@travetto/registry/README.md

Patterns and utilities for handling registration of metadata and functionality for run-time use

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# Registry

## Patterns and utilities for handling registration of metadata and functionality for run-time use

**Install: @travetto/registry**
```bash
npm install @travetto/registry

# or

yarn add @travetto/registry
```

This module is the backbone for all "discovered" and "registered" behaviors within the framework. This is primarily used for building modules within the framework and not directly useful for application development.

## Flows
Registration, within the framework flows throw two main use cases:

### Initial Flows
The primary flow occurs on initialization of the application. At that point, the module will:
   1. Initialize [RootRegistry](https://github.com/travetto/travetto/tree/main/module/registry/src/service/root.ts#L10) and will automatically register/load all relevant files
   1. As files are imported, decorators within the files will record various metadata relevant to the respective registries
   1. When all files are processed, the [RootRegistry](https://github.com/travetto/travetto/tree/main/module/registry/src/service/root.ts#L10) is finished, and it will signal to anything waiting on registered data that its free to use it.
This flow ensures all files are loaded and processed before application starts. A sample registry could like:

**Code: Sample Registry**
```typescript
import { Class } from '@travetto/runtime';
import { MetadataRegistry } from '@travetto/registry';

interface Group {
  class: Class;
  name: string;
}

interface Child {
  name: string;
  method: Function;
}

function isComplete(o: Partial<Group>): o is Group {
  return !!o;
}

export class SampleRegistry extends MetadataRegistry<Group, Child> {
  /**
   * Finalize class after all metadata is collected
   */
  onInstallFinalize<T>(cls: Class<T>): Group {
    const pending: Partial<Group> = this.getOrCreatePending(cls);
    if (isComplete(pending)) {
      return pending;
    } else {
      throw new Error('Invalid Group');
    }
  }

  /**
   * Create scaffolding on first encounter of a class
   */
  createPending(cls: Class): Partial<Group> {
    return {
      class: cls,
      name: cls.name
    };
  }
}
```

The registry is a [MetadataRegistry](https://github.com/travetto/travetto/tree/main/module/registry/src/service/metadata.ts#L13) that similar to the [Schema](https://github.com/travetto/travetto/tree/main/module/schema#readme "Data type registry for runtime validation, reflection and binding.")'s Schema registry and [Dependency Injection](https://github.com/travetto/travetto/tree/main/module/di#readme "Dependency registration/management and injection support.")'s Dependency registry.

### Live Flow
At runtime, the registry is designed to listen for changes and to propagate the changes as necessary. In many cases the same file is handled by multiple registries. 

As the [DynamicFileLoader](https://github.com/travetto/travetto/tree/main/module/registry/src/internal/file-loader.ts#L17) notifies that a file has been changed, the [RootRegistry](https://github.com/travetto/travetto/tree/main/module/registry/src/service/root.ts#L10) will pick it up, and process it accordingly.

## Supporting Metadata
As mentioned in [Manifest](https://github.com/travetto/travetto/tree/main/module/manifest#readme "Support for project indexing, manifesting, along with file watching")'s readme, the framework produces hashes of methods, classes, and functions, to allow for detecting changes to individual parts of the codebase. During the live flow, various registries will inspect this information to determine if action should be taken.

**Code: Sample Class Diffing**
```typescript

#handleFileChanges(importFile: string, classes: Class[] = []): number {
    const next = new Map<string, Class>(classes.map(cls => [cls.Ⲑid, cls] as const));

    let prev = new Map<string, Class>();
    if (this.#classes.has(importFile)) {
      prev = new Map(this.#classes.get(importFile)!.entries());
    }

    const keys = new Set([...Array.from(prev.keys()), ...Array.from(next.keys())]);

    if (!this.#classes.has(importFile)) {
      this.#classes.set(importFile, new Map());
    }

    let changes = 0;

    // Determine delta based on the various classes (if being added, removed or updated)
    for (const k of keys) {
      if (!next.has(k)) {
        changes += 1;
        this.emit({ type: 'removing', prev: prev.get(k)! });
        this.#classes.get(importFile)!.delete(k);
      } else {
        this.#classes.get(importFile)!.set(k, next.get(k)!);
        if (!prev.has(k)) {
          changes += 1;
          this.emit({ type: 'added', curr: next.get(k)! });
        } else {
          const prevHash = describeFunction(prev.get(k)!)?.hash;
          const nextHash = describeFunction(next.get(k)!)?.hash;
          if (prevHash !== nextHash) {
            changes += 1;
            this.emit({ type: 'changed', curr: next.get(k)!, prev: prev.get(k) });
          }
        }
      }
    }
    return changes;
  }
```