[**@blocksuite/store**](../../../@blocksuite/store/README.md)

***

[BlockSuite API Documentation](../../../README.md) / [@blocksuite/store](../README.md) / Extension

# Class: `abstract` Extension

# Understanding Extensions

Extensions provide a way to extend the functionality of a system using dependency injection.
They allow you to register services, implementations, and factories in the DI container,
which can then be retrieved and used by different parts of the application.

Extensions are particularly useful for:
- Registering different implementations for different types
- Creating pluggable architecture where components can be added or removed
- Managing dependencies between different parts of the application

# Usage Example: Fruit Processing System

Let's consider a fruit processing system where different types of fruits need
different processing methods. We'll show how to implement this using extensions.

## Step 1: Define the interfaces

```ts
interface FruitProcessor {
  process(fruit: Fruit): void;
}

interface Fruit {
  type: string;
  // other properties
}
```

## Step 2: Create a service identifier

```ts
import { createIdentifier } from '@blocksuite/global/di';

const FruitProcessorProvider = createIdentifier<FruitProcessor>('fruit-processor-provider');
```

## Step 3: Create implementations

```ts
class AppleProcessor implements FruitProcessor {
  process(fruit: Fruit): void {
    console.log('Slicing apple');
    // Apple-specific processing
  }
}

class BananaProcessor implements FruitProcessor {
  process(fruit: Fruit): void {
    console.log('Peeling banana');
    // Banana-specific processing
  }
}
```

## Step 4: Create an extension factory

```ts
const FruitProcessorExtension = (
  fruitType: string,
  implementation: new () => FruitProcessor
): ExtensionType => {
  return {
    setup: di => {
      di.addImpl(FruitProcessorProvider(fruitType), implementation);
    }
  };
};
```

## Step 5: Create concrete extensions

```ts
export const AppleProcessorExtension = FruitProcessorExtension('apple', AppleProcessor);
export const BananaProcessorExtension = FruitProcessorExtension('banana', BananaProcessor);
```

## Step 6: Use the extensions

```ts
import { Container } from '@blocksuite/global/di';

class FruitProcessingSystem {
  provider: ServiceProvider;

  constructor(extensions: ExtensionType[]) {
    const container = new Container();

    // Set up all extensions
    extensions.forEach(ext => ext.setup(container));

    // Create a provider from the container
    this.provider = container.provider();
  }

  processFruit(fruit: Fruit) {
    // Get the appropriate processor based on fruit type
    const processor = this.provider.get(FruitProcessorProvider(fruit.type));

    // Process the fruit
    processor.process(fruit);
  }
}

// Initialize the system with extensions
const system = new FruitProcessingSystem([
  AppleProcessorExtension,
  BananaProcessorExtension
]);

// Use the system
system.processFruit({ type: 'apple' });  // Output: Slicing apple
system.processFruit({ type: 'banana' }); // Output: Peeling banana
```

Note: We deliberately used a non-block specific example here. In BlockSuite, the extension
pattern can be applied to any entity that can be configured by third parties, not just blocks.
This includes different tools in the whiteboard, different column types in database blocks,
and many other extensible components. The pattern remains the same regardless of what you're extending.

## Extended by

- [`StoreExtension`](StoreExtension.md)
- [`BlockService`](../../block-std/index/classes/BlockService.md)
- [`LifeCycleWatcher`](../../block-std/index/classes/LifeCycleWatcher.md)