Add an egui editor adapter

nih_plug_egui/Cargo.toml

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+[package]
+name = "nih_plug_egui"
+version = "0.0.0"
+edition = "2021"
+authors = ["Robbert van der Helm <mail@robbertvanderhelm.nl>"]
+license = "GPL-3.0-or-later"
+
+description = "An adapter to use egui GUIs with NIH-plug"
+
+[dependencies]
+nih_plug = { path = ".." }
+
+baseview = { git = "https://github.com/RustAudio/baseview.git" }
+crossbeam = "0.8"
+egui = "0.16"
+# Upstream doesn't work with the current baseview and egui versions
+egui-baseview = { git = "https://github.com/robbert-vdh/egui-baseview.git", branch = "fix/update-dependencies" }

nih_plug_egui/README.md

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+# NIH-plug: egui support
+
+This provides an adapter to create [egui](https://github.com/emilk/egui) based
+GUIs with NIH-plug through
+[egui-baseview](https://github.com/BillyDM/egui-baseview).

nih_plug_egui/src/lib.rs

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+// nih-plug: plugins, but rewritten in Rust
+// Copyright (C) 2022 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program 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 General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+//! [egui](https://github.com/emilk/egui) editor support for NIH plug.
+//!
+//! TODO: Proper usage example
+
+use baseview::{Size, WindowHandle, WindowOpenOptions, WindowScalePolicy};
+use egui::CtxRef;
+use egui_baseview::{EguiWindow, Queue, RenderSettings, Settings};
+use nih_plug::{Editor, EditorWindowHandle, GuiContext};
+use std::sync::Arc;
+
+/// Re-export for convenience.
+pub use crossbeam::atomic::AtomicCell;
+
+/// Create an [Editor] instance using an [::egui] GUI. The size passed to this function is the GUI's
+/// intiial size, and this is kept in sync whenever the GUI gets resized. You should return the same
+/// size value in your plugin' [nih_plug::Plugin::editor_size()] implementation..
+//
+// TODO: Figure out if the build function and [Queue] things we're omitting now are actually useful
+//       to the user
+// TODO: Provide 'advanced' versions that expose more of the low level settings and details here
+// TODO: DPI scaling, this needs to be implemented on the framework level
+pub fn create_egui_editor<'context, T, U>(
+    parent: EditorWindowHandle,
+    context: Arc<dyn GuiContext + 'context>,
+    size: Arc<AtomicCell<(u32, u32)>>,
+    initial_state: T,
+    mut update: U,
+) -> Option<Box<dyn Editor + 'context>>
+where
+    T: 'static + Send,
+    U: FnMut(&CtxRef, &dyn GuiContext, &mut T) + 'static + Send,
+{
+    // TODO: Also pass the context reference to the update callback
+    let (width, height) = size.load();
+    let window = EguiWindow::open_parented(
+        &parent,
+        Settings {
+            window: WindowOpenOptions {
+                title: String::from("egui window"),
+                size: Size::new(width as f64, height as f64),
+                // TODO: What happens when we use the system scale factor here? I'd assume this
+                //       would work everywhere, even if the window may be tiny in some cases.
+                scale: WindowScalePolicy::ScaleFactor(1.0),
+            },
+            render_settings: RenderSettings {
+                version: (3, 2),
+                red_bits: 8,
+                blue_bits: 8,
+                green_bits: 8,
+                // If the window was not created with the correct visual, then specifying 8 bits
+                // here will cause creating the context to fail
+                alpha_bits: 0,
+                depth_bits: 24,
+                stencil_bits: 8,
+                samples: None,
+                srgb: true,
+                double_buffer: true,
+                vsync: true,
+                ..Default::default()
+            },
+        },
+        initial_state,
+        |_, _, _| {},
+        move |ctx, _, state| update(ctx, context.as_ref(), state),
+    );
+
+    // There's no error handling here, so let's just pray it worked
+    if window.is_open() {
+        Some(Box::new(EguiEditor {
+            _window: window,
+            size,
+        }))
+    } else {
+        None
+    }
+}
+
+/// An [Editor] implementation that calls an egui draw loop.
+pub struct EguiEditor {
+    _window: WindowHandle,
+    size: Arc<AtomicCell<(u32, u32)>>,
+}
+
+/// The window handle enum stored within 'WindowHandle' contains raw pointers. Is there a way around
+/// having this requirement?
+unsafe impl Send for EguiEditor {}
+unsafe impl Sync for EguiEditor {}
+
+impl Editor for EguiEditor {
+    fn size(&self) -> (u32, u32) {
+        self.size.load()
+    }
+}