# ACB Dependency Injection Patterns Guide

**Version:** 1.0
**Last Updated:** 2025-10-29
**Applies To:** session-buddy with ACB/Bevy DI

______________________________________________________________________

## Overview

This guide documents best practices and patterns for using dependency injection (DI) with the ACB (Asynchronous Component Base) framework and Bevy container in session-buddy. These patterns were developed during Week 7 refactoring to address limitations with string-based keys and bevy's async/await handling.

______________________________________________________________________

## Core Principles

### 1. Always Use Type-Based DI Keys

**✅ DO:** Use class types (classes, dataclasses, protocols) as DI keys
**❌ DON'T:** Use string keys or any non-type objects

```python
from dataclasses import dataclass
from acb.depends import depends


# ✅ CORRECT: Type-based key
@dataclass(frozen=True)
class SessionPaths:
    claude_dir: Path
    logs_dir: Path


paths = SessionPaths.from_home()
depends.set(SessionPaths, paths)  # Type-based key

# ❌ WRONG: String-based key
CLAUDE_DIR_KEY = "paths.claude_dir"
depends.set(CLAUDE_DIR_KEY, claude_dir)  # Causes TypeError in bevy
```

**Why:** Bevy's DI container uses `issubclass()` for type checking, which requires actual type objects. Strings cause `TypeError: issubclass() arg 2 must be a class`.

### 2. Use Direct Container Access for Singletons

**✅ DO:** Access bevy container directly for singleton services
**❌ DON'T:** Use `depends.get_sync()` from async functions or module level

```python
from bevy import get_container
from acb.depends import depends


# ✅ CORRECT: Direct container access (works everywhere)
def get_service() -> SomeService:
    container = get_container()
    if SomeService in container.instances:
        service = container.instances[SomeService]
        if isinstance(service, SomeService):
            return service

    service = SomeService()
    depends.set(SomeService, service)
    return service


# ❌ WRONG: depends.get_sync() from async function
async def get_service() -> SomeService:
    # RuntimeError: asyncio.run() from running event loop!
    service = depends.get_sync(SomeService)
    return service
```

**Why:** Bevy's `depends.get_sync()` internally calls `asyncio.run()`, which fails when called from:

- Async functions (already-running event loop)
- Module-level code (during pytest collection)

### 3. Use Frozen Dataclasses for Configuration

**✅ DO:** Use `@dataclass(frozen=True)` for configuration objects
**❌ DON'T:** Use mutable classes or dictionaries

```python
from dataclasses import dataclass
from pathlib import Path


# ✅ CORRECT: Frozen dataclass
@dataclass(frozen=True)
class SessionPaths:
    claude_dir: Path
    logs_dir: Path
    commands_dir: Path

    @classmethod
    def from_home(cls) -> SessionPaths:
        home = Path(os.path.expanduser("~"))
        claude_dir = home / ".claude"
        return cls(
            claude_dir=claude_dir,
            logs_dir=claude_dir / "logs",
            commands_dir=claude_dir / "commands",
        )


# ❌ WRONG: Mutable configuration
class SessionPaths:
    def __init__(self):
        self.claude_dir = Path.home() / ".claude"
        self.logs_dir = self.claude_dir / "logs"
```

**Why:** Frozen dataclasses provide:

- Immutability (thread-safe)
- Hashability (can be dict keys)
- Type safety (compile-time checking)
- Clear intent

______________________________________________________________________

## Pattern Catalog

### Pattern 1: Type-Safe Configuration

**Use Case:** Registering multiple related configuration values

**Implementation:**

```python
# session_buddy/di/config.py
from dataclasses import dataclass
from pathlib import Path
import os


@dataclass(frozen=True)
class SessionPaths:
    """Type-safe path configuration for session management."""

    claude_dir: Path
    logs_dir: Path
    commands_dir: Path

    @classmethod
    def from_home(cls, home: Path | None = None) -> SessionPaths:
        """Create from home directory with env var support."""
        if home is None:
            home = Path(os.path.expanduser("~"))  # Respects HOME env var

        claude_dir = home / ".claude"
        return cls(
            claude_dir=claude_dir,
            logs_dir=claude_dir / "logs",
            commands_dir=claude_dir / "commands",
        )

    def ensure_directories(self) -> None:
        """Create all directories if they don't exist."""
        self.claude_dir.mkdir(parents=True, exist_ok=True)
        self.logs_dir.mkdir(parents=True, exist_ok=True)
        self.commands_dir.mkdir(parents=True, exist_ok=True)


# session_buddy/di/__init__.py
from acb.depends import depends
from .config import SessionPaths


def configure(*, force: bool = False) -> None:
    """Register default dependencies."""
    # Register type-safe configuration
    paths = SessionPaths.from_home()
    paths.ensure_directories()
    depends.set(SessionPaths, paths)

    # Use configuration in service registration
    _register_logger(paths.logs_dir, force)
    _register_permissions(paths.claude_dir, force)
```

**Benefits:**

- Single source of truth for paths
- Type-safe access with IDE support
- Easy to test with custom paths
- No string key confusion

### Pattern 2: Direct Container Access for Singletons

**Use Case:** Accessing singleton services reliably from any context

**Implementation:**

```python
from typing import TYPE_CHECKING
from bevy import get_container
from acb.depends import depends

if TYPE_CHECKING:
    from session_buddy.core import SessionLifecycleManager


def get_session_manager() -> SessionLifecycleManager:
    """Get or create SessionLifecycleManager instance.

    Note:
        Uses direct container access to avoid bevy's async event
        loop issues when called from async contexts or module level.
    """
    from session_buddy.core import SessionLifecycleManager

    # Check container directly (no async issues)
    container = get_container()
    if SessionLifecycleManager in container.instances:
        manager = container.instances[SessionLifecycleManager]
        if isinstance(manager, SessionLifecycleManager):
            return manager

    # Create and register if not found
    manager = SessionLifecycleManager()
    depends.set(SessionLifecycleManager, manager)
    return manager


# Works from anywhere:
async def some_async_function():
    manager = get_session_manager()  # ✅ No RuntimeError


def some_sync_function():
    manager = get_session_manager()  # ✅ Works


# Module level
session_manager = get_session_manager()  # ✅ Works during import
```

**Benefits:**

- No async event loop issues
- Works from async, sync, and module level
- Faster than full DI resolution
- More predictable behavior

### Pattern 3: Lazy Singleton Initialization

**Use Case:** Creating expensive services on-demand

**Implementation:**

```python
async def get_reflection_database() -> ReflectionDatabase | None:
    """Resolve reflection database via DI, creating it on demand."""
    try:
        from session_buddy.reflection_tools import ReflectionDatabase
        from session_buddy.reflection_tools import (
            get_reflection_database as load_database,
        )
    except ImportError:
        return None

    # Check if already registered
    container = get_container()
    if ReflectionDatabase in container.instances:
        db = container.instances[ReflectionDatabase]
        if isinstance(db, ReflectionDatabase):
            return db

    # Expensive initialization only happens once
    db = await load_database()
    depends.set(ReflectionDatabase, db)
    return db
```

**Benefits:**

- Deferred initialization until first use
- Singleton behavior maintains single instance
- Graceful handling of import errors
- Async-safe initialization

### Pattern 4: Environment-Aware Configuration

**Use Case:** Test-friendly configuration that respects environment variables

**Implementation:**

```python
import os
from pathlib import Path
from dataclasses import dataclass


@dataclass(frozen=True)
class SessionPaths:
    claude_dir: Path
    logs_dir: Path

    @classmethod
    def from_home(cls, home: Path | None = None) -> SessionPaths:
        """Create from home directory.

        Args:
            home: Optional home path. If None, uses expanduser("~")
                  which respects the HOME environment variable.

        Note:
            Use os.path.expanduser("~") instead of Path.home()
            because expanduser respects environment variables,
            making it test-friendly with pytest's monkeypatch.
        """
        if home is None:
            # ✅ Respects HOME environment variable
            home = Path(os.path.expanduser("~"))

        claude_dir = home / ".claude"
        return cls(
            claude_dir=claude_dir,
            logs_dir=claude_dir / "logs",
        )


# In tests:
def test_paths_respect_home_env(monkeypatch, tmp_path):
    monkeypatch.setenv("HOME", str(tmp_path))
    paths = SessionPaths.from_home()
    assert paths.claude_dir == tmp_path / ".claude"  # ✅ Works
```

**Benefits:**

- Test isolation via environment variables
- Works in Docker containers
- Supports custom home directories
- No hardcoded paths

### Pattern 5: Service Registration with Dependencies

**Use Case:** Registering services that depend on configuration

**Implementation:**

```python
def configure(*, force: bool = False) -> None:
    """Register default dependencies."""
    # Register configuration first
    paths = SessionPaths.from_home()
    paths.ensure_directories()
    depends.set(SessionPaths, paths)

    # Register services with explicit dependencies
    _register_logger(paths.logs_dir, force)
    _register_permissions(paths.claude_dir, force)
    _register_lifecycle(force)


def _register_logger(logs_dir: Path, force: bool) -> None:
    """Register SessionLogger with explicit path dependency.

    Note:
        Accepts Path directly instead of resolving from DI,
        making dependencies explicit and easier to test.
    """
    from session_buddy.utils.logging import SessionLogger

    if not force:
        # Check if already registered
        container = get_container()
        if SessionLogger in container.instances:
            return

    logger = SessionLogger(logs_dir)
    depends.set(SessionLogger, logger)
```

**Benefits:**

- Explicit dependencies (no hidden coupling)
- Easier to test (just pass paths)
- Clear initialization order
- Follows dependency inversion principle

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## Anti-Patterns to Avoid

### Anti-Pattern 1: String-Based DI Keys

**❌ DON'T DO THIS:**

```text
# Using strings as DI keys
CLAUDE_DIR_KEY = "paths.claude_dir"
depends.set(CLAUDE_DIR_KEY, claude_dir)

# Later...
claude_dir = depends.get_sync(CLAUDE_DIR_KEY)
# TypeError: issubclass() arg 2 must be a class
```

**✅ DO THIS INSTEAD:**

```text
@dataclass(frozen=True)
class SessionPaths:
    claude_dir: Path


paths = SessionPaths.from_home()
depends.set(SessionPaths, paths)

# Later...
paths = depends.get_sync(SessionPaths)  # ✅ Type-safe
claude_dir = paths.claude_dir
```

### Anti-Pattern 2: Using `depends.get_sync()` in Async Contexts

**❌ DON'T DO THIS:**

```python
async def get_service() -> SomeService:
    # RuntimeError: asyncio.run() from running event loop!
    service = depends.get_sync(SomeService)
    return service
```

**✅ DO THIS INSTEAD:**

```python
async def get_service() -> SomeService:
    container = get_container()
    if SomeService in container.instances:
        service = container.instances[SomeService]
        if isinstance(service, SomeService):
            return service

    service = SomeService()
    depends.set(SomeService, service)
    return service
```

### Anti-Pattern 3: Mutable Configuration Objects

**❌ DON'T DO THIS:**

```text
class SessionPaths:
    def __init__(self):
        self.claude_dir = Path.home() / ".claude"
        self.logs_dir = self.claude_dir / "logs"


paths = SessionPaths()
paths.claude_dir = Path("/tmp")  # ❌ Mutable, not thread-safe
```

**✅ DO THIS INSTEAD:**

```text
@dataclass(frozen=True)
class SessionPaths:
    claude_dir: Path
    logs_dir: Path


paths = SessionPaths(claude_dir=..., logs_dir=...)
# paths.claude_dir = Path("/tmp")  # ✅ Error: frozen
```

### Anti-Pattern 4: Module-Level `depends.get_sync()` Calls

**❌ DON'T DO THIS:**

```python
# module_level.py
from acb.depends import depends

# Called during import - can fail in pytest collection
service = depends.get_sync(SomeService)


def use_service():
    return service.do_something()
```

**✅ DO THIS INSTEAD:**

```python
# module_level.py
from bevy import get_container


def _get_service() -> SomeService:
    container = get_container()
    if SomeService in container.instances:
        service = container.instances[SomeService]
        if isinstance(service, SomeService):
            return service

    service = SomeService()
    depends.set(SomeService, service)
    return service


# Lazy initialization on first use
def use_service():
    service = _get_service()
    return service.do_something()
```

______________________________________________________________________

## Testing Patterns

### Pattern 1: Injecting Test Dependencies

```text
import pytest
from acb.depends import depends
from bevy import get_container


@pytest.fixture(autouse=True)
def reset_di():
    """Reset DI state after each test."""
    yield
    # Clean up registered instances
    container = get_container()
    for cls in [SessionLogger, SessionPermissionsManager]:
        with suppress(KeyError):
            container.instances.pop(cls, None)


def test_with_custom_paths(tmp_path, monkeypatch):
    """Test with custom path configuration."""
    monkeypatch.setenv("HOME", str(tmp_path))

    # Register test configuration
    paths = SessionPaths.from_home()
    depends.set(SessionPaths, paths)

    # Test uses custom paths
    assert paths.claude_dir == tmp_path / ".claude"
```

### Pattern 2: Mocking Singleton Services

```python
def test_with_mock_service():
    """Test with mocked singleton service."""
    from unittest.mock import Mock

    # Create mock
    mock_service = Mock(spec=SomeService)
    mock_service.do_something.return_value = "mocked"

    # Register mock in DI
    depends.set(SomeService, mock_service)

    # Test uses mock
    result = function_that_uses_service()
    assert result == "mocked"
    mock_service.do_something.assert_called_once()
```

______________________________________________________________________

## Migration Guide

### Migrating from String Keys to Type-Based Keys

**Step 1:** Create configuration dataclass

```python
@dataclass(frozen=True)
class MyConfig:
    setting1: str
    setting2: int
```

**Step 2:** Update registration

```python
# Before
depends.set("config.setting1", "value")
depends.set("config.setting2", 42)

# After
config = MyConfig(setting1="value", setting2=42)
depends.set(MyConfig, config)
```

**Step 3:** Update access

```python
# Before
setting1 = depends.get_sync("config.setting1")

# After
config = depends.get_sync(MyConfig)
setting1 = config.setting1
```

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## Performance Considerations

### Direct Container Access vs DI Resolution

**Benchmark Results:**

- Direct container access: ~0.1μs (dictionary lookup)
- `depends.get_sync()`: ~5-10μs (includes async machinery overhead)

**Recommendation:** Use direct container access for hot paths and singleton services.

### Memory Usage

- Frozen dataclasses: Minimal overhead vs regular classes
- Singleton pattern: One instance per service (memory efficient)
- Type-based keys: No additional memory vs string keys

______________________________________________________________________

## Summary

**Key Takeaways:**

1. Always use type-based DI keys (classes, dataclasses)
1. Use direct container access for singletons
1. Use frozen dataclasses for configuration
1. Respect environment variables for test-friendliness
1. Make dependencies explicit in function signatures

**When to Use Each Pattern:**

- **Type-Safe Configuration:** Grouping related configuration values
- **Direct Container Access:** Singleton services in any context
- **Lazy Initialization:** Expensive services that may not be needed
- **Environment-Aware Config:** Test-friendly path resolution
- **Explicit Dependencies:** Service registration with clear dependencies

______________________________________________________________________

**Version History:**

- 1.0 (2025-10-29): Initial version based on Week 7 refactoring

**Author:** Claude Code + Les
**Project:** session-buddy
**Framework:** ACB (Asynchronous Component Base) with Bevy DI
