scroll_bar_widget.hpp

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// Copyright Take Vos 2020-2022.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#pragma once
 
#include "widget.hpp"
#include "../GUI/gui_event.hpp"
#include "../geometry/axis.hpp"
#include "../observer.hpp"
#include "../numbers.hpp"
#include <memory>
#include <string>
#include <array>
#include <optional>
#include <future>
 
namespace hi { inline namespace v1 {
 
template<axis Axis>
class scroll_bar_widget final : public widget {
public:
    using super = widget;
 
    static constexpr hi::axis axis = Axis;
 
    observer<int> offset;
    observer<int> aperture;
    observer<int> content;
 
    scroll_bar_widget(
        widget *parent,
        forward_of<observer<int>> auto&& content,
        forward_of<observer<int>> auto&& aperture,
        forward_of<observer<int>> auto&& offset) noexcept :
        widget(parent), content(hi_forward(content)), aperture(hi_forward(aperture)), offset(hi_forward(offset))
    {
        _content_cbt = this->content.subscribe([&](auto...) {
            ++global_counter<"scroll_bar_widget:content:relayout">;
            process_event({gui_event_type::window_relayout});
        });
        _aperture_cbt = this->aperture.subscribe([&](auto...) {
            ++global_counter<"scroll_bar_widget:aperture:relayout">;
            process_event({gui_event_type::window_relayout});
        });
        _offset_cbt = this->offset.subscribe([&](auto...) {
            ++global_counter<"scroll_bar_widget:offset:relayout">;
            process_event({gui_event_type::window_relayout});
        });
    }
 
    ~scroll_bar_widget() {}
 
    [[nodiscard]] box_constraints update_constraints() noexcept override
    {
        _layout = {};
 
        if (*mode <= widget_mode::collapse) {
            return {};
        }
 
        // The minimum size is twice the length of the slider, which is twice the theme().size()
        if constexpr (axis == axis::vertical) {
            return {
                extent2i{theme().icon_size, theme().size * 4},
                extent2i{theme().icon_size, theme().size * 4},
                extent2i{theme().icon_size, large_number_v<int>}};
        } else {
            return {
                extent2i{theme().size * 4, theme().icon_size},
                extent2i{theme().size * 4, theme().icon_size},
                extent2i{large_number_v<int>, theme().icon_size}};
        }
    }
 
    void set_layout(widget_layout const& context) noexcept override
    {
        _layout = context;
 
        if (*mode <= widget_mode::collapse) {
            _slider_rectangle = {};
            return;
        }
 
        // Calculate the position of the slider.
        hilet slider_offset = narrow_cast<int>(std::round(*offset * travel_vs_hidden_content_ratio()));
        if constexpr (axis == axis::vertical) {
            _slider_rectangle = aarectanglei{0, slider_offset, context.width(), slider_length()};
        } else {
            _slider_rectangle = aarectanglei{slider_offset, 0, slider_length(), context.height()};
        }
    }
 
    [[nodiscard]] bool visible() const noexcept
    {
        return *aperture < *content;
    }
 
    void draw(draw_context const& context) noexcept override
    {
        if (*mode > widget_mode::invisible and overlaps(context, layout()) and visible()) {
            draw_rails(context);
            draw_slider(context);
        }
    }
 
    hitbox hitbox_test(point2i position) const noexcept override
    {
        hi_axiom(loop::main().on_thread());
 
        if (*mode >= widget_mode::partial and layout().contains(position) and visible() and
            _slider_rectangle.contains(position)) {
            return {this, _layout.elevation, hitbox_type::scroll_bar};
        } else {
            return {};
        }
    }
 
    bool handle_event(gui_event const& event) noexcept
    {
        switch (event.type()) {
        case gui_event_type::mouse_down:
            if (event.mouse().cause.left_button) {
                // Record the original scroll-position before the drag starts.
                _offset_before_drag = *offset;
                return true;
            }
            break;
 
        case gui_event_type::mouse_drag:
            if (event.mouse().cause.left_button) {
                // The distance the slider has to move relative to the slider position at the
                // start of the drag.
                hilet slider_movement =
                    narrow_cast<int>(axis == axis::vertical ? event.drag_delta().y() : event.drag_delta().x());
                hilet content_movement = narrow_cast<int>(std::round(slider_movement * hidden_content_vs_travel_ratio()));
                hilet new_offset = _offset_before_drag + content_movement;
                offset = clamp_offset(new_offset);
                return true;
            }
            break;
 
        default:;
        }
 
        return super::handle_event(event);
    }
 
    [[nodiscard]] bool accepts_keyboard_focus(keyboard_focus_group group) const noexcept override
    {
        return false;
    }
 
    [[nodiscard]] color background_color() const noexcept override
    {
        return theme().color(semantic_color::fill, semantic_layer);
    }
 
    [[nodiscard]] color foreground_color() const noexcept override
    {
        if (*hover) {
            return theme().color(semantic_color::fill, semantic_layer + 2);
        } else {
            return theme().color(semantic_color::fill, semantic_layer + 1);
        }
    }
 
private:
    aarectanglei _slider_rectangle;
 
    int _offset_before_drag;
 
    typename decltype(content)::callback_token _content_cbt;
    typename decltype(aperture)::callback_token _aperture_cbt;
    typename decltype(offset)::callback_token _offset_cbt;
 
    [[nodiscard]] int clamp_offset(int new_offset) const noexcept
    {
        hilet scrollable_distance = std::max(0, *content - *aperture);
        return std::clamp(new_offset, 0, scrollable_distance);
    }
 
    [[nodiscard]] int rail_length() const noexcept
    {
        hi_axiom(loop::main().on_thread());
        return axis == axis::vertical ? layout().height() : layout().width();
    }
 
    [[nodiscard]] int slider_length() const noexcept
    {
        hi_axiom(loop::main().on_thread());
 
        hilet preferred_length = [&] {
            if (*content == 0) {
                return rail_length();
            } else {
                return *aperture * rail_length() / *content;
            }
        }();
 
        return std::clamp(preferred_length, narrow_cast<int>(theme().size) * 2, rail_length());
    }
 
    [[nodiscard]] int slider_travel_range() const noexcept
    {
        hi_axiom(loop::main().on_thread());
        return rail_length() - slider_length();
    }
 
    [[nodiscard]] int hidden_content() const noexcept
    {
        hi_axiom(loop::main().on_thread());
        return *content - *aperture;
    }
 
    [[nodiscard]] float hidden_content_vs_travel_ratio() const noexcept
    {
        hi_axiom(loop::main().on_thread());
 
        hilet _slider_travel_range = slider_travel_range();
        return _slider_travel_range != 0 ? narrow_cast<float>(hidden_content()) / _slider_travel_range : 0.0f;
    }
 
    [[nodiscard]] float travel_vs_hidden_content_ratio() const noexcept
    {
        hi_axiom(loop::main().on_thread());
 
        hilet _hidden_content = hidden_content();
        return _hidden_content != 0 ? narrow_cast<float>(slider_travel_range()) / _hidden_content : 0.0f;
    }
 
    void draw_rails(draw_context const& context) noexcept
    {
        hilet corner_radii =
            axis == axis::vertical ? hi::corner_radii{layout().width() * 0.5f} : hi::corner_radii{layout().height() * 0.5f};
        context.draw_box(layout(), layout().rectangle(), background_color(), corner_radii);
    }
 
    void draw_slider(draw_context const& context) noexcept
    {
        hilet corner_radii = axis == axis::vertical ? hi::corner_radii{narrow_cast<float>(_slider_rectangle.width() / 2)} :
                                                      hi::corner_radii{narrow_cast<float>(_slider_rectangle.height() / 2)};
 
        context.draw_box(
            layout(), translate_z(0.1f) * narrow_cast<aarectangle>(_slider_rectangle), foreground_color(), corner_radii);
    }
};
 
}} // namespace hi::v1