# @ngrx/store v3

### Problems:
* Users want to [compose reducer tree across modules](https://github.com/ngrx/store/issues/241)
* Idea of a single reducer function makes it difficult for the library to dynamically augment
  the shape of the state tree
* Turning control over to the library to build the root reducer limits the 
  use of meta-reducers
* Feature modules may inadvertently collide with the state of the root module
* Library authors may want to leverage @ngrx/store in their projects and provide an easy way
  for their libraries to work in apps already built with @ngrx/store

### Proposal:
* Add a fractal state mechanism for isolating child modules to @ngrx/store
* Shift control over creating the root reducer function to @ngrx/store. Developers would have to provide an action
  reducer map instead of a root reducer.
* Allow developers to optionally provide a `reducerFactory` function for turning an action reducer 
  map into an action reducer.
* Scope `Store` such that a child module's services and components only have access to the state of that module.
* All actions continue to move through a global dispatcher. Every reducer continues to receive every action.
* Add an API for dynamically inserting a reducer at a specific key
* If a feature module is isolated (as in there is no parent module that uses @ngrx/store), intelligently work like a root module

At the core this is still redux making it easy to instrument, rehydrate, etc.

### API:
Root module:
```ts
@NgModule({
  imports: [
    StoreModule.forRoot({ todos: todoReducer, layout: layoutReducer }, {
      initialState?: { ... },
      reducerFactory?: combineReducers
    })
  ]
})
export class AppModule { }
```

Global state shape:
```ts
{
  todos: { ... },
  layout: { ... }
}
```

Child module:
```ts
@NgModule({
  imports: [
    StoreModule.forFeature('feature', { blog: blogReducer }, {
      initialState?: { ... },
      reducerFactory?: combineReducers
    })
  ]
})
export class FeatureModule { }
```

New global state shape:
```ts
{
  todos: { ... },
  layout: { ... },
  feature: {
    blog: { ... }
  }
}
```

Injecting `Store` in a component or service belonging to `FeatureModule` only gives you access to the local state:
```ts
{
  blog: { ... }
}
```

### Implementation
I have a rough but complete implementation available in a branch here: https://github.com/MikeRyan52/store/tree/feat/fractal-statemanagement

### Prior Art:
* [cycle-onionify](https://github.com/staltz/cycle-onionify)
* [vuejs/vuex](https://github.com/vuejs/vuex)