FLUI Architecture Expert

FLUI Architecture Expert

Deep expertise in FLUI's Flutter-inspired three-tree architecture.

When to Use

Activate this skill when the user:

• Asks about View, Element, or RenderObject
• Discusses widget composition or tree structure
• Wants to understand build/​layout/​paint phases
• Needs to create custom render objects
• Has questions about the BuildContext

Three-Tree Architecture

View Tree (Immutable)

Purpose: Declarative UI description
Lifecycle: Created fresh each build
Key trait: View with build() -> impl IntoElement

Rules:

• Views must be 'static but NOT necessarily Clone
• Use cheap-to-clone types (String, Arc<T>)
• Never mutate state directly - use signals

Element Tree (Mutable)

Purpose: Manages widget lifecycle and state
Storage: Slab<Node<Element>> with ElementId keys
Key struct: Element with view_object: Box<dyn ViewObject>

Critical Pattern - ElementId Offset:

// Slab uses 0-based, ElementId uses 1-based (NonZeroUsize)
let id = slab.insert(node);
ElementId::new(id + 1)  // +1 for insertion

slab.get(element_id.get() - 1)  // -1 for access

Render Tree (Layout/​Paint)

Purpose: Layout calculation and painting
Arity system: Leaf, Single, Optional, Variable
Key trait: RenderBox<Arity>

Arity Types:

pub struct Leaf;        // 0 children (Text, Image)
pub struct Single;      // 1 child (Container, Padding)
pub struct Optional;    // 0-1 children (Conditional)
pub struct Variable;    // N children (Row, Column)

Pipeline Phases

Build Phase

• Traverses dirty elements
• Calls view.build(ctx) to generate new view tree
• Reconciles with existing element tree

Layout Phase

• Top-down constraint passing
• Bottom-up size determination
• Uses BoxConstraints for flexible sizing

Paint Phase

• Generates display list
• Layer composition
• GPU command generation via wgpu

Widget Patterns

Stateless Widget

pub struct MyWidget {
    pub text: String,
}

impl View for MyWidget {
    fn build(&self, ctx: &mut BuildContext) -> impl IntoElement {
        Text::new(&self.text)
    }
}

Stateful Widget (with hooks)

impl View for Counter {
    fn build(&self, ctx: &mut BuildContext) -> impl IntoElement {
        let count = use_signal(ctx, 0);
        
        Button::new(format!("Count: {}", count.get()))
            .on_press(move || count.set(count.get() + 1))
    }
}

Custom RenderObject

impl RenderBox<Single> for CustomRender {
    fn perform_layout(&mut self, constraints: BoxConstraints) -> Size {
        let child_size = self.child().layout(constraints);
        Size::new(child_size.width + 20.0, child_size.height + 20.0)
    }
    
    fn paint(&self, context: &mut PaintContext) {
        context.canvas.draw_rect(self.local_rect(), &self.paint);
        self.child().paint(context);
    }
}

Common Issues

Issue: Hook order changes

// BAD: Conditional hook
if condition {
    let signal = use_signal(ctx, 0);  // Will panic!
}

// GOOD: Always call hooks in same order
let signal = use_signal(ctx, 0);
if condition {
    // use signal
}

Issue: Expensive rebuilds

// BAD: Large subtree rebuilt on every change
Column::new(vec![
    expensive_widget_tree(),  // Rebuilt every time
])

// GOOD: Use memoization or separate signals
let memoized = use_memo(ctx, || expensive_widget_tree());

Architecture Verification Checklist

• Views are immutable
• State managed through signals
• ElementId uses +1/​-1 offset correctly
• RenderObject arity matches child count
• No mutable references across tree boundaries
• BuildContext only accessed during build phase