# 9. UI Architecture

> Building a complex, interactive terminal UI with React and Ink.

---

## Overview

Claude Code uses React (via [Ink](https://github.com/vadimdemedes/ink)) to render its terminal interface. This allows it to use React's declarative component model, hooks, and state management in a CLI context.

```mermaid
%%{init: {'theme': 'dark', 'themeVariables': { 'primaryColor': '#1a1a2e', 'primaryTextColor': '#e0e0e0', 'lineColor': '#4a9eff', 'primaryBorderColor': '#4a9eff'}}}%%
graph TB
    CLI["main.tsx<br/>Parses args, boots React"]:::entry
    INK["Ink rendered<br/>Terminal output"]:::external

    subgraph ReactApp["React Application"]
        APP["App.tsx<br/>Providers and Routing"]
        REPL["REPL.tsx (896KB)<br/>Main Interface"]

        subgraph Components["113 UI Components"]
            MESSAGES["Message Rendering<br/>User, Assistant, Tool"]
            DIALOGS["Permission Dialogs<br/>Y/n Prompts"]
            INPUT["Input Area<br/>Text, Voice, Paste"]
            INDICATORS["Spinners & Progress Bars"]
        end

        subgraph Hooks["83 Custom Hooks"]
            USE_STATE["useAppState"]
            USE_TOOL["useCanUseTool"]
            USE_INPUT["useInput"]
            USE_VOICE["useVoice"]
        end
    end

    CLI --> APP
    APP --> REPL
    REPL --> Components
    REPL --> Hooks
    Components --> INK

    classDef entry fill:#0d4f4f,stroke:#17a2b8,color:#e0e0e0,stroke-width:2px
    classDef external fill:#333,stroke:#888,color:#aaa,stroke-width:1px,stroke-dasharray: 5 5
```

---

## The REPL

The core of the interface is `REPL.tsx` (896KB). It manages the main interaction loop from the UI perspective, orchestrating input, rendering the conversation history, and handling background tasks.

### Key Responsibilities

1. **Virtual Scrolling**: The terminal can't hold infinite text. The REPL implements virtual scrolling, only rendering the messages that fit in the viewport plus a small buffer, while maintaining the illusion of a continuous scrollback.
2. **Input Handling**: Manages the text input area, handling multiline input, pasting files/images, keyboard shortcuts (including vim mode), and voice input.
3. **Task Foregrounding**: Only one agent task can write to the terminal at a time. The REPL manages which task is "foregrounded" and visible.
4. **State Synchronization**: Subscribes to the single immutable `AppState` store to trigger re-renders when data changes.

---

## Component Architecture

Claude Code breaks down the UI into 113 specialized components.

### Message Rendering

Each message type has a dedicated component:

- `UserMessage`: Renders user input.
- `AssistantMessage`: Renders markdown text from the model, handling streaming updates gracefully.
- `ToolUseMessage`: Displays the execution in progress (e.g., a spinner and the command being run).
- `ToolResultMessage`: Shows the outcome, often truncating long outputs and providing "Show More" functionality.

### Layout and Styling

Ink uses Yoga (a Flexbox engine) for layout. Components are composed using Flexbox principles, allowing for responsive terminal designs that adapt to the window size.

---

## State and Hooks

The UI is deeply integrated with the `AppState` store.

```mermaid
%%{init: {'theme': 'dark', 'themeVariables': { 'primaryColor': '#1a1a2e', 'primaryTextColor': '#e0e0e0', 'lineColor': '#28a745', 'primaryBorderColor': '#28a745'}}}%%
flowchart LR
    STORE["AppState<br/>Immutable Store"]:::state

    REPL["REPL Component"]:::ui
    COMP["Child Components"]:::ui

    HOOKS["Selectors<br/>useAppState(selector)"]:::hook

    STORE --> HOOKS
    HOOKS --> REPL
    HOOKS --> COMP

    classDef state fill:#1a2d4a,stroke:#4a9eff,color:#e0e0e0,stroke-width:2px
    classDef hook fill:#1b3a1b,stroke:#28a745,color:#e0e0e0,stroke-width:2px
    classDef ui fill:#2d1b4e,stroke:#6f42c1,color:#e0e0e0,stroke-width:2px
```

Custom hooks encapsulate complex logic:
- `useInput`: Handles raw stdin data, necessary for keyboard shortcuts that bypass normal text input.
- `useCanUseTool`: Connects a UI prompt to the permission system, pausing the engine until the user responds.

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