Bubble Tea Patterns

Elm Architecture

type Model struct {
    content  string
    viewport viewport.Model
    ready    bool
}

func (m Model) Init() tea.Cmd { return nil }

func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
    switch msg := msg.(type) {
    case tea.KeyMsg:
        if msg.String() == "q" {
            return m, tea.Quit
        }
    case tea.WindowSizeMsg:
        // Initialize viewport on first size message
        if !m.ready {
            m.viewport = viewport.New(msg.Width, msg.Height)
            m.viewport.SetContent(m.content)
            m.ready = true
        } else {
            m.viewport.Width = msg.Width
            m.viewport.Height = msg.Height
        }
    }
    var cmd tea.Cmd
    m.viewport, cmd = m.viewport.Update(msg)
    return m, cmd
}

func (m Model) View() string {
    if !m.ready {
        return "Loading..."
    }
    return m.viewport.View()
}

Critical: Wait for tea.WindowSizeMsg before initializing viewport - dimensions arrive async.

Stdin Piping (git diff | myapp)

func main() {
    stat, _ := os.Stdin.Stat()
    if stat.Mode()&os.ModeNamedPipe == 0 && stat.Size() == 0 {
        fmt.Println("Usage: git diff | diffview")
        os.Exit(1)
    }

    content, _ := io.ReadAll(os.Stdin)
    m := Model{content: string(content)}

    p := tea.NewProgram(m,
        tea.WithAltScreen(),       // Full-screen, restores on exit
        tea.WithMouseCellMotion(), // Mouse wheel support
    )
    p.Run()
}

Keyboard Handling

Simple matching:

case tea.KeyMsg:
    switch msg.String() {
    case "j", "down":
        m.viewport.LineDown(1)
    case "k", "up":
        m.viewport.LineUp(1)
    case "ctrl+d":
        m.viewport.HalfViewDown()
    case "ctrl+u":
        m.viewport.HalfViewUp()
    case "G":
        m.viewport.GotoBottom()
    case "q", "ctrl+c":
        return m, tea.Quit
    }

Multi-key sequences (gg):

type Model struct {
    pendingKey string
    // ...
}

case tea.KeyMsg:
    if m.pendingKey == "g" && msg.String() == "g" {
        m.viewport.GotoTop()
        m.pendingKey = ""
        return m, nil
    }
    if msg.String() == "g" {
        m.pendingKey = "g"
        return m, nil
    }
    m.pendingKey = ""

Customizable keymaps with bubbles/key:

import "github.com/charmbracelet/bubbles/key"

type KeyMap struct {
    Down key.Binding
    Up   key.Binding
    Quit key.Binding
}

var DefaultKeyMap = KeyMap{
    Down: key.NewBinding(key.WithKeys("j", "down"), key.WithHelp("j/↓", "down")),
    Up:   key.NewBinding(key.WithKeys("k", "up"), key.WithHelp("k/↑", "up")),
    Quit: key.NewBinding(key.WithKeys("q", "ctrl+c"), key.WithHelp("q", "quit")),
}

// Usage: key.Matches(msg, m.keymap.Down)

Viewport Built-in Keys

Lipgloss Styling

import "github.com/charmbracelet/lipgloss"

// Diff line styles with adaptive colors
addedStyle := lipgloss.NewStyle().
    Foreground(lipgloss.AdaptiveColor{Light: "28", Dark: "34"}).
    Background(lipgloss.AdaptiveColor{Light: "194", Dark: "22"})

removedStyle := lipgloss.NewStyle().
    Foreground(lipgloss.AdaptiveColor{Light: "160", Dark: "203"}).
    Background(lipgloss.AdaptiveColor{Light: "224", Dark: "52"})

// Line numbers
lineNumStyle := lipgloss.NewStyle().
    Foreground(lipgloss.Color("245")).
    Width(6).
    Align(lipgloss.Right)

// Side-by-side layout
joined := lipgloss.JoinHorizontal(lipgloss.Top, leftPanel, rightPanel)

// Measure ANSI-aware width
width := lipgloss.Width(styledString)

Layering styles (syntax + diff): Render inner style first, wrap with outer.

Header/Footer Pattern

func (m Model) View() string {
    return fmt.Sprintf("%s\n%s\n%s",
        m.headerView(),
        m.viewport.View(),
        m.footerView(),
    )
}

// Calculate viewport height accounting for margins
case tea.WindowSizeMsg:
    headerHeight := lipgloss.Height(m.headerView())
    footerHeight := lipgloss.Height(m.footerView())
    m.viewport.Height = msg.Height - headerHeight - footerHeight

Testing

Package: github.com/charmbracelet/x/exp/teatest

Deterministic Color Output

Use explicit renderer to avoid terminal auto-detection:

// Test helper - creates renderer with fixed TrueColor profile
func trueColorRenderer() *lipgloss.Renderer {
    r := lipgloss.NewRenderer(io.Discard)
    r.SetColorProfile(termenv.TrueColor)
    return r
}

// Pass to model via option
m := NewModel(content,
    WithTheme(lipgloss.TestTheme()),      // Stable colors
    WithRenderer(trueColorRenderer()),     // Deterministic output
)

Why: Without explicit renderer, Lipgloss auto-detects terminal capabilities. Tests become flaky across environments.

Test Theme Pattern

Use TestTheme() with stable, predictable colors. Production themes can evolve without breaking tests:

// In lipgloss/theme.go
func TestTheme() diffview.Theme {
    return newTheme(diffview.Palette{
        Added:   "#00ff00",  // Pure green - easy to verify
        Deleted: "#ff0000",  // Pure red
        // ... stable values that won't change
    })
}

Principle: TestTheme() is a stable contract. DefaultTheme() can change aesthetically.

Behavior Tests vs Color Tests

Behavior tests - verify functionality, not appearance:

func TestNavigation(t *testing.T) {
    t.Parallel()

    m := NewModel(diff, WithTheme(lipgloss.TestTheme()))
    tm := teatest.NewTestModel(t, m,
        teatest.WithInitialTermSize(80, 24),
    )

    tm.Send(tea.KeyMsg{Runes: []rune{'j'}})

    // Check content presence, ignore colors
    teatest.WaitFor(t, tm.Output(), func(out []byte) bool {
        return bytes.Contains(out, []byte("expected content"))
    })
}

Color integration tests - verify colors apply correctly:

func TestColorsApplied(t *testing.T) {
    t.Parallel()

    m := NewModel(diff,
        WithTheme(lipgloss.TestTheme()),
        WithRenderer(trueColorRenderer()),
    )
    tm := teatest.NewTestModel(t, m,
        teatest.WithInitialTermSize(80, 24),
    )

    teatest.WaitFor(t, tm.Output(), func(out []byte) bool {
        // TrueColor format: ESC[48;2;R;G;Bm (background)
        hasBackground := bytes.Contains(out, []byte("48;2;"))
        hasContent := bytes.Contains(out, []byte("+added"))
        return hasBackground && hasContent
    })
}

Golden File Testing

func TestView(t *testing.T) {
    m := NewModel(testContent)
    tm := teatest.NewTestModel(t, m,
        teatest.WithInitialTermSize(80, 24),
    )

    tm.Send(tea.KeyMsg{Runes: []rune{'j'}})
    tm.Send(tea.KeyMsg{Runes: []rune{'q'}})

    out, _ := io.ReadAll(tm.FinalOutput(t))
    teatest.RequireEqualOutput(t, out)  // Compares to testdata/TestView.golden
}

Workflow:

1. go test -update → creates/updates testdata/TestName.golden
2. Golden files include ANSI codes - use TestTheme() for stability
3. Tests fail with unified diff when output changes

Testing Principles

1. Behavior tests use TestTheme() - decouples from aesthetic changes
2. Always use explicit renderer - no terminal auto-detection in tests
3. Check content, not colors for most tests - colors are implementation detail
4. Color tests verify ANSI presence - bytes.Contains(out, []byte("48;2;")) not specific RGB values
5. One theme change shouldn't break behavior tests - only color-specific tests

Gotchas

1. Always return model from Update, even if modified via receiver
2. View() must be pure - no side effects
3. Commands run async - don't assume order
4. No line wrapping - viewport truncates long lines
5. Pass all messages to viewport for built-in scrolling to work
6. Never use len(string) for display width - use lipgloss.Width() instead:

   // WRONG: len() counts bytes, not display width
   padding := strings.Repeat(" ", maxWidth - len(line))
   
   // CORRECT: lipgloss.Width() handles Unicode properly
   padding := strings.Repeat(" ", maxWidth - lipgloss.Width(line))
   

   • len("日本語") = 9 bytes, but displays as 6 cells (CJK are double-width)
   • len("emoji 😀") = 10 bytes, but displays as 8 cells
   • lipgloss.Width() uses go-runewidth internally for correct display width