computed fields are working perfectly well now

canvas/Cargo.toml

@@ -34,6 +34,7 @@ gui = ["ratatui", "crossterm"]
 suggestions = ["tokio"]
 cursor-style = ["crossterm"]
 validation = ["regex"]
+computed = []
 
 [[example]]
 name = "suggestions"
@@ -64,3 +65,7 @@ required-features = ["gui", "validation"]
 [[example]]
 name = "validation_5"
 required-features = ["gui", "validation"]
+
+[[example]]
+name = "computed_fields"
+required-features = ["gui", "computed"]

canvas/examples/computed_fields.rs

@@ -0,0 +1,620 @@
+// examples/computed_fields.rs - COMPLETE WORKING VERSION
+//! Demonstrates computed fields with the canvas library - Invoice Calculator Example
+//!
+//! This example REQUIRES the `computed` feature to compile.
+//!
+//! Run with:
+//! cargo run --example computed_fields --features "gui,computed"
+
+#[cfg(not(feature = "computed"))]
+compile_error!(
+    "This example requires the 'computed' feature. \
+     Run with: cargo run --example computed_fields --features \"gui,computed\""
+);
+
+use std::io;
+use crossterm::{
+    event::{
+        self, DisableMouseCapture, EnableMouseCapture, Event, KeyCode, KeyModifiers,
+    },
+    execute,
+    terminal::{disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen},
+};
+use ratatui::{
+    backend::{Backend, CrosstermBackend},
+    layout::{Constraint, Direction, Layout, Rect},
+    style::{Color, Style, Modifier},
+    text::{Line, Span},
+    widgets::{Block, Borders, Paragraph, Wrap},
+    Frame, Terminal,
+};
+
+use canvas::{
+    canvas::{gui::render_canvas_default, modes::AppMode},
+    DataProvider, FormEditor,
+    computed::{ComputedProvider, ComputedContext},
+};
+
+/// Invoice data with computed fields
+struct InvoiceData {
+    fields: Vec<(String, String)>,
+    computed_indices: std::collections::HashSet<usize>,
+}
+
+impl InvoiceData {
+    fn new() -> Self {
+        let mut computed_indices = std::collections::HashSet::new();
+        
+        // Mark computed fields (read-only, calculated)
+        computed_indices.insert(4); // Subtotal
+        computed_indices.insert(5); // Tax Amount
+        computed_indices.insert(6); // Total
+
+        Self {
+            fields: vec![
+                ("📦 Product Name".to_string(), "".to_string()),
+                ("🔢 Quantity".to_string(), "".to_string()),
+                ("💰 Unit Price ($)".to_string(), "".to_string()),
+                ("📊 Tax Rate (%)".to_string(), "".to_string()),
+                ("➕ Subtotal ($)".to_string(), "".to_string()),        // COMPUTED
+                ("🧾 Tax Amount ($)".to_string(), "".to_string()),      // COMPUTED
+                ("💳 Total ($)".to_string(), "".to_string()),           // COMPUTED
+                ("📝 Notes".to_string(), "".to_string()),
+            ],
+            computed_indices,
+        }
+    }
+}
+
+impl DataProvider for InvoiceData {
+    fn field_count(&self) -> usize {
+        self.fields.len()
+    }
+
+    fn field_name(&self, index: usize) -> &str {
+        &self.fields[index].0
+    }
+
+    fn field_value(&self, index: usize) -> &str {
+        &self.fields[index].1
+    }
+
+    fn set_field_value(&mut self, index: usize, value: String) {
+        // 🔥 FIXED: Allow computed fields to be updated for display purposes
+        // The editing protection happens at the editor level, not here
+        self.fields[index].1 = value;
+    }
+
+    fn supports_suggestions(&self, _field_index: usize) -> bool {
+        false
+    }
+
+    fn display_value(&self, _index: usize) -> Option<&str> {
+        None
+    }
+    
+    /// Mark computed fields
+    fn is_computed_field(&self, field_index: usize) -> bool {
+        self.computed_indices.contains(&field_index)
+    }
+
+    /// Get computed field values
+    fn computed_field_value(&self, field_index: usize) -> Option<String> {
+        if self.computed_indices.contains(&field_index) {
+            Some(self.fields[field_index].1.clone())
+        } else {
+            None
+        }
+    }
+}
+
+/// Invoice calculator - computes totals based on input fields
+struct InvoiceCalculator;
+
+impl ComputedProvider for InvoiceCalculator {
+    fn compute_field(&mut self, context: ComputedContext) -> String {
+        // Helper to parse field values safely
+        let parse_field = |index: usize| -> f64 {
+            let value = context.field_values[index].trim();
+            if value.is_empty() {
+                0.0
+            } else {
+                value.parse().unwrap_or(0.0)
+            }
+        };
+
+        match context.target_field {
+            4 => {
+                // Subtotal = Quantity × Unit Price
+                let qty = parse_field(1);
+                let price = parse_field(2);
+                let subtotal = qty * price;
+                
+                if qty == 0.0 || price == 0.0 {
+                    "".to_string()  // Show empty if no meaningful calculation
+                } else {
+                    format!("{:.2}", subtotal)
+                }
+            }
+            5 => {
+                // Tax Amount = Subtotal × (Tax Rate / 100)
+                let qty = parse_field(1);
+                let price = parse_field(2);
+                let tax_rate = parse_field(3);
+                let subtotal = qty * price;
+                let tax_amount = subtotal * (tax_rate / 100.0);
+                
+                if subtotal == 0.0 || tax_rate == 0.0 {
+                    "".to_string()
+                } else {
+                    format!("{:.2}", tax_amount)
+                }
+            }
+            6 => {
+                // Total = Subtotal + Tax Amount
+                let qty = parse_field(1);
+                let price = parse_field(2);
+                let tax_rate = parse_field(3);
+                let subtotal = qty * price;
+                
+                if subtotal == 0.0 {
+                    "".to_string()
+                } else {
+                    let tax_amount = subtotal * (tax_rate / 100.0);
+                    let total = subtotal + tax_amount;
+                    format!("{:.2}", total)
+                }
+            }
+            _ => "".to_string(),
+        }
+    }
+    
+    fn handles_field(&self, field_index: usize) -> bool {
+        matches!(field_index, 4 | 5 | 6) // Subtotal, Tax Amount, Total
+    }
+    
+    fn field_dependencies(&self, field_index: usize) -> Vec<usize> {
+        match field_index {
+            4 => vec![1, 2],       // Subtotal depends on Quantity, Unit Price
+            5 => vec![1, 2, 3],    // Tax Amount depends on Quantity, Unit Price, Tax Rate
+            6 => vec![1, 2, 3],    // Total depends on Quantity, Unit Price, Tax Rate
+            _ => vec![],
+        }
+    }
+}
+
+/// Enhanced editor with computed fields
+struct ComputedFieldsEditor<D: DataProvider> {
+    editor: FormEditor<D>,
+    calculator: InvoiceCalculator,
+    debug_message: String,
+    last_computed_values: Vec<String>,
+}
+
+impl<D: DataProvider> ComputedFieldsEditor<D> {
+    fn new(data_provider: D) -> Self {
+        let mut editor = FormEditor::new(data_provider);
+        editor.set_computed_provider(InvoiceCalculator);
+
+        let calculator = InvoiceCalculator;
+        let last_computed_values = vec!["".to_string(); 8];
+
+        Self {
+            editor,
+            calculator,
+            debug_message: "💰 Invoice Calculator - Start typing in fields to see calculations!".to_string(),
+            last_computed_values,
+        }
+    }
+
+    fn is_computed_field(&self, field_index: usize) -> bool {
+        self.editor.ui_state().is_computed_field(field_index)
+    }
+
+    fn update_computed_fields(&mut self) {
+        // Trigger recomputation of all computed fields
+        self.editor.recompute_all_fields(&mut self.calculator);
+
+        // 🔥 CRITICAL FIX: Sync computed values to DataProvider so GUI shows them!
+        for i in [4, 5, 6] { // Computed field indices  
+            let computed_value = self.editor.effective_field_value(i);
+            self.editor.data_provider_mut().set_field_value(i, computed_value.clone());
+        }
+
+        // Check if values changed to show feedback
+        let mut changed = false;
+        let mut has_calculations = false;
+        
+        for i in [4, 5, 6] {
+            let new_value = self.editor.effective_field_value(i);
+            if new_value != self.last_computed_values[i] {
+                changed = true;
+                self.last_computed_values[i] = new_value.clone();
+            }
+            if !new_value.is_empty() {
+                has_calculations = true;
+            }
+        }
+
+        if changed {
+            if has_calculations {
+                let subtotal = &self.last_computed_values[4];
+                let tax = &self.last_computed_values[5];
+                let total = &self.last_computed_values[6];
+                
+                let mut parts = Vec::new();
+                if !subtotal.is_empty() {
+                    parts.push(format!("Subtotal=${}", subtotal));
+                }
+                if !tax.is_empty() {
+                    parts.push(format!("Tax=${}", tax));
+                }
+                if !total.is_empty() {
+                    parts.push(format!("Total=${}", total));
+                }
+                
+                if !parts.is_empty() {
+                    self.debug_message = format!("🧮 Calculated: {}", parts.join(", "));
+                } else {
+                    self.debug_message = "💰 Enter Quantity and Unit Price to see calculations".to_string();
+                }
+            } else {
+                self.debug_message = "💰 Enter Quantity and Unit Price to see calculations".to_string();
+            }
+        }
+    }
+
+    fn insert_char(&mut self, ch: char) -> anyhow::Result<()> {
+        let current_field = self.editor.current_field();
+        let result = self.editor.insert_char(ch);
+
+        if result.is_ok() && matches!(current_field, 1 | 2 | 3) {
+            self.editor.on_field_changed(&mut self.calculator, current_field);
+            self.update_computed_fields();
+        }
+
+        result
+    }
+
+    fn delete_backward(&mut self) -> anyhow::Result<()> {
+        let current_field = self.editor.current_field();
+        let result = self.editor.delete_backward();
+
+        if result.is_ok() && matches!(current_field, 1 | 2 | 3) {
+            self.editor.on_field_changed(&mut self.calculator, current_field);
+            self.update_computed_fields();
+        }
+
+        result
+    }
+
+    fn delete_forward(&mut self) -> anyhow::Result<()> {
+        let current_field = self.editor.current_field();
+        let result = self.editor.delete_forward();
+
+        if result.is_ok() && matches!(current_field, 1 | 2 | 3) {
+            self.editor.on_field_changed(&mut self.calculator, current_field);
+            self.update_computed_fields();
+        }
+
+        result
+    }
+
+    fn next_field(&mut self) {
+        let old_field = self.editor.current_field();
+        let _ = self.editor.next_field();
+        let new_field = self.editor.current_field();
+
+        if old_field != new_field {
+            let field_name = self.editor.data_provider().field_name(new_field);
+            let field_type = if self.is_computed_field(new_field) {
+                "computed (read-only)"
+            } else {
+                "editable"
+            };
+            self.debug_message = format!("→ {} - {} field", field_name, field_type);
+        }
+    }
+
+    fn prev_field(&mut self) {
+        let old_field = self.editor.current_field();
+        let _ = self.editor.prev_field();
+        let new_field = self.editor.current_field();
+
+        if old_field != new_field {
+            let field_name = self.editor.data_provider().field_name(new_field);
+            let field_type = if self.is_computed_field(new_field) {
+                "computed (read-only)"
+            } else {
+                "editable"
+            };
+            self.debug_message = format!("← {} - {} field", field_name, field_type);
+        }
+    }
+
+    fn enter_edit_mode(&mut self) {
+        let current = self.editor.current_field();
+        
+        // Double protection: check both ways
+        if self.editor.data_provider().is_computed_field(current) || self.is_computed_field(current) {
+            let field_name = self.editor.data_provider().field_name(current);
+            self.debug_message = format!(
+                "🚫 {} is computed (read-only) - Press Tab to move to editable fields",
+                field_name
+            );
+            return;
+        }
+        
+        self.editor.enter_edit_mode();
+        let field_name = self.editor.data_provider().field_name(current);
+        self.debug_message = format!("✏️ Editing {} - Type to see calculations update", field_name);
+    }
+
+    fn enter_append_mode(&mut self) {
+        let current = self.editor.current_field();
+        
+        if self.editor.data_provider().is_computed_field(current) || self.is_computed_field(current) {
+            let field_name = self.editor.data_provider().field_name(current);
+            self.debug_message = format!(
+                "🚫 {} is computed (read-only) - Press Tab to move to editable fields",
+                field_name
+            );
+            return;
+        }
+        
+        self.editor.enter_append_mode();
+        let field_name = self.editor.data_provider().field_name(current);
+        self.debug_message = format!("✏️ Appending to {} - Type to see calculations", field_name);
+    }
+
+    fn exit_edit_mode(&mut self) {
+        let current_field = self.editor.current_field();
+        self.editor.exit_edit_mode();
+
+        if matches!(current_field, 1 | 2 | 3) {
+            self.editor.on_field_changed(&mut self.calculator, current_field);
+            self.update_computed_fields();
+        }
+
+        self.debug_message = "🔒 Normal mode - Press 'i' to edit fields".to_string();
+    }
+
+    // Delegate methods
+    fn current_field(&self) -> usize { self.editor.current_field() }
+    fn cursor_position(&self) -> usize { self.editor.cursor_position() }
+    fn mode(&self) -> AppMode { self.editor.mode() }
+    fn current_text(&self) -> &str { self.editor.current_text() }
+    fn data_provider(&self) -> &D { self.editor.data_provider() }
+    fn ui_state(&self) -> &canvas::EditorState { self.editor.ui_state() }
+    fn move_left(&mut self) { self.editor.move_left(); }
+    fn move_right(&mut self) { self.editor.move_right(); }
+    fn move_up(&mut self) { let _ = self.editor.move_up(); }
+    fn move_down(&mut self) { let _ = self.editor.move_down(); }
+}
+
+fn handle_key_press(
+    key: KeyCode,
+    modifiers: KeyModifiers,
+    editor: &mut ComputedFieldsEditor<InvoiceData>,
+) -> anyhow::Result<bool> {
+    let mode = editor.mode();
+
+    if (key == KeyCode::Char('q') && modifiers.contains(KeyModifiers::CONTROL))
+        || (key == KeyCode::Char('c') && modifiers.contains(KeyModifiers::CONTROL))
+        || key == KeyCode::F(10)
+    {
+        return Ok(false);
+    }
+
+    match (mode, key, modifiers) {
+        (AppMode::ReadOnly, KeyCode::Char('i'), _) => {
+            editor.enter_edit_mode();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('a'), _) => {
+            editor.enter_append_mode();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('A'), _) => {
+            editor.editor.move_line_end();
+            editor.enter_edit_mode();
+        }
+        (_, KeyCode::Esc, _) => {
+            if mode == AppMode::Edit {
+                editor.exit_edit_mode();
+            }
+        }
+
+        // Movement
+        (AppMode::ReadOnly, KeyCode::Char('h'), _) | (AppMode::ReadOnly, KeyCode::Left, _) => {
+            editor.move_left();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('l'), _) | (AppMode::ReadOnly, KeyCode::Right, _) => {
+            editor.move_right();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('j'), _) | (AppMode::ReadOnly, KeyCode::Down, _) => {
+            editor.move_down();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('k'), _) | (AppMode::ReadOnly, KeyCode::Up, _) => {
+            editor.move_up();
+        }
+
+        // Edit mode movement
+        (AppMode::Edit, KeyCode::Left, _) => { editor.move_left(); }
+        (AppMode::Edit, KeyCode::Right, _) => { editor.move_right(); }
+        (AppMode::Edit, KeyCode::Up, _) => { editor.move_up(); }
+        (AppMode::Edit, KeyCode::Down, _) => { editor.move_down(); }
+
+        // Navigation
+        (_, KeyCode::Tab, _) => {
+            editor.next_field();
+        }
+        (_, KeyCode::BackTab, _) => {
+            editor.prev_field();
+        }
+
+        // Editing
+        (AppMode::Edit, KeyCode::Char(c), m) if !m.contains(KeyModifiers::CONTROL) => {
+            editor.insert_char(c)?;
+        }
+        (AppMode::Edit, KeyCode::Backspace, _) => {
+            editor.delete_backward()?;
+        }
+        (AppMode::Edit, KeyCode::Delete, _) => {
+            editor.delete_forward()?;
+        }
+
+        // Debug info
+        (AppMode::ReadOnly, KeyCode::Char('?'), _) => {
+            let current = editor.current_field();
+            let field_name = editor.data_provider().field_name(current);
+            let field_type = if editor.is_computed_field(current) {
+                "COMPUTED (read-only)"
+            } else {
+                "EDITABLE"
+            };
+            editor.debug_message = format!(
+                "{} - {} - Position {} - Mode: {:?}",
+                field_name, field_type, editor.cursor_position(), mode
+            );
+        }
+
+        _ => {}
+    }
+
+    Ok(true)
+}
+
+fn run_app<B: Backend>(
+    terminal: &mut Terminal<B>,
+    mut editor: ComputedFieldsEditor<InvoiceData>,
+) -> io::Result<()> {
+    editor.update_computed_fields(); // Initial computation
+
+    loop {
+        terminal.draw(|f| ui(f, &editor))?;
+
+        if let Event::Key(key) = event::read()? {
+            match handle_key_press(key.code, key.modifiers, &mut editor) {
+                Ok(should_continue) => {
+                    if !should_continue {
+                        break;
+                    }
+                }
+                Err(e) => {
+                    editor.debug_message = format!("Error: {}", e);
+                }
+            }
+        }
+    }
+
+    Ok(())
+}
+
+fn ui(f: &mut Frame, editor: &ComputedFieldsEditor<InvoiceData>) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([Constraint::Min(8), Constraint::Length(10)])
+        .split(f.area());
+
+    render_enhanced_canvas(f, chunks[0], editor);
+    render_computed_status(f, chunks[1], editor);
+}
+
+fn render_enhanced_canvas(f: &mut Frame, area: Rect, editor: &ComputedFieldsEditor<InvoiceData>) {
+    render_canvas_default(f, area, &editor.editor);
+}
+
+fn render_computed_status(f: &mut Frame, area: Rect, editor: &ComputedFieldsEditor<InvoiceData>) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([Constraint::Length(3), Constraint::Length(7)])
+        .split(area);
+
+    let mode_text = match editor.mode() {
+        AppMode::Edit => "INSERT",
+        AppMode::ReadOnly => "NORMAL",
+        _ => "OTHER",
+    };
+
+    let current = editor.current_field();
+    let field_status = if editor.is_computed_field(current) {
+        "📊 COMPUTED FIELD (read-only)"
+    } else {
+        "✏️ EDITABLE FIELD"
+    };
+
+    let status_text = format!("-- {} -- {} | {}", mode_text, field_status, editor.debug_message);
+
+    let status = Paragraph::new(Line::from(Span::raw(status_text)))
+        .block(Block::default().borders(Borders::ALL).title("💰 Invoice Calculator"));
+
+    f.render_widget(status, chunks[0]);
+
+    let help_text = match editor.mode() {
+        AppMode::ReadOnly => {
+            "💰 COMPUTED FIELDS DEMO: Real-time invoice calculations!\n\
+             🔢 EDITABLE: Product, Quantity, Unit Price, Tax Rate, Notes\n\
+             📊 COMPUTED: Subtotal, Tax Amount, Total (calculated automatically)\n\
+             \n\
+             🚀 START: Press 'i' to edit Quantity, type '5', Tab to Unit Price, type '19.99'\n\
+             Watch Subtotal and Total appear! Add Tax Rate to see tax calculations.\n\
+             Navigation: Tab/Shift+Tab skips computed fields automatically"
+        }
+        AppMode::Edit => {
+            "✏️ EDIT MODE: Type numbers to see calculations appear!\n\
+             \n\
+             💡 EXAMPLE: Type '5' in Quantity, then Tab to Unit Price and type '19.99'\n\
+             • Subtotal appears: $99.95\n\
+             • Total appears: $99.95\n\
+             • Add Tax Rate (like '10') to see tax: $9.99, Total: $109.94\n\
+             \n\
+             Esc=normal, Tab=next field (auto-skips computed fields)"
+        }
+        _ => "💰 Invoice Calculator with Computed Fields"
+    };
+
+    let help_style = if editor.is_computed_field(editor.current_field()) {
+        Style::default().fg(Color::Yellow).add_modifier(Modifier::ITALIC)
+    } else {
+        Style::default().fg(Color::Gray)
+    };
+
+    let help = Paragraph::new(help_text)
+        .block(Block::default().borders(Borders::ALL).title("🚀 Try It Now!"))
+        .style(help_style)
+        .wrap(Wrap { trim: true });
+
+    f.render_widget(help, chunks[1]);
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    println!("💰 Canvas Computed Fields Demo - Invoice Calculator");
+    println!("✅ computed feature: ENABLED");
+    println!("🚀 QUICK TEST:");
+    println!("   1. Press 'i' to edit Quantity");
+    println!("   2. Type '5' and press Tab");
+    println!("   3. Type '19.99' in Unit Price");
+    println!("   4. Watch Subtotal ($99.95) and Total ($99.95) appear!");
+    println!();
+
+    enable_raw_mode()?;
+    let mut stdout = io::stdout();
+    execute!(stdout, EnterAlternateScreen, EnableMouseCapture)?;
+    let backend = CrosstermBackend::new(stdout);
+    let mut terminal = Terminal::new(backend)?;
+
+    let data = InvoiceData::new();
+    let editor = ComputedFieldsEditor::new(data);
+
+    let res = run_app(&mut terminal, editor);
+
+    disable_raw_mode()?;
+    execute!(terminal.backend_mut(), LeaveAlternateScreen, DisableMouseCapture)?;
+    terminal.show_cursor()?;
+
+    if let Err(err) = res {
+        println!("{:?}", err);
+    }
+
+    println!("💰 Demo completed! Computed fields should have updated in real-time!");
+    Ok(())
+}

canvas/src/canvas/state.rs

@@ -23,6 +23,10 @@ pub struct EditorState {
     // Validation state (only available with validation feature)
     #[cfg(feature = "validation")]
     pub(crate) validation: crate::validation::ValidationState,
+
+    /// Computed fields state (only when computed feature is enabled)
+    #[cfg(feature = "computed")]
+    pub(crate) computed: Option<crate::computed::ComputedState>,
 }
 
 #[derive(Debug, Clone)]
@@ -56,6 +60,8 @@ impl EditorState {
             selection: SelectionState::None,
             #[cfg(feature = "validation")]
             validation: crate::validation::ValidationState::new(),
+            #[cfg(feature = "computed")]
+            computed: None,
         }
     }
 
@@ -68,6 +74,15 @@ impl EditorState {
         self.current_field
     }
 
+    /// Check if field is computed
+    #[cfg(feature = "computed")]
+    pub fn is_computed_field(&self, field_index: usize) -> bool {
+        self.computed
+            .as_ref()
+            .map(|state| state.is_computed_field(field_index))
+            .unwrap_or(false)
+    }
+
     /// Get current cursor position (for user's business logic)
     pub fn cursor_position(&self) -> usize {
         self.cursor_pos

canvas/src/computed/mod.rs

@@ -0,0 +1,5 @@
+pub mod provider;
+pub mod state;
+
+pub use provider::{ComputedContext, ComputedProvider};
+pub use state::ComputedState;

canvas/src/computed/provider.rs

@@ -0,0 +1,31 @@
+// ================================================================================================
+// COMPUTED FIELDS - Provider and Context
+// ================================================================================================
+
+/// Context information provided to computed field calculations
+#[derive(Debug, Clone)]
+pub struct ComputedContext<'a> {
+    /// All field values in the form (index -> value)
+    pub field_values: &'a [&'a str],
+    /// The field index being computed
+    pub target_field: usize,
+    /// Current field that user is editing (if any)
+    pub current_field: Option<usize>,
+}
+
+/// User implements this to provide computed field logic
+pub trait ComputedProvider {
+    /// Compute value for a field based on other field values.
+    /// Called automatically when any field changes.
+    fn compute_field(&mut self, context: ComputedContext) -> String;
+
+    /// Check if this provider handles the given field.
+    fn handles_field(&self, field_index: usize) -> bool;
+
+    /// Get list of field dependencies for optimization.
+    /// If field A depends on fields [1, 3], only recompute A when fields 1 or 3 change.
+    /// Default: depend on all fields (always recompute) with a reasonable upper bound.
+    fn field_dependencies(&self, _field_index: usize) -> Vec<usize> {
+        (0..100).collect()
+    }
+}

canvas/src/computed/state.rs

@@ -0,0 +1,88 @@
+/* file: canvas/src/computed/state.rs */
+/*
+Add computed state module file implementing caching and dependencies
+*/
+
+// ================================================================================================
+// COMPUTED FIELDS - State: caching and dependencies
+// ================================================================================================
+
+use std::collections::{HashMap, HashSet};
+
+/// Internal state for computed field management
+#[derive(Debug, Clone)]
+pub struct ComputedState {
+    /// Cached computed values (field_index -> computed_value)
+    computed_values: HashMap<usize, String>,
+    /// Field dependency graph (field_index -> depends_on_fields)
+    dependencies: HashMap<usize, Vec<usize>>,
+    /// Track which fields are computed (display-only)
+    computed_fields: HashSet<usize>,
+}
+
+impl ComputedState {
+    /// Create a new, empty computed state
+    pub fn new() -> Self {
+        Self {
+            computed_values: HashMap::new(),
+            dependencies: HashMap::new(),
+            computed_fields: HashSet::new(),
+        }
+    }
+
+    /// Register a field as computed with its dependencies
+    ///
+    /// - `field_index`: the field that is computed (display-only)
+    /// - `dependencies`: indices of fields this computed field depends on
+    pub fn register_computed_field(&mut self, field_index: usize, mut dependencies: Vec<usize>) {
+        // Deduplicate dependencies to keep graph lean
+        dependencies.sort_unstable();
+        dependencies.dedup();
+
+        self.computed_fields.insert(field_index);
+        self.dependencies.insert(field_index, dependencies);
+    }
+
+    /// Check if a field is computed (read-only, skip editing/navigation)
+    pub fn is_computed_field(&self, field_index: usize) -> bool {
+        self.computed_fields.contains(&field_index)
+    }
+
+    /// Get cached computed value for a field, if available
+    pub fn get_computed_value(&self, field_index: usize) -> Option<&String> {
+        self.computed_values.get(&field_index)
+    }
+
+    /// Update cached computed value for a field
+    pub fn set_computed_value(&mut self, field_index: usize, value: String) {
+        self.computed_values.insert(field_index, value);
+    }
+
+    /// Get fields that should be recomputed when `changed_field` changed
+    ///
+    /// This scans the dependency graph and returns all computed fields
+    /// that list `changed_field` as a dependency.
+    pub fn fields_to_recompute(&self, changed_field: usize) -> Vec<usize> {
+        self.dependencies
+            .iter()
+            .filter_map(|(field, deps)| {
+                if deps.contains(&changed_field) {
+                    Some(*field)
+                } else {
+                    None
+                }
+            })
+            .collect()
+    }
+
+    /// Iterator over all computed field indices
+    pub fn computed_fields(&self) -> impl Iterator<Item = usize> + '_ {
+        self.computed_fields.iter().copied()
+    }
+}
+
+impl Default for ComputedState {
+    fn default() -> Self {
+        Self::new()
+    }
+}

canvas/src/data_provider.rs

@@ -34,6 +34,20 @@ pub trait DataProvider {
     fn validation_config(&self, _field_index: usize) -> Option<crate::validation::ValidationConfig> {
         None
     }
+
+    /// Check if field is computed (display-only, skip in navigation)
+    /// Default: not computed
+    #[cfg(feature = "computed")]
+    fn is_computed_field(&self, _field_index: usize) -> bool {
+        false
+    }
+
+    /// Get computed field value if this is a computed field.
+    /// Returns None for regular fields. Default: not computed.
+    #[cfg(feature = "computed")]
+    fn computed_field_value(&self, _field_index: usize) -> Option<String> {
+        None
+    }
 }
 
 /// Optional: User implements this for suggestions data

canvas/src/editor.rs

@@ -668,6 +668,45 @@ impl<D: DataProvider> FormEditor<D> {
             return Ok(());
         }
 
+        // Skip computed fields during navigation when feature enabled
+        #[cfg(feature = "computed")]
+        {
+            if let Some(computed_state) = &self.ui_state.computed {
+                // Find previous non-computed field
+                let mut candidate = self.ui_state.current_field;
+                for _ in 0..field_count {
+                    candidate = candidate.saturating_sub(1);
+                    if !computed_state.is_computed_field(candidate) {
+                        // Validate and move as usual
+                        #[cfg(feature = "validation")]
+                        {
+                            let current_text = self.current_text();
+                            if !self.ui_state.validation.allows_field_switch(self.ui_state.current_field, current_text) {
+                                if let Some(reason) = self.ui_state.validation.field_switch_block_reason(self.ui_state.current_field, current_text) {
+                                    tracing::debug!("Field switch blocked: {}", reason);
+                                    return Err(anyhow::anyhow!("Cannot switch fields: {}", reason));
+                                }
+                            }
+                        }
+                        #[cfg(feature = "validation")]
+                        {
+                            let current_text = self.current_text().to_string();
+                            let _validation_result = self.ui_state.validation.validate_field_content(
+                                self.ui_state.current_field,
+                                &current_text,
+                            );
+                        }
+                        self.ui_state.move_to_field(candidate, field_count);
+                        self.clamp_cursor_to_current_field();
+                        return Ok(());
+                    }
+                    if candidate == 0 {
+                        break;
+                    }
+                }
+            }
+        }
+
         // Check if field switching is allowed (minimum character enforcement)
         #[cfg(feature = "validation")]
         {
@@ -705,6 +744,45 @@ impl<D: DataProvider> FormEditor<D> {
             return Ok(());
         }
 
+        // Skip computed fields during navigation when feature enabled
+        #[cfg(feature = "computed")]
+        {
+            if let Some(computed_state) = &self.ui_state.computed {
+                // Find next non-computed field
+                let mut candidate = self.ui_state.current_field;
+                for _ in 0..field_count {
+                    candidate = (candidate + 1).min(field_count - 1);
+                    if !computed_state.is_computed_field(candidate) {
+                        // Validate and move as usual
+                        #[cfg(feature = "validation")]
+                        {
+                            let current_text = self.current_text();
+                            if !self.ui_state.validation.allows_field_switch(self.ui_state.current_field, current_text) {
+                                if let Some(reason) = self.ui_state.validation.field_switch_block_reason(self.ui_state.current_field, current_text) {
+                                    tracing::debug!("Field switch blocked: {}", reason);
+                                    return Err(anyhow::anyhow!("Cannot switch fields: {}", reason));
+                                }
+                            }
+                        }
+                        #[cfg(feature = "validation")]
+                        {
+                            let current_text = self.current_text().to_string();
+                            let _validation_result = self.ui_state.validation.validate_field_content(
+                                self.ui_state.current_field,
+                                &current_text,
+                            );
+                        }
+                        self.ui_state.move_to_field(candidate, field_count);
+                        self.clamp_cursor_to_current_field();
+                        return Ok(());
+                    }
+                    if candidate == field_count - 1 {
+                        break;
+                    }
+                }
+            }
+        }
+
         // Check if field switching is allowed (minimum character enforcement)
         #[cfg(feature = "validation")]
         {
@@ -763,6 +841,112 @@ impl<D: DataProvider> FormEditor<D> {
         self.move_up()
     }
 
+    // ================================================================================================
+    // COMPUTED FIELDS (behind 'computed' feature)
+    // ================================================================================================
+
+    /// Initialize computed fields from provider and computed provider
+    #[cfg(feature = "computed")]
+    pub fn set_computed_provider<C>(&mut self, mut provider: C)
+    where
+        C: crate::computed::ComputedProvider,
+    {
+        // Initialize computed state
+        self.ui_state.computed = Some(crate::computed::ComputedState::new());
+
+        // Register computed fields and their dependencies
+        let field_count = self.data_provider.field_count();
+        for field_index in 0..field_count {
+            if provider.handles_field(field_index) {
+                let deps = provider.field_dependencies(field_index);
+                if let Some(computed_state) = &mut self.ui_state.computed {
+                    computed_state.register_computed_field(field_index, deps);
+                }
+            }
+        }
+
+        // Initial computation of all computed fields
+        self.recompute_all_fields(&mut provider);
+    }
+
+    /// Recompute specific computed fields
+    #[cfg(feature = "computed")]
+    pub fn recompute_fields<C>(&mut self, provider: &mut C, field_indices: &[usize])
+    where
+        C: crate::computed::ComputedProvider,
+    {
+        if let Some(computed_state) = &mut self.ui_state.computed {
+            // Collect all field values for context
+            let field_values: Vec<String> = (0..self.data_provider.field_count())
+                .map(|i| {
+                    if computed_state.is_computed_field(i) {
+                        // Use cached computed value
+                        computed_state
+                            .get_computed_value(i)
+                            .cloned()
+                            .unwrap_or_default()
+                    } else {
+                        // Use regular field value
+                        self.data_provider.field_value(i).to_string()
+                    }
+                })
+                .collect();
+
+            let field_refs: Vec<&str> = field_values.iter().map(|s| s.as_str()).collect();
+
+            // Recompute specified fields
+            for &field_index in field_indices {
+                if provider.handles_field(field_index) {
+                    let context = crate::computed::ComputedContext {
+                        field_values: &field_refs,
+                        target_field: field_index,
+                        current_field: Some(self.ui_state.current_field),
+                    };
+
+                    let computed_value = provider.compute_field(context);
+                    computed_state.set_computed_value(field_index, computed_value);
+                }
+            }
+        }
+    }
+
+    /// Recompute all computed fields
+    #[cfg(feature = "computed")]
+    pub fn recompute_all_fields<C>(&mut self, provider: &mut C)
+    where
+        C: crate::computed::ComputedProvider,
+    {
+        if let Some(computed_state) = &self.ui_state.computed {
+            let computed_fields: Vec<usize> = computed_state.computed_fields().collect();
+            self.recompute_fields(provider, &computed_fields);
+        }
+    }
+
+    /// Trigger recomputation when field changes (call this after set_field_value)
+    #[cfg(feature = "computed")]
+    pub fn on_field_changed<C>(&mut self, provider: &mut C, changed_field: usize)
+    where
+        C: crate::computed::ComputedProvider,
+    {
+        if let Some(computed_state) = &self.ui_state.computed {
+            let fields_to_update = computed_state.fields_to_recompute(changed_field);
+            if !fields_to_update.is_empty() {
+                self.recompute_fields(provider, &fields_to_update);
+            }
+        }
+    }
+
+    /// Enhanced getter that returns computed values for computed fields when available
+    #[cfg(feature = "computed")]
+    pub fn effective_field_value(&self, field_index: usize) -> String {
+        if let Some(computed_state) = &self.ui_state.computed {
+            if let Some(computed_value) = computed_state.get_computed_value(field_index) {
+                return computed_value.clone();
+            }
+        }
+        self.data_provider.field_value(field_index).to_string()
+    }
+
     /// Move to next field (alternative to move_down)
     pub fn next_field(&mut self) -> Result<()> {
         self.move_down()
@@ -960,6 +1144,15 @@ impl<D: DataProvider> FormEditor<D> {
 
     /// Enter edit mode from read-only mode (vim i/a/o)
     pub fn enter_edit_mode(&mut self) {
+        #[cfg(feature = "computed")]
+        {
+            if let Some(computed_state) = &self.ui_state.computed {
+                if computed_state.is_computed_field(self.ui_state.current_field) {
+                    // Can't edit computed fields - silently ignore
+                    return;
+                }
+            }
+        }
         self.set_mode(AppMode::Edit);
     }
 

canvas/src/lib.rs

@@ -12,6 +12,10 @@ pub mod suggestions;
 #[cfg(feature = "validation")]
 pub mod validation;
 
+// Only include computed module if feature is enabled
+#[cfg(feature = "computed")]
+pub mod computed;
+
 #[cfg(feature = "cursor-style")]
 pub use canvas::CursorManager;
 
@@ -41,6 +45,10 @@ pub use validation::{
     CustomFormatter, FormattingResult, PositionMapper, DefaultPositionMapper,
 };
 
+// Computed exports (only when computed feature is enabled)
+#[cfg(feature = "computed")]
+pub use computed::{ComputedProvider, ComputedContext, ComputedState};
+
 // Theming and GUI
 #[cfg(feature = "gui")]
 pub use canvas::theme::{CanvasTheme, DefaultCanvasTheme};