2. State and rebuilds

A live UI = state + a function from state to tree. The App holds the state, runs your view(app), and on a state change rebuilds and diffs — emitting patches through an apply_patches callback.

from dataclasses import dataclass

from tempest_core import App, Column, Text, Widget


@dataclass
class State:
    value: int = 0


emitted = []


def view(app: App[State]) -> Widget:
    return Column(children=[Text(content=f"Count: {app.state.value}", key="lbl")])


app = App(state=State(), view=view, apply_patches=emitted.append)  # (1)!
app.start()                                   # (2)!
app.set_state(lambda s: setattr(s, "value", 1))  # (3)!

1. apply_patches receives each tick's patch list. A renderer applies it; here we just collect.
2. start() builds the initial scene.
3. set_state mutates state and schedules a coalesced rebuild — several changes in one tick become a single diff.

The view is pure

view() only reads app.state and describes the UI. Changing state is the handlers' job, via set_state. The view never mutates anything.

Navigation is state too

The App owns a NavStack (app.nav): push / pop / replace / reset change the top route and schedule a rebuild — the view reads app.nav.top and draws the screen. No new IR node: changing routes is just the view producing a different tree.

Recap

• App(state, view, apply_patches) + start() starts the UI.
• set_state(mutator) schedules a coalesced rebuild → diff → patches.
• app.nav (push/pop/reset) is navigation as state.
• Next: styling.