Castella Core UI Development

Castella is a pure Python cross-platform UI framework for desktop (GLFW/SDL2), web (PyScript/Pyodide), and terminal (prompt-toolkit) applications. Write once, run everywhere with GPU-accelerated rendering via Skia.

When to use: "create a Castella app", "build a Castella UI", "Castella component", "add a button/input/text", "use reactive state", "layout with Row/Column", "change the theme", "handle click events", "preserve scroll position", "animate a widget"

Quick Start

Create a minimal Castella app:

from castella import App, Text
from castella.frame import Frame

App(Frame("Hello", 800, 600), Text("Hello, Castella!")).run()

Install and run:

uv sync --extra glfw   # Desktop with GLFW
uv run python app.py

Core Concepts

App and Frame

• Frame(title, width, height) - Window/container for the UI
• App(frame, widget) - Application entry point with .run()
• Frame auto-selects platform: GLFW (desktop), Web, or Terminal

from castella import App
from castella.frame import Frame

frame = Frame("My App", 800, 600)
app = App(frame, my_widget)
app.run()

Widgets

Base building blocks for UI elements:

Widget	Description	Key Methods
Text(content)	Display text	.font_size(n)
Button(label)	Clickable button	.on_click(handler)
Input(initial)	Single-line input	.on_change(handler)
MultilineInput(state)	Multi-line editor	.on_change(handler)
CheckBox(state)	Toggle checkbox	.on_change(handler)
Slider(state)	Range slider	.on_change(handler)
Image(path)	Local image	-
NetImage(url)	Remote image	-
Markdown(content)	Rich markdown	.on_link_click(handler)

Layout Containers

Arrange widgets hierarchically:

from castella import Column, Row, Box

# Vertical stack
Column(
    Text("Header"),
    Button("Click me"),
    Text("Footer"),
)

# Horizontal stack
Row(
    Button("Left"),
    Button("Right"),
)

# Overlapping (z-index support)
Box(
    main_content,
    modal_overlay.z_index(10),
)

Component Pattern

Build reactive UIs with the Component class:

from castella import Component, State, Column, Text, Button

class Counter(Component):
    def __init__(self):
        super().__init__()
        self._count = State(0)
        self._count.attach(self)  # Trigger view() on change

    def view(self):
        return Column(
            Text(f"Count: {self._count()}"),
            Button("+1").on_click(lambda _: self._count.set(self._count() + 1)),
        )

State Management

State[T] is an observable value that triggers UI rebuilds:

from castella import State

count = State(0)           # Create with initial value
value = count()            # Read current value
count.set(42)              # Set new value
count += 1                 # Operator support: +=, -=, *=, /=

ListState for Collections

ListState is an observable list:

from castella import ListState

items = ListState(["a", "b", "c"])
items.append("d")          # Triggers rebuild
items.set(["x", "y"])      # Atomic replace (single rebuild)

Multiple States Pattern

When using multiple states, attach each to the component:

class MultiStateComponent(Component):
    def __init__(self):
        super().__init__()
        self._tab = State("home")
        self._counter = State(0)
        # Attach each state
        self._tab.attach(self)
        self._counter.attach(self)

    def view(self):
        return Column(
            Text(f"Tab: {self._tab()}"),
            Text(f"Count: {self._counter()}"),
        )

Size Policies

Control how widgets size themselves:

Policy	Behavior
SizePolicy.FIXED	Exact size specified
SizePolicy.EXPANDING	Fill available space
SizePolicy.CONTENT	Size to fit content

Fluent API Shortcuts

from castella import SizePolicy

# Fixed sizing
widget.fixed_width(100)
widget.fixed_height(40)
widget.fixed_size(200, 100)

# Content sizing
widget.fit_content()          # Both dimensions
widget.fit_content_width()    # Width only
widget.fit_content_height()   # Height only

# Fill parent
widget.fit_parent()

Important Constraint

A Layout with CONTENT height_policy cannot have EXPANDING height children:

# This will raise RuntimeError:
Column(
    Text("Hello"),  # Text defaults to EXPANDING height
).height_policy(SizePolicy.CONTENT)

# Fix by setting children to FIXED or CONTENT:
Column(
    Text("Hello").fixed_height(24),
).height_policy(SizePolicy.CONTENT)

Styling

Widget Styling Methods

Chain style methods on widgets:

Text("Hello")
    .bg_color("#1a1b26")
    .text_color("#c0caf5")
    .fixed_height(40)
    .padding(10)

Theme System

Access and toggle themes:

from castella.theme import ThemeManager

manager = ThemeManager()
theme = manager.current           # Get current theme
manager.toggle_dark_mode()        # Toggle dark/light
manager.prefer_dark(True)         # Force dark mode

Built-in themes: Tokyo Night (default), Cupertino, Material Design 3

See references/theme.md for custom themes.

Event Handling

Click Events

Button("Click me").on_click(lambda event: print("Clicked!"))

Input Changes

Input("initial").on_change(lambda text: print(f"New value: {text}"))

Important: Input Widget Pattern

Do NOT attach states that Input/MultilineInput manages:

class FormComponent(Component):
    def __init__(self):
        super().__init__()
        self._text = State("initial")
        # DON'T attach - causes focus loss on every keystroke
        # self._text.attach(self)

    def view(self):
        return Input(self._text()).on_change(lambda t: self._text.set(t))

Animation

AnimatedState

Values that animate smoothly on change:

from castella import AnimatedState

class AnimatedCounter(Component):
    def __init__(self):
        super().__init__()
        self._value = AnimatedState(0, duration_ms=300)
        self._value.attach(self)

    def view(self):
        return Column(
            Text(f"Value: {self._value():.1f}"),
            Button("+10").on_click(lambda _: self._value.set(self._value() + 10)),
        )

Widget Animation Methods

# Animate to position/size
widget.animate_to(x=200, y=100, duration_ms=400)

# Slide animations
widget.slide_in("left", distance=100, duration_ms=300)
widget.slide_out("right", distance=100, duration_ms=300)

See references/animation.md for more animation patterns.

Scrollable Containers

Make layouts scrollable:

from castella import Column, ScrollState, SizePolicy

class ScrollableList(Component):
    def __init__(self, items):
        super().__init__()
        self._items = ListState(items)
        self._items.attach(self)
        self._scroll = ScrollState()  # Preserves scroll position

    def view(self):
        return Column(
            *[Text(item).fixed_height(30) for item in self._items],
            scrollable=True,
            scroll_state=self._scroll,
        ).fixed_height(300)

Z-Index Stacking

Layer widgets with z-index:

from castella import Box

Box(
    main_content.z_index(1),
    modal_dialog.z_index(10),  # Appears on top
)

Semantic IDs for MCP

Assign semantic IDs for MCP accessibility:

Button("Submit").semantic_id("submit-btn")
Input("").semantic_id("email-input")

Best Practices

1. Attach states: Use state.attach(self) for each observable state
2. Fixed heights in scrollable containers: Use .fixed_height() for list items
3. Preserve scroll: Use ScrollState to maintain scroll position
4. Atomic list updates: Use ListState.set(items) for single rebuild
5. Don't attach Input states: Avoid attaching states managed by Input widgets
6. Semantic IDs: Add .semantic_id() for MCP integration

Running Scripts

# Counter example
uv run python scripts/counter.py

# Hot reload during development
uv run python tools/hot_restarter.py scripts/counter.py

Reference

• references/widgets.md - Complete widget API
• references/theme.md - Theme system details
• references/animation.md - Animation patterns
• references/state.md - State management patterns
• scripts/ - Executable examples (counter.py, form.py, scrollable_list.py)