Core Concepts

Essential concepts for understanding Presentar.

Widget

The fundamental building block. Everything on screen is a widget.

pub trait Widget {
    fn measure(&self, constraints: Constraints) -> Size;
    fn layout(&mut self, bounds: Rect) -> LayoutResult;
    fn paint(&self, canvas: &mut dyn Canvas);
    fn event(&mut self, event: &Event) -> Option<Box<dyn Any + Send>>;
}

Unidirectional Data Flow

Event → State → Widget → Draw
  │                        │
  └────────────────────────┘

1. Event: User interaction (click, type, scroll)
2. State: Application data updates
3. Widget: UI tree rebuilds
4. Draw: Canvas receives commands

Constraints

Minimum and maximum size bounds:

// Tight: exact size
Constraints::tight(Size::new(100.0, 50.0))

// Loose: 0 to maximum
Constraints::loose(Size::new(400.0, 300.0))

// Unbounded
Constraints::unbounded()

Layout Phases

Canvas

Abstract drawing surface:

canvas.fill_rect(bounds, color);
canvas.draw_text(text, position, style);
canvas.fill_circle(center, radius, color);

Events

User interactions:

Event::MouseDown { position, button }
Event::MouseUp { position, button }
Event::KeyDown { key }
Event::TextInput { text }
Event::FocusIn
Event::FocusOut

Messages

Widgets emit messages on interaction:

// Button emits ButtonClicked
if let Some(msg) = button.event(&event) {
    if msg.downcast_ref::<ButtonClicked>().is_some() {
        // Handle click
    }
}

Verified Test

#[test]
fn test_core_concepts() {
    use presentar_core::{Constraints, Size};

    // Constraints work
    let c = Constraints::loose(Size::new(100.0, 100.0));
    assert_eq!(c.biggest(), Size::new(100.0, 100.0));
    assert_eq!(c.smallest(), Size::new(0.0, 0.0));
}