XCEngine/new_editor/app/Features/Shared/GridItemDragDrop.h

#pragma once

#include <XCEngine/UI/Types.h>

#include <string>
#include <string_view>
#include <vector>

namespace XCEngine::UI::Editor::App::GridItemDragDrop {

using ::XCEngine::UI::UIInputEvent;
using ::XCEngine::UI::UIInputEventType;
using ::XCEngine::UI::UIPoint;
using ::XCEngine::UI::UIPointerButton;

inline constexpr float kDefaultDragThreshold = 4.0f;

struct State {
    std::string armedItemId = {};
    std::string draggedItemId = {};
    std::string dropTargetItemId = {};
    UIPoint pressPosition = {};
    bool armed = false;
    bool dragging = false;
    bool validDropTarget = false;
    bool requestPointerCapture = false;
    bool requestPointerRelease = false;
};

struct ProcessResult {
    bool selectionForced = false;
    bool dropCommitted = false;
    std::string draggedItemId = {};
    std::string dropTargetItemId = {};
};

inline void ResetTransientRequests(State& state) {
    state.requestPointerCapture = false;
    state.requestPointerRelease = false;
}

inline bool HasActivePointerCapture(const State& state) {
    return state.dragging;
}

inline float ComputeSquaredDistance(const UIPoint& lhs, const UIPoint& rhs) {
    const float dx = lhs.x - rhs.x;
    const float dy = lhs.y - rhs.y;
    return dx * dx + dy * dy;
}

template <typename Callbacks>
ProcessResult ProcessInputEvents(
    State& state,
    const std::vector<UIInputEvent>& inputEvents,
    Callbacks& callbacks,
    float dragThreshold = kDefaultDragThreshold) {
    ProcessResult result = {};

    for (const UIInputEvent& event : inputEvents) {
        switch (event.type) {
        case UIInputEventType::PointerButtonDown:
            if (event.pointerButton == UIPointerButton::Left) {
                state.armedItemId = callbacks.ResolveDraggableItem(event.position);
                state.pressPosition = event.position;
                state.armed = !state.armedItemId.empty();
                if (!state.armed) {
                    state.draggedItemId.clear();
                    state.dropTargetItemId.clear();
                    state.validDropTarget = false;
                }
            }
            break;

        case UIInputEventType::PointerMove:
            if (state.armed &&
                !state.dragging &&
                ComputeSquaredDistance(event.position, state.pressPosition) >=
                    dragThreshold * dragThreshold) {
                state.dragging = !state.armedItemId.empty();
                state.draggedItemId = state.armedItemId;
                state.dropTargetItemId.clear();
                state.validDropTarget = false;
                if (state.dragging) {
                    state.requestPointerCapture = true;
                    if (!callbacks.IsItemSelected(state.draggedItemId)) {
                        result.selectionForced =
                            callbacks.SelectDraggedItem(state.draggedItemId);
                    }
                }
            }

            if (state.dragging) {
                state.dropTargetItemId =
                    callbacks.ResolveDropTargetItem(
                        state.draggedItemId,
                        event.position);
                state.validDropTarget =
                    !state.dropTargetItemId.empty() &&
                    callbacks.CanDropOnItem(
                        state.draggedItemId,
                        state.dropTargetItemId);
            }
            break;

        case UIInputEventType::PointerButtonUp:
            if (event.pointerButton != UIPointerButton::Left) {
                break;
            }

            if (state.dragging) {
                if (state.validDropTarget) {
                    result.draggedItemId = state.draggedItemId;
                    result.dropTargetItemId = state.dropTargetItemId;
                    result.dropCommitted =
                        callbacks.CommitDropOnItem(
                            state.draggedItemId,
                            state.dropTargetItemId);
                }

                state.armed = false;
                state.dragging = false;
                state.armedItemId.clear();
                state.draggedItemId.clear();
                state.dropTargetItemId.clear();
                state.validDropTarget = false;
                state.requestPointerRelease = true;
            } else {
                state.armed = false;
                state.armedItemId.clear();
            }
            break;

        case UIInputEventType::PointerLeave:
            if (state.dragging) {
                state.dropTargetItemId.clear();
                state.validDropTarget = false;
            }
            break;

        case UIInputEventType::FocusLost:
            {
                const bool requestPointerRelease = state.dragging;
                state = {};
                state.requestPointerRelease = requestPointerRelease;
            }
            break;

        default:
            break;
        }
    }

    return result;
}

} // namespace XCEngine::UI::Editor::App::GridItemDragDrop