Getting started

This guide takes you from an empty directory to a running Day app. You will install the CLI, choose one platform, and edit the app’s first screen. You need a terminal and a text editor; the examples assume you can read basic Rust.

1. Install the CLI

Install Rust through rustup and Git, then install Day’s command-line tool:

Check the installation and available toolchains:

day --version
day doctor

day doctor reports missing tools. You only need to install the tools for your chosen target. See system requirements for supported versions and installation details.

2. Choose a platform and run your app

Select your development computer and the platform you want to run on. Install the listed requirements, then run the commands in a terminal. The first build downloads and compiles dependencies, so allow more time than for later builds.

day new app hello --toolkit macos-appkit --appid com.example.hello
cd hello
day launch -p macos-appkit

day new app hello --toolkit ios-uikit --appid com.example.hello
cd hello
day launch -p ios-uikit

day new app hello --toolkit android-mdc --appid com.example.hello
cd hello
day launch -p android-mdc

day new app hello --toolkit linux-gtk --appid com.example.hello
cd hello
day launch -p linux-gtk

day new app hello --toolkit linux-qt --appid com.example.hello
cd hello
day launch -p linux-qt

day new app hello --toolkit windows-xaml --appid com.example.hello
cd hello
day launch -p windows-xaml

day new app hello --toolkit harmony-arkui --appid com.example.hello
cd hello
day launch -p harmony-arkui

day new app hello --toolkit web-dom --appid com.example.hello
cd hello
day launch -p web-dom

The selector covers the main targets. GTK and Qt also run on macOS and Windows for development; see all platform combinations for those options.

The generated project contains a Day.toml app manifest, Cargo dependencies pinned to a Day release, and platform files for your chosen toolkit. Open src/lib.rs to find root(), the function that defines your app’s interface. Project structure explains the other files.

If the app does not launch, use the troubleshooting guide, run day doctor again and check the target’s setup instructions above. Mobile targets need a running emulator, simulator, or connected device as described there.

3. Make your first change

Open src/lib.rs. The scaffold’s root() returns the whole UI; replace it with:

pub fn root() -> impl Piece {
    column((
        label("Hello from my first edit"),
        button("Tap me").action(|| println!("tapped")),
    ))
    .spacing(12.0)
    .padding(16.0)
}

Run the day launch command for your selected target again to see the change. Pieces explains how column, label, and button compose the interface.

4. Make the screen respond

Replace root() again to give the button something visible to do:

pub fn root() -> impl Piece {
    let count = Signal::new(0i64);

    column((
        label(move || format!("Count: {}", count.get())),
        button("Add one").action(move || count.update(|n| *n += 1)),
    ))
    .spacing(12.0)
    .padding(16.0)
}

Rebuild and launch. Pressing Add one should change the count on screen. The Signal holds the value, and the label’s closure reads it whenever it changes. Keep the read inside the closure: label(count.get().to_string()) would display only the initial value.

Optional: add another target

Once the app works on your first platform, install another target’s tools and add it to the project:

day project add-target web-dom
day launch -p web-dom

This example adds the experimental web target. Use a different target name if you prefer. Your UI code is shared, but platform capabilities differ; test the features your app uses on each target you plan to ship.

Optional: editor integration

The Day VS Code extension adds project creation, build and run controls, and diagnostics to the editor. You can also use AI-assisted development to edit and test your app. Neither is required to follow this guide.

Where to go next

• Reactivity explains how state changes update the UI.
• Layout covers arranging and sizing controls.
• Gallery connects sample app screenshots to source code and related guides.
• CLI & projects documents build, run, and project commands.