plotter/adapters/imgui/imgui_plot.hpp

#pragma once

#include <optional>

#include <plot/core/rect.hpp>
#include <plot/input/controller.hpp>
#include <plot/math/vec2.hpp>
#include <plot/plot/plot.hpp>
#include <plot/render/renderer.hpp>

namespace plot::imgui {

/// Embeds a `Plot` in a Dear ImGui window as a child region, and drives that plot
/// from ImGui's own input state.
///
/// `draw()` opens a child window of the requested size, translates its screen
/// rectangle into framebuffer pixels, hands that rectangle to
/// `Renderer::begin_frame()` and draws the plot into it: a scissor- and
/// viewport-bounded sub-rectangle of the very framebuffer ImGui is about to draw
/// its chrome into, not an offscreen texture. Nothing is rendered through a
/// texture or an FBO, and ImGui's frame is left undisturbed -- the renderer
/// clears only its own rect and restores the GL state it changed, and ImGui's
/// backend re-establishes its viewport and blend state before its first draw.
///
/// Two pixel spaces meet here, exactly as they do in the GLFW adapter. ImGui
/// lays out and reports in logical *points* -- `io.DisplaySize`, the cursor's
/// screen position, a child's content region -- while this library speaks
/// *framebuffer pixels*. The bridge is `io.DisplayFramebufferScale`, which the
/// platform backend fills in from the window and framebuffer sizes; on an
/// unscaled display it is (1, 1) and nothing changes.
///
/// ## The coordinate assumption
///
/// Points -> pixels through `io.DisplayFramebufferScale` is *the whole
/// framebuffer's* mapping only when both of these hold:
///
///   * the GL viewport the host is drawing its frame with is the whole
///     framebuffer -- origin (0, 0), size `DisplaySize * DisplayFramebufferScale`
///     -- and
///   * the bound draw target is the default framebuffer, not an offscreen one.
///
/// Under that assumption -- the one every ImGui platform backend documents, and
/// what a window that spans its framebuffer gives -- the child's point rectangle
/// is the pixel rectangle the plot is laid out and scissored into, and a gesture
/// lands on the pixel it was aimed at. A host that renders into an FBO, or with
/// a viewport that is not the whole framebuffer (a tiled compositor, a scaled
/// surface, a pass into a texture), breaks the assumption. This widget cannot
/// detect it: it has no view of the host's viewport and no way to ask. Such a
/// host must place the plot itself -- `Renderer::begin_frame()` takes the
/// framebuffer size and the plot's rect directly, so it needs no widget -- or
/// apply its own points -> pixels mapping on top of `plot.set_viewport()`.
///
/// ## What the widget draws over
///
/// The widget renders *immediately*, from inside the child, before ImGui renders
/// its draw data (see the frame order in the demo). It therefore writes the
/// child's pixels before every ImGui command of that frame, and ImGui has no way
/// to be told the child is opaque. The host must not paint over the child's
/// rectangle:
///
///   * the hosting window must not fill its background across the child -- use
///     `ImGui::SetNextWindowBgAlpha(0.0f)` or `ImGuiWindowFlags_NoBackground`
///     (`ImGuiCol_WindowBg`'s default alpha would otherwise dim the plot), and
///   * no other window may overlap the rectangle with opaque content.
///
/// The child's own background is forced transparent by the widget, whatever the
/// host's `ImGuiCol_ChildBg`, so the widget itself never paints over the plot.
///
/// ## Input
///
/// Pointer, wheel and key state are translated into `plot::input` events and
/// delivered to the plot's `Controller` -- the one the widget owns, reachable
/// through `controller()`. ImGui reports levels (`io.MouseDown`) where the
/// controller wants edges (`PointerEvent::pressed`/`released`), so the widget
/// remembers the previous frame's buttons and synthesises the transitions.
///
/// Events are forwarded while an invisible item covering the child's region is
/// hovered -- `IsItemHovered()` rather than `IsWindowHovered()`, because the
/// item is the child's whole rectangle (the child has no padding) and its hover
/// is false while the host is dragging some other widget, so dragging a slider
/// next to the plot cannot pan it. A press that started over the child keeps
/// being delivered until every button is up again, so a drag that leaves the
/// region still ends instead of leaving the plot mid-pan.
///
/// Wheel input goes to the plot while it is hovered, and the plot's zoom wins
/// over ImGui's window scrolling: `SetItemKeyOwner(ImGuiKey_MouseWheelY)` on that
/// invisible item makes ImGui withhold the wheel from the hosting window. The
/// claim takes effect from the frame after it is made, because ImGui decides
/// window scrolling in its own `NewFrame()` before any widget runs -- so a
/// hosting window with a scroll range can still scroll on the very first wheel of
/// a hover that has only just started. (The child's
/// `ImGuiWindowFlags_NoScrollWithMouse` would not achieve this on its own: ImGui
/// deliberately bubbles the wheel up to the parent when a child refuses it.)
/// ImGui consumes `io.MouseWheel` itself at the end of the frame, so a widget
/// cannot consume it any later than this.
///
/// Not thread-safe: like everything else in the render thread's frame.
class PlotWidget {
 public:
  PlotWidget() noexcept = default;
  PlotWidget(const PlotWidget&) = delete;
  PlotWidget& operator=(const PlotWidget&) = delete;

  /// Allocates a child region of `size` (ImGui points) identified by `id`, and
  /// renders `plot` into the framebuffer pixels it covers with `renderer`, which
  /// must already be initialised on the current context.
  ///
  /// Must be called between `ImGui::NewFrame()` and `ImGui::Render()`, from a
  /// window; the plot is drawn (GL calls issued) during the call, inside the
  /// child, so the host's later ImGui commands never have to restore anything.
  ///
  /// The plot's viewport is the child's content rectangle, so it fills the child
  /// exactly; the widget sets it every frame through the controller, and the plot
  /// is re-laid out to match. A degenerate region (a collapsed or clipped child)
  /// draws nothing and forwards nothing.
  void draw(const char* id, Plot& plot, Vec2 size, Renderer& renderer);

  /// The controller the widget feeds, or null before the first `draw()`. It is
  /// bound to whichever plot the last `draw()` was given; a `draw()` for a
  /// different plot re-binds it and drops any drag in progress.
  input::Controller* controller() noexcept { return controller_ ? &*controller_ : nullptr; }
  const input::Controller* controller() const noexcept {
    return controller_ ? &*controller_ : nullptr;
  }

 private:
  /// Synthesises this frame's `plot::input` events from ImGui's levels.
  void forward_input(bool hovered);

  /// Forgets the previous frame's pointer state, so a re-bound plot cannot see an
  /// edge that belongs to the old one.
  void reset_pointer() noexcept;

  Plot* plot_ = nullptr;
  std::optional<input::Controller> controller_;

  /// The previous frame's button levels, in ImGui's order (left, right, middle).
  bool prev_down_[3] = {false, false, false};
  f32 prev_x_ = 0.0f;  ///< previous pointer position, in framebuffer pixels
  f32 prev_y_ = 0.0f;
  bool have_prev_pos_ = false;
  /// True while a button pressed over the child is still down: keeps the release
  /// flowing even after the pointer has left the region.
  bool capturing_ = false;
};

}  // namespace plot::imgui