList is enough — no member access here.
        virtual void Emit(DrawList& dl) const {
            for (auto& c : children_) c->Emit(dl);
        }

        // Walk all descendants in pre-order.
        template<typename F>
        void ForEach(F&& f) {
            f(*this);
            for (auto& c : children_) c->ForEach(f);
        }
    };

    // CRTP base providing fluent setters that return the concrete widget type.
    template<typename Self>
    struct WidgetBuilder : Widget {
        Self& self() { return static_cast<Self&>(*this); }

        Self& width(Length l)            { width_  = l; return self(); }
        Self& height(Length l)           { height_ = l; return self(); }
        Self& size(Length w, Length h)   { width_ = w; height_ = h; return self(); }
        Self& padding(Edges e)           { padding_ = e; return self(); }
        Self& padding(float all)         { padding_ = Edges(all); return self(); }
        Self& padding(float v, float h)  { padding_ = Edges(v, h); return self(); }
        Self& margin(Edges e)            { margin_ = e; return self(); }
        Self& margin(float all)          { margin_ = Edges(all); return self(); }
        Self& anchor(Anchor a)           { anchor_ = a; return self(); }
        Self& expand()                   { width_ = Length::Frac(1); height_ = Length::Frac(1); return self(); }

        // Take ownership of a parameter pack of widgets and append them as children.
        template<typename... Ws>
            requires (std::derived_from<std::decay_t<Ws>, Widget> && ...)
        Self& children(Ws&&... ws) {
            children_.reserve(children_.size() + sizeof...(Ws));
            (AppendChild(std::forward<Ws>(ws)), ...);
            return self();
        }

    private:
        // .children(...) takes ownership of each widget argument unconditionally;
        // builder chains like `Button{"X"}.font(f)` return Self& (lvalue ref to
        // the temporary), so we always move rather than std::forward.
        template<typename W>
        void AppendChild(W&& w) {
            using T = std::remove_cvref_t<W>;
            auto p = std::make_unique<T>(std::move(w));
            p->parent = this;
            children_.push_back(std::move(p));
        }
    };

    // Stable typed handle into the scene; populated by the scene when a
    // widget tree is mounted.
    template<typename T>
    struct WidgetRef {
        T* node = nullptr;

        T* operator->() const { return node; }
        T& operator*()  const { return *node; }
        explicit operator bool() const { return node != nullptr; }
    };

    // Mutable observable value. Setting a new value invokes any registered
    // watchers; widgets register watchers in their mount step to mark
    // themselves dirty when the underlying value changes.
    template<typename T>
    class Observable {
    public:
        Observable() = default;
        Observable(T v) : value_(std::move(v)) {}

        Observable(const Observable&) = delete;
        Observable& operator=(const Observable&) = delete;

        Observable& operator=(T v) {
            if constexpr (std::equality_comparable<T>) {
                if (value_ == v) return *this;
            }
            value_ = std::move(v);
            Notify();
            return *this;
        }

        const T& Get() const { return value_; }
        operator const T&() const { return value_; }

        // Register a watcher; returned token unregisters on destruction.
        // For V1 there is no unsubscribe — watchers live as long as the
        // Observable does. The scene clears watchers when widgets are torn
        // down by destroying the Observable they were watching.
        void Watch(std::function<void()> fn) {
            watchers_.push_back(std::move(fn));
        }

    private:
        T value_{};
        std::vector<std::function<void()>> watchers_;

        void Notify() {
            for (auto& w : watchers_) w();
        }
    };
}