Reactive State management in the angular - NgRx Store, actions, selectors

NgRx framework helps to build reactive angular applications.

Basic Concepts

NgRx store provides reactive state management for the angular application. NgRx store is the redux implementation developed specifically for angular applications and provides RxJS observable API.

state is an immutable data structure that is a single source of truth for the whole application.

NgRx Actions represent the unique events in the application which may be used to perform state transition or trigger side-effects.

NgRx Reducersare pure functions that react to Actionsto perform state transitions.

NgRx Selectorsare pure functions that select, derive, or compose a slice of the state.

NgRx Effects allow the isolation of side-effects.

Prerequisites -

• you have a fair understanding of the angular framework.
• You have a basic understanding of redux architecture.
• you have a fair knowledge of RxJs Observable API and various operators.

Table of contents

• Installation
• State
  • Design the State
  • Initialize the State
• NgRx Actions
• NgRx Reducer
  • createReducer function
  • Create ActionReducerMap
  • Register the State
• NgRx selectors
  • Selectors
  • createSelector function
  • String selectors.

Installation

If you already have an angular app, you can directly go to step - 4

# 1) install angular cli
npm install -g @angular/cli

# 2) create a demo app
ng new ngrx-angular-demo

# 3) navigate to demo app
cd ngrx-angular-demo

# 4) install ngrx store
ng add @ngrx/store@latest

# 5) run angular in dev mode
ng serve

To begin with , let us have a look at an example file structure. A structure like this would be helpful to split up each feature of NgRxstate management in your app. I usually replicate the same structure in each feature module.

    ──store
        |_ app.actions.ts
        |_ app.effects.ts
        |_ app.reducer.ts
        |_ app.selectors.ts

• app.actions.ts file will contain the NgRX actions
• app.effects.tsfile will contain the NgRx effects.
• app.reducer.ts file will contain the State design and its initialization. it will also contain a reducer function.
• app.selectors.ts will contain the NgRx selectors.

Here is the complete project setup.

State

The state represents an immutable object that contains the state of an application. It is read-only, so every state transition will return a new state rather than modifying the existing state. As the application grows, each feature should contain a separate state which are part of the global app state. As such, the application state contains one or more feature states.

The state is similar to Javascript objects. It contains the feature states as the key-value pairs _where the key represents a feature state and the value is the feature state object.

The state related to a feature module is referred to as feature state.

interface State{
    feature_1: FeatureOneState,
    feature_2: FeatureTwoState,
    feature_3: FeatureThreeState
}

Design the State

Let’s assume, our angular application has many feature modules. One of the feature modules is responsible for user’s profile. The profilemodule is responsible for rendering the list of usersand the related posts.

To design the state, we can assume that the state required for the profile module should contain List of users and List of posts.

Let’s call the profile state as ProfileFeatureState.

/** User modal */
export interface User {

  id: number;
  email: string;
  first_name: string;
  last_name: string;
  avatar: string;
}

/ **post modal** /
export interface Post {
  id: number;
  userId: number;
  title: string;
  body: string;
}

/ **the modals should ideally be in their own ts file** /

export interface ProfileFeatureState {
  users: User[];
  posts: Post[];
}

We defined the type for Userand Post and also created an interface for ProfileFeatureState.

Finally, we would add ProfileFeatureState to the applications root state -AppState. The profile key represents the profileFeatureState.

interface AppState{    
    profile: UserFeatureState,    
//..other features here
}

Initialize the State

Initially, the state of the application is nullsince there would be no data. As such, both the users array and posts array would be initialized to null.

export const initialProfileFeatureState: ProfileFeatureState = {
  users: null,
  addresses: null
};

At this point, the app.reducer.ts file should look like -

/*app.reducer.ts*/

/** User modal */
export interface User {
  id: number;
  email: string;
  first_name: string;
  last_name: string;
  avatar: string;
}

/** Post Modal */

export interface Post {
  id: number;
  userId: number;
  title: string;
  body: string;
}

export interface ProfileFeatureState {
  users: User[];
  addresses: Address[];
}

export const initialProfileFeatureState: ProfileFeatureState = {
  users: null,
  addresses: null
};

export interface AppState {
  profile: ProfileFeatureState;
}

Actions:

NgRx Actions represent events in the application. They may trigger a state transition or trigger a side-effect in the NgRx Effect services.

interface Action{
    type:string
    //optional metadata properties
}

The Actioninterface contains a property called Type. The Typeproperty identifies the action. Actions can also contain optional metadata.

createActionfunction is used to create the actions and it returns an ActionCreator function. ActionCreator function when called returns an action of type TypedAction. Optionally, we can also supply additional metadata using props function.

Let’s go ahead and create an action to add users to ProfileFeatureState.

export const addUsers = createAction(
 '[profile] add users',
 props<{ users: User[] }>()
);

Notice the type of addUsers action as [profile] add users. The [profile] represents the source of action. Also, the props contain the array of users as the metadata.

Similarly, we can create an action for adding posts to the feature state.

//file: app.actions.ts

export const addPosts = createAction(
  '[profile] add posts',
  props<{ posts: Post[] }>()
);

addUsers action is dispatched to indicate that the users should be added to the state. It will also contain user[] as metadata.

Similarly the Actions related to posts are dispatched

Actions represent the events and not the commands or operations. A single command or operation may generate many types of Actions. For example, An operation that creates a new user would at least generate Actions for success and failure such as [profile] user created or [profile] user creation failed.

NgRx Reducer -

Reducers are pure functions which perform transitions from one state to another state based on the latest action dispatched. The reducer functions do not modify the existing state, rather it returns a new state for every state transition. Hence all the reducer functions perform immutable operations.

CreateReducer

NgRx provides a createReducer function to create reducers. It takes initialState as the first param and any number of on functions. The on function provide association between actions and the state changes.

When an action is dispatched, all the reducers receive the action. The on function mapping determines whether the reducer should handle the action.

createReducer function returns an ActionReducer function . ActionReducer function takes an Action and a State as input, and returns a new computed State.

Let’s go ahead a create reducer that handles transitions for ProfileFeatureState.

import * as AppActions from './app.actions';

const theProfFeatureReducer = createReducer(
  initialProfileFeatureState,
  on(AppActions.addUsers, (state, { users }) => ({
    ...state,
    users: [...users]
  })),
  on(AppActions.addPosts, (state, { posts }) => ({
    ...state,
    posts: [...posts]
  })),

);

createReducer function maps many actions and returns an ActionReducer function.

addUsers action is mapped to a function that creates a new User array and returns a newly computed state. Similarly the addPosts action is mapped.

The […] spread operator copies the properties of the object and returns a new object. It only performs the shallow copying and does not copy the nested structures. You should always consider a better alternative if you are dealing with a state that contains nested data structures. Libraries like lodash provide methods to clone nested structures.

Create ActionReducerMap

ActionReducerMap provides the mapping as key-value pairs where the key represents the feature name as a string and the value is the ActionReducer function returned by createReducer function.

In our case, the ActionReducerMap will contain profile as a key and value as theProfFeatureReducer.

/**The profileFeatureReducer function is necessary as function calls are not supported in the View Engine AOT compiler. It is no longer required if you use the default Ivy AOT compiler (or JIT)**/

function profileFeatureReducer
(state: ProfileFeatureState = initialState, action: Action) {
  return theProfFeatureReducer(state, action);
}

/ **AppActionReducer Map** /
export const AppActionReducerMap: ActionReducerMap<AppState> = {
  profile: profileFeatureReducer
  // ... other feature go here

};

It is not necessary to create an ActionReducerMap. You can directly provide the mapping in StoreModule.forRoot({key: ActionReducer})while registering the reducer in app.module.ts. You can also separately register the feature state in the feature module. I prefer creating the ActionReducerMapseparately as it provides a better type checking in Typescript.

At this point, our app.reducer.ts file should look like :

/ **app.reducer.ts** /
export interface ProfileFeatureState {
  users: User[];
  posts: Post[];
}

export const initialProfileFeatureState: ProfileFeatureState = {
  users: null,
  posts: null
};

export interface AppState {
  profile: ProfileFeatureState;
}

const theProfFeatureReducer = createReducer(
  initialProfileFeatureState,
  on(AppActions.addUsers, (state, { users }) => ({
    ...state,
    users: [...users]
  })),
  on(AppActions.addPosts, (state, { posts }) => ({
    ...state,
    posts: [...posts]
  }))
);

function profileFeatureReducer(state: ProfileFeatureState = initialProfileFeatureState, action: Action) {
  return theProfFeatureReducer(state, action);
}

export const AppActionReducerMap: ActionReducerMap<AppState> = {
  profile: profileFeatureReducer
};

Register the State

Once the reducer is created, It should be registered in the Module . The state can be registered using one of the two options:

Register Root state

To register the global store in the application

StoreModule.forRoot({ AppActionReducerMap })

StoreModule.forRoot() takes ActionReducerMap as an argument. The map contains key and ActionReducer Object returned by createReducer function.

Register each feature state separately -

Feature states are similar to root states but they represent the state of specific features of an application. Typically, each feature should be registed in its own module.

StoreModule.forFeature({ profile: profileFeatureReducer })

If you have come this far. You might also want to read about NgRx Selectors.

• How to create NgRx Selectors
• How to use createSelector function to compose selectors with - single or multiple slices of state
• How to use a projector function to return only a part within the slice of state.

NgRx selectors are used to select a slice of state. I have a detailed post about it here