A new realm appears on the horizon, waiting for you with plenty of new features to discover! I’m exited to announce Relm4 v0.5, by far our biggest release so far.

About Relm4

Relm4 is an idiomatic GUI library inspired by Elm and based on gtk4-rs.

We believe that GUI development should be easy, productive and delightful.
The gtk4-rs crate already provides everything you need to write modern, beautiful and cross-platform applications. Built on top of this foundation, Relm4 makes developing more idiomatic, simpler and faster and enables you to become productive in just a few hours.

What’s new?

Relm4 v0.5 features some fundamental changes that greatly simplify the API and solve some longstanding issues. More than half a year ago, Michael Murphy from System76 started off the development of 0.5 with some great ideas to improve the structure of Relm4 applications. Over time, more developers joined the effort to redesign and refine Relm4’s API. Now - almost 300 commits later - I’m proud to present the results of our hard work.

Fewer traits, more fun!

So far, Relm4 had several traits for different use-cases. In total, 7 traits were unified into one interface, the Component trait. Furthermore, the new trait covers not only all previous use-cases, it even adds some features on top.

Maximum flexibility

In version 0.4, Relm4’s macros made Relm4 more attractive for gtk4-rs based applications. I’m happy this trend continues in version 0.5. You can now mix Relm4 into any gtk-rs application and the other way around with hardly any limitations.


Commands are a concept from the Elm programming language that is often used to run web-requests in the background without blocking other UI updates. With Relm4, you now can take full advantage of Rust’s async ecosystem to run asynchronous tasks in the background without blocking your application.

// Add a new command future to be executed in the backgroud
sender.command(|out, shutdown| {
    // Cancel the future if the component is shut down in the meantime
    shutdown.register(async move {
        // Wait for one second
        // Emit a delayed increment event

Seamless communication

Many applications written in Relm4 rely upon communication between components. Often one message needs to be forwarded to another component, e.g. to show a dialog. Previously, this required forwarding the message from the application logic. Now you can simply register a closure to modify and forward messages.

let component = MyComponent::builder()
    // Start the component service with an initial parameter
    .launch("Hello world")
    // Attach the returned receiver's messages to this closure.
    .connect_receiver(move |sender, message| match message {
        // Transform and forward the message
        Output::HelloThere => {
            sender.send(Input::Answer("General Kenobi!"))
        _ => (),

Macroscopic macro improvements

The view! macro has been largely rewritten, making the codebase smaller, faster and more maintainable. Additionally, the macro gained a lot of exiting features and can continue the code generation even if it encounters errors.

Rusty blueprints

With 0.5, the macro syntax was cleaned up and became even easier. By pure coincidence, it looks very similar to the Blueprint language that’s specifically designed for GTK4 UIs.

Yet, Relm4 has more to offer than just creating UIs. It integrates well into the surrounding Rust code by allowing you to update values automatically, to use local variables and to connect message handlers directly with the UI declaration. You can even use match and if conditions to show widgets depending upon certain conditions.

// Create a new container
gtk::Box {
    // Arrange widgets vertically
    set_orientation: Vertical,

    // Watch `counter.value` to make the box invisible
    // if the value reaches 42
    set_visible: counter.value != 42,

    // Append a new label
    gtk::Label {
        set_label: "Hello world!",

    // Append a new label using a builder pattern
        .label("Builder pattern works!")

    // Use different widgets depending on a pattern
    // and show a transition in between
    match counter.value {
        (0..=2) => {
            // First match arm: show a button
            gtk::Button {
                set_label: "Value is smaller than 3, click to increment it!",
                connect_clicked[sender] => move |_| {
        _ => {
            // Second match arm: show a label
            gtk::Label {
                set_label: "Value is higher than 2",

Further features added in this release include:

  • Support for multiple top-level widgets
  • Support for adding widgets from local variables
  • Automatically block signals while updating values

Fabulous factories

Along with components, factories saw a big update. The new FactoryComponent trait is very similar to Component and allows every entry of a factory to manage its own state.

The updated FactoryVecDeque type comes extremely close to the goal of making collections of UI elements just as accessible as regular data collections. With implementations for Index and IndexMut as well as most methods from VecDeque, you’ll hardly notice you’re actually working with UI elements. Even external updates, e.g. when a user swaps two tabs, are now automatically synchronized with the order of elements in the FactoryVecDeque. A new RAII-guard completes the list of improvements and makes it impossible to forget to render the changes after modifying the data.

let mut guard = factory.guard();
guard.swap(1, 2);
let last_value = guard.pop_back().unwrap();
println!("Value of the first element {}", guard[0]);
// Changes are rendered as soon as the scope ends

Let’s see the new factories in action! The new tab_game example takes advantage of the new features to build a small game.

Try it out!

Try out the new beta by adding the following lines to your project’s Cargo.toml file:

relm4 = { git = "https://github.com/Relm4/Relm4", tag = "v0.5.0-beta.1" }

Yet, keep in mind that this is still a beta, so some things might be slightly adjusted before the stable release. Also, the book isn’t fully ported to v0.5, but the examples in the repository are always up to date.

Remaining tasks

We’re looking forward to release 0.5 as stable soon. However, there are still a few things left to do. Feedback and contributions are highly appreciated!

  • Polish some rough edges in the API
  • Finish updating the book
  • Port more examples to 0.5
  • Port the rest of relm4-components to 0.5

Where to get started

Special thanks

We highly appreciate feedback and contributions to Relm4. In the name of all members of the Relm4 organization I thank those who helped with this release:

  • Michael Murphy for coming up with several brilliant ideas and contributing most of the component rework.
  • Maksym Shcherbak for his ongoing and outstanding contributions around factories and other parts of Relm4.
  • Andy Russell for contributing improvements all across Relm4.
  • Eduardo Flores for joining the discussions and porting the book to 0.5.
  • Everyone else who contributed or gave feedback.
  • The whole gtk-rs team for providing awesome Rust bindings for GTK and always being helpful.