Your first app

A browser dashboard on a live counter that persists across restarts. One Nu tree, two Fabrics.

import nu


class Counter(nu.Shape):
    value: nu.kv.IntRef


class Dashboard(nu.ui.Page):
    count: nu.ui.TextRef


class App(nu.ui.Index):
    pages = nu.ui.Pages({"/": Dashboard})


app = nu.With(
    nu.kv.rocksdb_navigator(".dbcounter"),
    nu.ui.server(
        nu.kv.auto_flow_atomic(
            nu.ReactForever(
                Counter.value.on_change(),
                Dashboard.count.set(Counter.value),
            ),
        ),
    ),
    body=nu.kv.auto_flow_atomic(
        nu.IfDo(Counter.value.missing(), Counter.value.set(0))
        >> nu.ForeverDo(
            Counter.value.inc() >> nu.Delay(1.0),
        )
    ),
)


if __name__ == "__main__":
    import asyncio

    asyncio.run(nu.arun(app))

Set up Nu with the nu.ui Fabric first: see Install. Then save the code above as counter.py, run python counter.py, open the browser tab that appears. The counter ticks once a second; the dashboard mirrors it live. Kill it, run again, it picks up where it left off.

What happened

Two Shapes, two Fabrics. Counter holds one int on the kv Fabric. Dashboard holds one text label on the UI Fabric. App is the site index with one page mounted at /. Same declaration style, different Fabric.

One tree, two Fabrics. nu.With binds nu.kv.rocksdb_navigator (RocksDB storage under a Navigator) and a UI server. The server runs ReactForever — wake on Counter.value change, mirror it into Dashboard.count. The body seeds the counter if missing, then ticks it forever, one second between beats.

Composition. >> runs children left-to-right. ForeverDo loops its body forever. ReactForever subscribes to a change source and re-runs on every fire. auto_flow_atomic wraps writes in the minimum atomic region so you get transactions without spelling them out.

One tree, two Fabrics

Same operators, same brackets, same loops. The Fabric changes; the composition doesn't. Swap rocksdb_navigator(".dbcounter") for memory_navigator() and the counter resets on every restart. Drop the UI server and it becomes a headless script.