Symbolic Dependency Injection

This package is meant to illustrate how a single package can accept injected behaviors (similar to classical OO Dependency Injection) via purely symbolic values.

The "host" package is a driver program. Build all three packages (run tsc in the consumer, then provider, then host) and run node index.js in the "host" directory to see the example.

The "consumer" package exposes a Consumer class that accepts an optional provider. However, rather than an interface specifying a contract of behavior, provider is a symbol that must be registered through the "consumer" package's registerProvider method. In order to ensure type safety, that symbol must be added to the __CONSUMER_PROVIDER_REGISTRY (otherwise it cannot be registered), a global type map that lists valid providers.

THe "provider" package extends the __CONSUMER_PROVIDER_REGISTRY and registers a secondary provider, exporting the symbol it used to register the provider through its API.

The "host" then imports both packages and can use whichever provider it prefers.

Notes:

• If "provider" is not imported, then the type of the provider option is limited to only the DefaultProvider. The type of the provider option expands dynamically when the "provider" package is imported.
• "host" can register its own provider if it creates a symbol, adds its unique symbol type to the provider registry, and then registers it with a handler (using the registerProvider method, same as the "provider" package does).

It is probably possible to accomplish a cleaner version of this (that does not leak __CONSUMER_PROVIDER_REGISTRY into the global scope) through module augmentation.