Service Injection

How Injection Works

Service injection is GL Plugin's mechanism for providing dependencies to your plugins automatically. Instead of manually instantiating services inside your plugins, you declare what you need and the framework delivers it.

Plugin Service Hierarchy: Mermaid Link

Key points:

• Services come from two sources: Handler (via create_injections) and Manager (via global_services)

• All services are collected in the Service Registry

• Injection happens at the __new__ level, before __init__ runs

• By the time your __init__ executes, all services are already available as instance attributes

• Services are identified by type, not by name

---

Type-Based Resolution

GL Plugin uses the class type to match services with plugin attributes. This means:

# In your handler
@classmethod
def create_injections(cls, instance):
    return {
        CalculatorService: CalculatorService()  # Key is the TYPE
    }

# In your plugin
class MyPlugin(Plugin):
    calculator: CalculatorService  # Matched by TYPE, not by name "calculator"

The framework looks at the type hint (CalculatorService) and finds the matching key in the injection dictionary. The attribute name (calculator) can be anything you want.

# These all work - the name doesn't matter, only the type
class MyPlugin(Plugin):
    calculator: CalculatorService    # ✓ Works
    calc: CalculatorService          # ✓ Works
    my_service: CalculatorService    # ✓ Works

---

Handler Services

Handler services are defined in your handler's create_injections() method. These services are specific to plugins attached to that handler.

Basic Definition

from gl_plugin.plugin.handler import PluginHandler
from typing import Any, Dict, Type

class MyHandler(PluginHandler):
    @classmethod
    def create_injections(cls, instance: "MyHandler") -> Dict[Type, Any]:
        return {
            ServiceA: ServiceA(),
            ServiceB: ServiceB(),
            ServiceC: lambda: ServiceC(),  # Factory
        }

Using Handler Properties

You can access handler properties via the instance parameter to configure services:

class DatabaseHandler(PluginHandler):
    def __init__(self, connection_string: str) -> None:
        super().__init__()
        self.connection_string = connection_string

    @classmethod
    def create_injections(cls, instance: "DatabaseHandler") -> Dict[Type, Any]:
        return {
            DatabaseService: DatabaseService(instance.connection_string)
        }

---

Global Services

Global services are shared across all plugins, regardless of which handler they're attached to. They're registered at the Manager level.

Registering Global Services

Pass services to the global_services parameter when creating the Manager:

from gl_plugin.plugin.manager import PluginManager

# Create shared services
config_service = ConfigService()
cache_service = CacheService()
logger_service = LoggerService()

# Register them globally
manager = PluginManager(
    handlers=[handler_a, handler_b],
    global_services=[
        config_service,
        cache_service,
        logger_service,
    ],
)

Consuming Global Services

Declare them in your plugin just like handler services:

@Plugin.for_handler(SomeHandler)
class MyPlugin(Plugin):
    name = "MyPlugin"
    version = "1.0.0"
    description = "A plugin using global services"

    # Handler service
    router: Router

    # Global services
    config: ConfigService
    cache: CacheService

    def __init__(self):
        super().__init__()
        # All services available here
        api_key = self.config.get("api_key")
        cached_data = self.cache.get("my_data")

When to Use Global vs Handler Services

Service Type	Scope	Use Case
Handler	Plugins of one handler	Router, handler-specific utilities
Global	All plugins	Config, cache, logging, authentication

Use global services for cross-cutting concerns that multiple plugin types need. Use handler services for functionality specific to a plugin category.

Example: Combining Both

# Services
class ConfigService:
    def get(self, key: str) -> str:
        # Returns config values
        pass

# Handler with its own service
class HttpHandler(PluginHandler):
    @classmethod
    def create_injections(cls, instance) -> Dict[Type, Any]:
        return {
            Router: lambda: Router()  # Handler-specific
        }

# Plugin using both
@Plugin.for_handler(HttpHandler)
class GithubPlugin(Plugin):
    name = "GithubPlugin"
    version = "1.0.0"
    description = "Github integration"

    # From handler
    router: Router

    # From global services
    config: ConfigService

    def __init__(self):
        super().__init__()

        @self.router.post("/webhook")
        async def handle_webhook():
            api_key = self.config.get("github_api_key")
            # ...

# Wiring it all together
handler = HttpHandler()
manager = PluginManager(
    handlers=[handler],
    global_services=[
        ConfigService(),
    ],
)
manager.register_plugin(GithubPlugin)

---

Consuming Services in Plugins

To receive an injected service, declare it as a class attribute with a type hint:

@Plugin.for_handler(MyHandler)
class MyPlugin(Plugin):
    name = "MyPlugin"
    version = "1.0.0"
    description = "A plugin with injected services"

    # Declare services you need - they'll be injected automatically
    service_a: ServiceA
    service_b: ServiceB

    def __init__(self):
        super().__init__()
        # Services are already available here!
        self.service_a.do_something()

Inheritance

When extending plugins, child classes inherit all service injections from their parents:

@Plugin.for_handler(CalculatorHandler)
class MathPlugin(Plugin):
    calculator: CalculatorService  # Injected

    @abstractmethod
    def calculate(self, a: float, b: float) -> float:
        pass


class AddPlugin(MathPlugin):
    # calculator is inherited - no need to declare again

    def calculate(self, a: float, b: float) -> float:
        return self.calculator.add(a, b)  # Just use it

---

Example: Service Injection in Action

Let's trace through the complete flow using our Calculator example.

1

Handler Defines Services

class CalculatorHandler(PluginHandler):
    @classmethod
    def create_injections(cls, instance: "CalculatorHandler") -> Dict[Type, Any]:
        return {
            CalculatorService: CalculatorService()
        }

2

Plugin Declares Dependencies

@Plugin.for_handler(CalculatorHandler)
class MathPlugin(Plugin):
    calculator: CalculatorService  # "I need a CalculatorService"

3

Manager Connects Them

When you register the plugin:

manager.register_plugin(AddPlugin)

The manager:

1. Sees AddPlugin is attached to CalculatorHandler

2. Calls CalculatorHandler.create_injections() to get handler services

3. Combines with global services from the Manager

4. Scans AddPlugin (and its parents) for type-hinted attributes

5. Matches calculator: CalculatorService with the CalculatorService key

6. Injects the service instance during __new__

7. Calls __init__ — service is ready to use

---

Multiple Services

Plugins can receive multiple services from both handler and global sources:

class AdvancedHandler(PluginHandler):
    @classmethod
    def create_injections(cls, instance) -> Dict[Type, Any]:
        return {
            CalculatorService: CalculatorService(),
            ValidatorService: ValidatorService(),
        }


@Plugin.for_handler(AdvancedHandler)
class AdvancedPlugin(Plugin):
    # Handler services
    calculator: CalculatorService
    validator: ValidatorService

    # Global services
    logger: LoggerService
    config: ConfigService

    def process(self, a: float, b: float) -> float:
        self.logger.info(f"Processing {a} and {b}")

        if not self.validator.validate(a, b):
            raise ValueError("Invalid input")

        return self.calculator.add(a, b)

---

What Happens If a Service Is Missing?

If a plugin declares a service that neither the handler nor global services provide, the attribute will not be set. Accessing it will raise an AttributeError:

@Plugin.for_handler(CalculatorHandler)
class BrokenPlugin(Plugin):
    calculator: CalculatorService  # ✓ Provided by handler
    logger: LoggerService          # ✗ NOT provided anywhere

    def do_something(self):
        self.calculator.add(1, 2)  # Works
        self.logger.info("Hello")  # AttributeError!

Always ensure your handler or global services provide all services that your plugins expect. Missing services will cause runtime errors when accessed.

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Summary

Concept	Description
Injection timing	Services are injected at __new__, before __init__
Resolution	Type-based matching (class type, not attribute name)
Declaration	Use type hints on class attributes
Inheritance	Child plugins inherit parent's service declarations
Handler services	Specific to plugins of that handler
Global services	Shared across all plugins