What is store.select in ngrx

Question

I'm new to Redux and started with ngrx. I'm unable to understand the meaning of this line of code store.select:

 clock: Observable<Date>;  this.clock = store.select('clock'); 

Answer 1

In very simple terms select gives you back a slice of data from the application state wrapped into an Observable.

What it means is, select operator gets the chunk of data you need and then it converts it into an Observable object. So, what you get back is an Observable that wraps the required data. To consume the data you need to subscribe to it.

Lets see a very basic example.

1. Lets define the model of our store

   export interface AppStore {    clock: Date } 

2. Import the Store into your component from '@ngrx/store'

3. Create a store by injecting into the constructor

   constructor(private _store: Store<AppStore>){} 

4. Select returns an Observable.

   So, declare the clock variable in your component as follows:-

   public clock: Observable<Date>; 

   Now you can do something like follows:-

   this.clock = this._store.select('clock'); 

Answer 2

Wow, this is a big topic. So basically "select" is really a RXJS operator that is used in this case to retrieve the value of a part of the application state object. So say your main app state has a array of users and a array of security functions. "Select" allows you to get a reference to a observable whose value is just that array of users. Before you get into ngrx you really need to study up on Observables and RXJS. I found this article linked off of the main Github project page for ngrx helpful.

https://gist.github.com/btroncone/a6e4347326749f938510

RXJS and redux can be a big topic but I suggest working on your knowledge in small bite size chunks. It took me about 2 months of working with it before I really started to feel comfortable. Even if you don't stay with ngrx, understanding how RXJS works is incredibly useful and is worth the time investment to learn it.

Here is a gist article that also gives a good intro into RXJS. https://gist.github.com/staltz/868e7e9bc2a7b8c1f754