Rust VST part 4: creating a GUI

2022 - Feb 27 7 minute read

So far, we've been controlling our plugin through the dials and knobs our host provides. However, most commercial audio plugins also include a custom GUI. In this chapter, we will create a simple user interface with egui and baseview related crates.

Setting up logging

Before we start, it may be useful to enable logging. VST plugins don't do anything with print statements (e.g. println!()). Instead, we must log messages to a file.

Because of Rust and fundsp designs, the risk of introducing a panic within our process block is slim. Yet, different systems and hosts can react unpredictably, and logs are a useful diagnostic. To capture logs, we'll be adding a few more crates:

# ..
log = "0.4"
simplelog = "0.11"
log-panics = "2"

In, implement the init method of the Plugin trait to set up file logging:

impl Plugin for Synthy {
    // ...
    fn init(&mut self) {
        // Set up logs, adapted from code from DGriffin91
        // MIT:
        let Info {
        } = self.get_info();
        let home = dirs::home_dir().unwrap().join("tmp");
        let id_string = format!("{name}-{version}-{unique_id}-log.txt");
        let log_file = std::fs::File::create(home.join(id_string)).unwrap();
        let log_config = ::simplelog::ConfigBuilder::new()
        simplelog::WriteLogger::init(simplelog::LevelFilter::Info, log_config, log_file).ok();
    // ...

This code will set up a file using the plugin's name, version, and ID to which to write logs. log_panics lets us capture any panic messages, too. We find the current home directory using dirs, and log to a subfolder named tmp, as simply adding a log adjacent to the plugin binary may result in issues 1. To test that this works, cargo build --release like usual, and load up the plugin in your host. If you open ~/tmp/synthy-{bunchofnumbers}-log.txt, it should read something like:

05:36:50 [INFO] init
05:36:50 [INFO] Host is asking if plugin can: ReceiveMidiEvent.
05:36:50 [INFO] Host is asking if plugin can: SendMidiEvent.
05:36:50 [INFO] Host is asking if plugin can: ReceiveSysExEvent.
05:36:50 [INFO] Host is asking if plugin can: Other("MPE").

Everything looks good (except for some CanDos that we haven't handled. But that's fine.) We now have logging, and can check if the plugin has issues loading or is crashing.

Getting a UI to show

We need to bring in a few more crates to get our UI working. Note the specific required rev, which matches the version used in egui-baseview.

# ...
egui = "0.15"
egui-baseview = {git = "" }
baseview = { git = "", rev = "f6e99e9aa6f5aeb6b721cb05e4d882a51d995909" }
raw-window-handle = "0.3"
# ...

Don't worry if you're not 100% sure what's going on in the next few code blocks. This is plumbing to get our UI to show up. Once it's set up, we won't need to touch a lot of this code again.

Create a new file called Add a mod editor; line somewhere in to include the new editor module. Initialize the file with the following code 2. Note that our code will not compile for the next few code blocks.

use crate::Parameters;
use baseview::*;
use egui::*;
use egui_baseview::*;
use std::sync::Arc;
use vst::{editor::Editor, plugin::PluginParameters};

// ------------------ //
// 1. Setting UI size //
// ------------------ //
const WINDOW_WIDTH: usize = 256;
const WINDOW_HEIGHT: usize = 256;

// --------------------------------- //
// 2. Creating `PluginEditor` struct //
// --------------------------------- //
pub struct PluginEditor {
    pub params: Arc<Parameters>,
    pub window_handle: Option<WindowParent>,
    pub is_open: bool,

// ------------------------ //
// 3. Implementing `Editor` //
// ------------------------ //
impl Editor for PluginEditor {
    fn position(&self) -> (i32, i32) {
        (0, 0)

    fn size(&self) -> (i32, i32) {
        (WINDOW_WIDTH as i32, WINDOW_HEIGHT as i32)

    fn is_open(&mut self) -> bool {

    fn close(&mut self) {
        self.is_open = false;
        if let Some(mut window_handle) = self.window_handle.take() {

    fn open(&mut self, parent: *mut ::std::ffi::c_void) -> bool {
        log::info!("Editor open");
        match self.is_open {
            true => false,
            false => {
                // ---------------------------- //
                // 4. Setting up `egui` for use //
                // ---------------------------- //
                self.is_open = true;
                let settings = Settings {
                    window: WindowOpenOptions {
                        title: String::from("synthy"),
                        size: Size::new(WINDOW_WIDTH as f64, WINDOW_HEIGHT as f64),
                        scale: WindowScalePolicy::SystemScaleFactor,
                    render_settings: RenderSettings::default(),

                let window_handle = EguiWindow::open_parented(
                    |_egui_ctx, _queue, _state| {},
                    |egui_ctx: &CtxRef, _, state: &mut Arc<Parameters>| {
                        draw_ui(egui_ctx, state);

                self.window_handle = Some(WindowParent(window_handle));

// ---------------------------- //
// 4. Wrapper types boilerplate //
// ---------------------------- //
use raw_window_handle::{HasRawWindowHandle, RawWindowHandle};

struct VstParent(*mut ::std::ffi::c_void);
unsafe impl Send for VstParent {}

pub struct WindowParent(pub WindowHandle);
unsafe impl Send for WindowParent {}

Most of this code is boilerplate, so let's cover the important parts only:

1. Setting UI size

2. Creating PluginEditor struct

3. Implementing Editor

4. Setting up egui for use

5. Wrapper types boilerplate

Because our Plugin must implement Send, we wrap a few needed pointers in newtypes. We then implement Send manually on the newtypes.

Right below that block, add the following boilerplate code for platform specific windowing:

// ...
#[cfg(target_os = "macos")]
unsafe impl HasRawWindowHandle for VstParent {
    fn raw_window_handle(&self) -> RawWindowHandle {
        use raw_window_handle::macos::MacOSHandle;

        RawWindowHandle::MacOS(MacOSHandle {
            ns_view: self.0 as *mut ::std::ffi::c_void,

#[cfg(target_os = "windows")]
unsafe impl HasRawWindowHandle for VstParent {
    fn raw_window_handle(&self) -> RawWindowHandle {
        use raw_window_handle::windows::WindowsHandle;

        RawWindowHandle::Windows(WindowsHandle {
            hwnd: self.0,

#[cfg(target_os = "linux")]
unsafe impl HasRawWindowHandle for VstParent {
    fn raw_window_handle(&self) -> RawWindowHandle {
        use raw_window_handle::unix::XcbHandle;

        RawWindowHandle::Xcb(XcbHandle {
            window: self.0 as u32,

Adjusting our to support the editor

We still need to make a few more changes in our file to enable showing the editor. Add the following field on our Synthy struct to keep track of our editor:

struct Synthy {
    audio: Box<dyn AudioUnit64 + Send>,
    sample_rate: f32,
    parameters: Arc<Parameters>,
    time: Duration,
    note: Option<(Note, Velocity)>,
    enabled: bool,
    // New field
    editor: Option<editor::PluginEditor>,

Modify the Plugin::new method implemented on Synthy to return the following struct:

// Plugin::new
// ...
let params: Arc<Parameters> = Arc::new(Default::default());
Self {
    audio: Box::new(audio_graph) as Box<dyn AudioUnit64 + Send>,
    parameters: params.clone(),
    note: None,
    time: Duration::default(),
    sample_rate: 41_000f32,
    enabled: false,
    editor: Some(editor::PluginEditor {
        window_handle: None,
        is_open: false,
// ...

Lastly, implement the Plugin::get_editor method on Synthy:

// Plugin
// ...
fn get_editor(&mut self) -> Option<Box<dyn vst::editor::Editor>> {
    if let Some(editor) = self.editor.take() {
        Some(Box::new(editor) as Box<dyn vst::editor::Editor>)
    } else {

We can finally create the draw_ui function and begin creating a UI, and get our code to compile.

Using egui to create a user interface

The hard, boring part is over! Time to create some cool UIs. Create a new function in your file titled draw_ui with the following parameters. Remember that we called this draw_ui function earlier in our Editor::open method.

fn draw_ui(ctx: &CtxRef, params: &mut Arc<Parameters>) -> egui::Response {
    egui::CentralPanel::default().show(ctx, |ui| {
        ui.vertical(|ui| {
            ui.label("hello rust");
                "Modulation: {}",
                params.get_parameter(crate::Parameter::Modulation as i32)

We use a CentralPanel as a base canvas for all our UI content. We then want to show two text labels, so we use vertical to position a few labels. In our second label, we get the current value of Parameter::Modulation and display it. If we want to modify our UI, we will revisit the draw_ui function. Almost everything else we've done so far is boilerplate that won't change much.

egui built-in widgets

Setting parameters



Keep in mind that many Windows users store plugins in C:\Program Files (x86)\VstPlugins which has strict permissions. In my testing, if the folder specified was incorrect, my host failed to even list the plugin as available.


It is important to mention the majority of the initialization is adapted from egui_baseview_test_vst2 by DGriffin91, licensed under MIT.