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imperative-to-declarative-flet

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Convert an imperative Flet Python app in which controls are mutated and then page.update is called to declarative style using flet.component, flet.observable and state hooks.

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Imperative → Declarative (Flet Python)

Port an existing imperative Flet app to “components mode” with @ft.component, hooks, and (optionally) @ft.observable models.

Outcomes

  • Original folder unchanged; new sibling folder created.
  • Entry point uses page.render(App) (components mode).
  • UI is derived from state; minimal/zero manual page.update() for normal UI updates.

Choose a state strategy

  • Use @ft.observable for nested app data you mutate in place (boards/lists/cards, drag+drop reorder).
  • Use ft.use_state for local ephemeral UI state (hover flags, input text, dialog selection).
  • Avoid storing live Control/component objects in state; store ids/enums and create controls during render.

Workflow

1) Create a new declarative copy

  • Copy the existing app folder to a new one (e.g. trolli → trolli-declarative-*).
  • Keep assets/ with the new folder.

2) Switch to components mode entrypoint

  • Create a root component @ft.component def App(): ....
  • Run with:
    • ft.run(lambda page: page.render(App), assets_dir=...)
  • Set page globals either:
    • in a main(page) function before page.render(App), or
    • in ft.on_mounted(...) (works, but ordering can be less obvious).

3) Centralize routing in App

  • Keep app.route: str as source of truth.
  • In one place, define:
    • route_change(e) to normalize/redirect/validate routes and set app.route
    • a render-time match app.route (or a derived active_screen) to pick content
  • Prefer: route_change mutates route state; App() render chooses UI based on that state.

4) Componentize UI

  • Move UI chunks into src/components/*.py.
  • Each component:
    • takes only the model(s) it needs
    • uses hooks for local UI state
    • mutates observable models for app data changes

5) Dialogs: reduce page.update() usage

If a dialog mutates existing controls and calls page.update(), convert it to:

  • @ft.component dialog content with ft.use_state for error, selected_color, etc.
  • event handlers call setters (no explicit updates)
  • show it via page.show_dialog(ft.AlertDialog(content=DialogContent(...)))

6) Assets/fonts checklist

  • Prefer an absolute assets_dir derived from __file__.
  • If running via flet run, be aware it can set FLET_ASSETS_DIR and override assets_dir=.
  • Font registration uses the dict key as the font family name:
    • page.fonts = {"Pacifico": "Pacifico-Regular.ttf"}
    • use font_family="Pacifico".

Common pitfalls (and fixes)

  • Event handler typing is invariant (Event[Sub] ≠ Event[Base]):
    • If on_click is declared on Button, annotate e as ft.Event[ft.Button] (or use def handler(): ...).
  • TemplateRoute params are dynamic:
    • raw = getattr(troute, "id", None) then isinstance(raw, str) before int(raw).
  • controls=[*[...], ...] can confuse type checkers with components:
    • build lists in two steps and annotate/cast, or return raw controls from non-component factories.