TUI Component Design Patterns

Best practices for building maintainable, testable TUI components using Bubbletea v2 and the Charm ecosystem, based on the hive diff viewer implementation.

Component Organization

Single Responsibility Per File

Each component should be in its own file with clear boundaries:

internal/tui/diff/
├── model.go           # Top-level compositor that orchestrates sub-components
├── diffviewer.go      # Diff content display with scrolling and selection
├── filetree.go        # File navigation tree with expand/collapse
├── lineparse.go       # Pure function utilities for parsing diff lines
├── delta.go           # External tool integration (syntax highlighting)
└── utils.go           # Shared utilities

Key principle: Each file should represent ONE component with its own Model, Update, and View methods.

Component Hierarchy Pattern

For complex UIs, use a compositor pattern:

// Top-level Model composes sub-components
type Model struct {
    fileTree      FileTreeModel      // Left panel
    diffViewer    DiffViewerModel    // Right panel
    focused       FocusedPanel       // Which component has focus
    helpDialog    *components.HelpDialog  // Modal overlay
    showHelp      bool               // Dialog visibility state
}

// Update delegates to focused component
func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    switch m.focused {
    case FocusFileTree:
        m.fileTree, cmd = m.fileTree.Update(msg)
    case FocusDiffViewer:
        m.diffViewer, cmd = m.diffViewer.Update(msg)
    }
    return m, cmd
}

Benefits:

• Each sub-component is independently testable
• Clear ownership of state and behavior
• Easy to reason about message flow

Component Structure

Standard Component Template

// 1. Model struct with all state
type ComponentModel struct {
    // Data
    items []Item

    // UI State
    selected int
    offset   int
    width    int
    height   int

    // Feature flags
    iconStyle IconStyle
    expanded  bool
}

// 2. Constructor with dependencies
func NewComponent(data []Item, cfg *config.Config) ComponentModel {
    return ComponentModel{
        items:     data,
        selected:  0,
        iconStyle: determineIconStyle(cfg),
    }
}

// 3. Update handles messages
func (m ComponentModel) Update(msg tea.Msg) (ComponentModel, tea.Cmd) {
    switch msg := msg.(type) {
    case tea.KeyPressMsg:
        return m.handleKeyPress(msg)
    case tea.WindowSizeMsg:
        m.width = msg.Width
        m.height = msg.Height
    }
    return m, nil
}

// 4. View renders output
func (m ComponentModel) View() string {
    return m.render()
}

// 5. Helper methods for complex logic
func (m ComponentModel) render() string {
    // Rendering logic here
}

State Management

Avoid Hidden State

Bad:

// State hidden in closures or package variables
var currentSelection int

func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    currentSelection++ // Modifying hidden state
}

Good:

// All state explicit in model
type Model struct {
    currentSelection int
}

func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    m.currentSelection++ // Clear, traceable state change
    return m, nil
}

Separate UI State from Data

type DiffViewerModel struct {
    // Immutable data
    file    *gitdiff.File
    content string
    lines   []string

    // Mutable UI state
    offset         int  // Scroll position
    cursorLine     int  // Current line
    selectionMode  bool // Visual mode active
    selectionStart int  // Selection anchor
}

Benefits:

• Easy to test rendering at different scroll positions
• Data can be shared/cached without UI state interference
• Clear separation of concerns

Async Operations and Caching

Pattern: Command-Based Async with Caching

For expensive operations like syntax highlighting or external tool calls:

type ComponentModel struct {
    cache   map[string]*CachedResult
    loading bool
}

// 1. Initiate async operation, return immediately
func (m *ComponentModel) SetData(data *Data) tea.Cmd {
    filePath := data.Path

    // Check cache first
    if cached, ok := m.cache[filePath]; ok {
        m.content = cached.content
        m.lines = cached.lines
        return nil
    }

    // Mark as loading, start async
    m.loading = true
    return func() tea.Msg {
        content, lines := generateContent(data)
        return contentGeneratedMsg{filePath, content, lines}
    }
}

// 2. Handle completion message
func (m ComponentModel) Update(msg tea.Msg) (ComponentModel, tea.Cmd) {
    switch msg := msg.(type) {
    case contentGeneratedMsg:
        // Cache result
        m.cache[msg.filePath] = &CachedResult{
            content: msg.content,
            lines:   msg.lines,
        }
        // Update display
        m.content = msg.content
        m.lines = msg.lines
        m.loading = false
    }
    return m, nil
}

Key points:

• Never block the UI thread
• Cache expensive computations
• Show loading state while processing
• Custom messages for async results

External Tool Integration

For tools like delta (syntax highlighting):

// 1. Check availability once at init
func NewDiffViewer(file *gitdiff.File) DiffViewerModel {
    deltaAvailable := CheckDeltaAvailable() == nil
    return DiffViewerModel{
        deltaAvailable: deltaAvailable,
    }
}

// 2. Separate pure function for testability
func generateDiffContent(file *gitdiff.File, deltaAvailable bool) (string, []string) {
    diff := buildUnifiedDiff(file)

    if !deltaAvailable {
        return diff, strings.Split(diff, "\n")
    }

    // Apply syntax highlighting
    return applyDelta(diff)
}

// 3. Make it async with proper error handling
func (m *ComponentModel) loadContent(file *gitdiff.File) tea.Cmd {
    return func() tea.Msg {
        content, lines := generateDiffContent(file, m.deltaAvailable)
        return contentReadyMsg{content, lines}
    }
}

Visual Modes and Complex Interactions

Mode-Based Keybindings

For vim-style interfaces with normal/visual modes:

type Model struct {
    mode          Mode  // Normal, Visual, Insert
    selectionMode bool  // Visual mode active
}

func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    switch msg := msg.(type) {
    case tea.KeyPressMsg:
        // Handle mode transitions first
        if msg.Code == 'v' && !m.selectionMode {
            m.selectionMode = true
            m.selectionStart = m.cursorLine
            return m, nil
        }

        if msg.Code == tea.KeyEscape && m.selectionMode {
            m.selectionMode = false
            return m, nil
        }

        // Handle mode-specific behavior
        if m.selectionMode {
            return m.handleVisualMode(msg)
        }
        return m.handleNormalMode(msg)
    }
    return m, nil
}

Selection State Management

For visual selection (highlighting lines):

type Model struct {
    selectionMode  bool
    selectionStart int  // Anchor point
    cursorLine     int  // Active end
}

// Helper to get normalized selection range
func (m Model) SelectionRange() (start, end int, active bool) {
    if !m.selectionMode {
        return 0, 0, false
    }

    start = m.selectionStart
    end = m.cursorLine
    if start > end {
        start, end = end, start
    }
    return start, end, true
}

// Use in rendering
func (m Model) View() string {
    start, end, active := m.SelectionRange()

    for i, line := range m.lines {
        if active && i >= start && i <= end {
            line = highlightStyle.Render(line)
        }
        // ... render line
    }
}

Scroll Management

Viewport Pattern

For scrollable content with fixed dimensions:

type Model struct {
    lines   []string
    offset  int  // Top visible line
    height  int  // Viewport height
}

// Calculate visible range
func (m Model) visibleLines() []string {
    start := m.offset
    end := min(m.offset + m.contentHeight(), len(m.lines))
    return m.lines[start:end]
}

// Content height (excluding fixed UI elements)
func (m Model) contentHeight() int {
    return m.height - headerHeight - footerHeight
}

// Scroll with cursor tracking
func (m Model) scrollDown() Model {
    // Move cursor first
    if m.cursorLine < len(m.lines)-1 {
        m.cursorLine++
    }

    // Adjust viewport if cursor moved out of view
    visibleBottom := m.offset + m.contentHeight() - 1
    if m.cursorLine > visibleBottom {
        m.offset++
    }

    return m
}

Key principle: Cursor moves first, viewport follows to keep cursor visible.

Editor Integration

Opening External Editors

Pattern for jumping to specific line in editor:

func (m Model) openInEditor(filePath string, lineNum int) tea.Cmd {
    return func() tea.Msg {
        editor := os.Getenv("EDITOR")
        if editor == "" {
            editor = "vim"
        }

        // Format: editor +line file
        arg := fmt.Sprintf("+%d", lineNum)
        cmd := exec.Command(editor, arg, filePath)

        // Important: Connect to terminal for interactive editors
        cmd.Stdin = os.Stdin
        cmd.Stdout = os.Stdout
        cmd.Stderr = os.Stderr

        err := cmd.Run()
        return editorFinishedMsg{err: err}
    }
}

Critical: For vim/interactive editors, you must connect stdin/stdout/stderr or the editor won't work properly.

Component Communication

Message-Based Coordination

// Custom messages for component coordination
type (
    fileSelectedMsg struct {
        file *gitdiff.File
    }

    diffLoadedMsg struct {
        content string
    }
)

// Parent handles coordination
func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    switch msg := msg.(type) {
    case fileSelectedMsg:
        // FileTree selected a file, tell DiffViewer
        return m, m.diffViewer.LoadFile(msg.file)
    }

    // Delegate to children
    var cmd tea.Cmd
    m.fileTree, cmd = m.fileTree.Update(msg)
    return m, cmd
}

Focus Management

type FocusedPanel int

const (
    FocusFileTree FocusedPanel = iota
    FocusDiffViewer
)

func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    if msg, ok := msg.(tea.KeyPressMsg); ok && msg.Code == tea.KeyTab {
        // Switch focus
        m.focused = (m.focused + 1) % 2
        return m, nil
    }

    // Only focused component handles input
    switch m.focused {
    case FocusFileTree:
        m.fileTree, cmd = m.fileTree.Update(msg)
    case FocusDiffViewer:
        m.diffViewer, cmd = m.diffViewer.Update(msg)
    }
    return m, cmd
}

Helper Modal Pattern

For overlays like help dialogs:

type Model struct {
    helpDialog *components.HelpDialog
    showHelp   bool
}

func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    // Help dialog intercepts input when visible
    if m.showHelp {
        if msg, ok := msg.(tea.KeyPressMsg); ok && msg.Code == '?' {
            m.showHelp = false
            return m, nil
        }
        // Help dialog handles all input
        *m.helpDialog, cmd = m.helpDialog.Update(msg)
        return m, cmd
    }

    // Toggle help
    if msg, ok := msg.(tea.KeyPressMsg); ok && msg.Code == '?' {
        m.showHelp = true
        return m, nil
    }

    // Normal input handling
    // ...
}

func (m Model) View() string {
    view := m.renderNormal()

    if m.showHelp {
        // Overlay help on top
        return m.helpDialog.View(view)
    }

    return view
}

Common Pitfalls

❌ Modifying State Outside Update

// BAD: State modified in View
func (m Model) View() string {
    m.offset++ // NEVER modify state in View!
    return m.render()
}

View must be pure - no side effects!

❌ Blocking Operations in Update

// BAD: Blocking I/O in Update
func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    content := os.ReadFile("large-file.txt") // BLOCKS UI!
    m.content = string(content)
    return m, nil
}

Use commands for I/O.

❌ Complex Logic in Update

// BAD: 200 lines of logic in Update
func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    switch msg := msg.(type) {
    case tea.KeyPressMsg:
        // ... 200 lines of key handling ...
    }
}

Extract to helper methods:

func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
    switch msg := msg.(type) {
    case tea.KeyPressMsg:
        return m.handleKeyPress(msg)
    }
}

func (m Model) handleKeyPress(msg tea.KeyPressMsg) (Model, tea.Cmd) {
    // Clear logic here
}

Summary

1. One component per file with clear boundaries
2. Compositor pattern for complex UIs (parent coordinates, children handle specifics)
3. All state in Model - no hidden variables
4. Commands for async - never block Update
5. Cache expensive operations - external tools, rendering
6. Mode-based behavior for complex interactions (vim-style)
7. Focus management for multi-panel UIs
8. Extract helper methods - keep Update readable
9. Pure View - no side effects, deterministic output
10. Message-based coordination - components communicate via messages