Cobalt SDK

Create an application

Install the CLI, create an application and start the browser simulator:

cargo install --path crates/kobo-cli
kobo new my-app
cd my-app
kobo dev

Implement KoboApp, render the current state and handle named actions:

use kobo_sdk::{action_id, ActionId, Context, KoboApp, ScreenBuilder};

#[derive(Default)]
struct Counter { taps: u32 }

impl KoboApp for Counter {
    fn on_start(&mut self, context: &mut Context) {
        self.show(context);
    }

    fn on_action(&mut self, context: &mut Context, action: ActionId) {
        if action == action_id("tap") {
            self.taps += 1;
        }
        self.show(context);
    }
}

impl Counter {
    fn show(&self, context: &mut Context) {
        context.set_screen(
            ScreenBuilder::new("counter")
                .top_bar("Counter")
                .heading(format!("{} taps", self.taps))
                .primary_button("tap", "Tap")
                .build(),
        );
    }
}

fn main() {
    let _ = kobo_sdk::run("counter", Counter::default());
}

Application callbacks

The runtime delivers events to one KoboApp. Rebuild the screen after state changes. Do not block callbacks or retain renderer objects.

Callbacks have a 250 ms deadline. Hand slow work to Context::spawn and render an activity or skeleton state while it runs.

UI components

ScreenBuilder provides 94 component and configuration methods. The renderer handles physical size, wrapping, grayscale tone, touch geometry and refresh strategy.

Runtime services

Capabilities

Capabilities use exact manifest names. Device results distinguish undeclared, battery-withheld and unsupported operations.

App launching, catalog management and platform updates are authorized by caller identity rather than by public manifest capabilities.

Application-facing crates

Use kobo-sdk for the application API. Add companion crates for specific data formats and reading surfaces.

CLI commands

The CLI creates and tests applications, manages developer devices, and builds Store artifacts. Commands accepting --device also accept -s.

Build and exercise applications

Work with a developer device

Credentials, trust and Store releases

Testing and shipping

# Browser simulator from an app directory
kobo dev

# Host runtime
cargo run -p kobo-cli -- run --sim --app my-app

# Drive taps, typing, expectations and screenshots
cargo run -p kobo-cli -- drive --script tour.kobo --shots target/shots

# Targeted application tests
cargo test -p kobo-my-app

# Package a device build
cargo run -p kobo-cli -- package

The simulator uses the production layout, renderer, hit testing and refresh planner. Failure scenarios cover offline, timeout, low battery, denied capability, missing credentials, storage pressure and image-cache pressure.

Platform constraints