what are selectors in ngrx

Full Answer

Why do we use selectors in ngrx?

We use selectors to avoid manually traversing the state tree over and over, and in return, we get powerful declarative functional programming for our state management. Now that the concept of selectors is clear, let’s take a look at why understanding it is important to grasp selectors in NGRX.

What is the use of feature name in ngrx?

The feature name plays a crucial role in creating a state selector for the feature state using a handy function provided by ngrx/store called createFeatureSelector. createFeatureSelector allows us to get a top-level feature state property of the state tree simply by calling it out by its feature name:

What are selectors in angular?

Learn Angular the right way. Premium Angular courses to skyrocket your skills to the top. Join 78,277 developers pushing their limits. Selectors are pure functions that take slices of state as arguments and return some state data that we can pass to our components.

What is ngrx in angular?

NgRx provides reactive state management utilizing RxJs and a global state object. In this guide, I'll explain how to implement the State, Actions, Reducer, Selectors, Effects, and how to use in a components. Angular architecture depends heavily on the observer pattern with heavy usage of RxJs.

How do I subscribe to a selector in NgRx?

3:2813:19Use of NgRx Selectors in Angular - YouTubeYouTubeStart of suggested clipEnd of suggested clipAnd you can pass or you can iterate between them you can write your logic. And we will see thatMoreAnd you can pass or you can iterate between them you can write your logic. And we will see that after some time now i'll go to our component. And here we'll use our selector means our movie selector.

Are selectors Memoized?

A selector function that does a direct lookup and return of a value should be a plain function, not memoized.

What are props in NgRx?

The props method is used to define any additional metadata needed for the handling of the action. Action creators provide a consistent, type-safe way to construct an action that is being dispatched. Creating actions for liking and disliking a photo could look like this: // src/app/store/photo.

What are reducers in NgRx?

Reducerslink. Reducers in NgRx are responsible for handling transitions from one state to the next state in your application. Reducer functions handle these transitions by determining which actions to handle based on the type.

What is the selector?

A selector is a chain of one or more simple selectors separated by combinators. Combinators are: white space, ">", and "+". White space may appear between a combinator and the simple selectors around it. The elements of the document tree that match a selector are called subjects of the selector.

How do you use selectors?

The id selector uses the id attribute of an HTML element to select a specific element. The id of an element is unique within a page, so the id selector is used to select one unique element! To select an element with a specific id, write a hash (#) character, followed by the id of the element.

What are actions in NgRx?

Actions are one of the main building blocks in NgRx. Actions express unique events that happen throughout your application. From user interaction with the page, external interaction through network requests, and direct interaction with device APIs, these and more events are described with actions.

How do I name my actions in NgRx?

naming actions (for state) DO name actions with a unique name and captial letters - VERB + NOUN. DO name actions values with - Prefix + VERB + NOUN.

What are effects in angular?

​ } ​@ngrx/effects provides an Angular actions$ service (which is also an Observable ) to emit every action that has been dispatched by your application in a single stream. Its ofType() method can be used to filter the one or more actions we're interesting in before adding a side-effect.

What is difference between NgRx and RxJS?

Using NgRx store you can create your store, effects , reducers & actions in any angular app. On the other hand RxJS is used for mainly for consuming api data and creating shared services using subject etc.

What is store in RxJS?

Store is RxJS powered state management for Angular applications, inspired by Redux. Store is a controlled state container designed to help write performant, consistent applications on top of Angular.

What is Dispatch in Angular?

Store Side-Effects For instance, you dispatch an action to load all Heroes data into the application. Such an action leads to a side-effect of having to communicate, via an Angular Service, with a Web API hosted on the server-side (or cloud) to return the Heroes data.

Are NGRX selectors Memoized?

Because selectors are pure functions, the last result can be returned when the arguments match without reinvoking your selector function. This can provide performance benefits, particularly with selectors that perform expensive computation. This practice is known as memoization.

What is a Memoized function?

In programming, memoization is an optimization technique that makes applications more efficient and hence faster. It does this by storing computation results in cache, and retrieving that same information from the cache the next time it's needed instead of computing it again.

What is selector in NGRX?

Selectors are pure functions that take slices of state as arguments and return some state data that we can pass to our components. To better understand what selectors are and what they do, it helps see ngrx state as a data structure - a tree that can be serialised to JSON. Data is added to the state tree by composing state in reducers - that’s ...

How many selectors does createselector take?

The createSelector function takes up to eight selector functions as arguments, each one referencing different slices of state. The last argumet to createSelector can be treated as our “projector function”. Let’s take a look at one of the TypeScript definitions for createSelector to further grasp this before continuing:

Why are selector functions composed together?

Due to being pure functions that return a slice of state, selector functions can be composed together to be consumed by components , they may be composed from various pieces of our overall state - and this is where state management becomes more important as we need to get things correct from the get-go.

What is a selector function?

We’ve seen that a selector function is a pure function that grants us direct access to the value of a state tree traversal. We use selectors to avoid manually traversing the state tree over and over, and in return, we get powerful declarative functional programming for our state management. Now that the concept of selectors is clear, let’s take a look at why understanding it is important to grasp selectors in NGRX. Let’s move on and see what this same data structure would look like with NGRX.

How to kickstart composition?

We are able to easily get a reference to a top-level state property by using createFeatureSelector. Once we have that reference, we can then compose it with other selectors that point to state further down in our feature state - effectively traversing the state tree till we get to a desired property. We did something similar in one of the examples of a previous section using pure functions. Let’s see how we can do the same within our store.

How to get to a particular property?

In order to get to a particular property, we have to traverse the tree. For example, we build our way to entities like building a chain and each level down is a link that connects us from state to entities. If we miss any link in the chain, it breaks and we cannot create the connection. Each link in this chain represents a reference to that state property. Hence, we need a reference to products, then a reference to pizzas, and finally a reference to entities. Once there, we can access the data held by entities.

Can selectors perform data transformation?

We may find ourselves writing complex data transformation logic in our components to get exactly what we need - however we don’t want to do that - and this is where selectors come into play. Just like with databases, we can perform data transformation from within the store by composing selectors to just return what we need. We’ll keep our components lean and decoupled from the Store.

What tools does NgRx use?

Use the tools provided by NgRx, like createFeatureSelector, createSelector and Entity.getSelectors (explained in more detail below).

Why use NgRX?

We use NgRx because we want a scalable state management solution independent of the view layer. But first of all, we need a clear understanding of what state is and how to manage it.

What is memoization in selectors?

Memoization. First of all, selectors created by createSelector are memoized, meaning that they won't be called unnecessarily unless the data in the store has changed. This provides an important performance boost

Why use selectors in UI?

Now let us use selectors to reduce code complexity. Imagine this scenario: we can select orders from the UI, and another part of the UI has to display their owners (each Order has a property owner ). We could create a property on our state called owners and update it each time an Order is selected (when the corresponding action is dispatched).

What is derived state in a selector?

As we have learned, derived state is a mapping of one form of original state into some result via a selector. Sometimes we can also map an existing selector to a new selector, as we have seen in the previous example. But more often than you would expect, some forms of derived state depend not on one, but two (or even more!) original states.

What is NGRX in Angular?

NgRx is a powerful tool with lots of features to make our lives easier when dealing with large Angular applications. And one thing in particular I find with NgRx is that it has almost no redundant or obscure features or tools. To truly harness its powers, one needs to know and correctly use all the tools it provides. And the most important tool it provides that is often overlooked by newcomers is selectors. Hopefully, with this article, we will be able to write better NgRx selectors, resulting in better experiences when developing with it.

What is state in NGRX?

In NgRx, the state usually contains Features - most top level objects in the hierarchy, branches in the form of properties of the global state object, which usually represent some real life features of our app . For example, an application can contain features like "users", "companies", "orders" and so on.

Display filtered data

Lets’s say we have our Todo app. We handle in the DTO an ”active” field that is a boolean. I’d like to know how many of my entities are “active”. There is a tool for that : filterFn. This is a property of the EntityMetadata.

Have an advanced filter on Entities

If you need some more advanced filtering, you can come with PropsFilterFnFactory that allows you to work with regexp. This could look like that if you want to filter on 2 different fields case insensitive :

The custom selectors from NgRx Data

NgRx Data has some extension points, and one that can be helpful here is the custom selector. The official doc has the code right, but it’s mixed with another entity selector and can be confusing. Here is a simple method :

What is @ngrx/router-store?

Another possibility is to use @ngrx/router-store. This module connects the Angular router module with the NgRx global store. In other words, all the route information will be available in the store and thus also in the selectors.

How to pass a parameter to a selector?

To pass a parameter to a selector we must wrap the selector in a factory method, this is what I call a factory selector. Throughout this post we'll use a simple example, which is a selector that returns a customer from the global store state based on a passed id parameter.

What is the selector id 47?

This results that the customer with id 47 is selector from the global store state.

What is a selectCustomerOrders?

In the example below, selectCustomerOrders is a factory selector that receives a customer id, and the selector invokes the factory selectors selectCustomer and selectCustomerOrders with the given customer id as parameter. With the results from both selectors, the selector creates a new object that consists of the customer and the customers' orders.

How to use state of URL?

To use the state of the URL, we make use of the getSelectors method to create all the router store selectors. In the example below, we then use the selectRouteParams selector to read the customer id from the URL, e.g. https//localhost:4200/customers/47.

Can you add memoization layer to selector?

For most cases this is fine and you won't notice the difference. But when the selector has to so expensive work, you can add a memoization layer on top as a countermeasure.

Can you pass a selector to a child?

If you need to pass a selector to a child selector, how would you do that? This is actually not that hard to implement. Because a factory selector is simply a method, it can be invoked from a parent selector.

Can you put a filter in the projection parameter?

So in some cases you can put a filter in the projection parameter (last) for createSelector. Here is an example:

Can create selector recompute projection?

In the documentation it mentions that the same state passed to the memoized selector created by createSelector will not recompute the projection. It will return the same value when invoked. I couldn't find documentation on this but through testing I noticed that if the projection results in the same value as the previous value then it will not be emitted if the subscription already received the previous value. Similar behavior to the rxjs operator distinctUntilChanged. I haven't had time to dig through their source code to understand where/how/why.

What is NgRX state management?

NgRx provides reactive state management utilizing RxJs and a global state object. In this guide, I'll explain how to implement the State, Actions, Reducer, Selectors, Effects, and how to use in a components.

What is a selector in a state?

Selectors provide a method to read slices of the state.

How to read data into a component?

In order to read data into our component, inject the Store service into the component. Create an observable property to read a slice of data from a selector. I prefer not to explicitly subscribe to the observable. There exist better methods handling the subscriptions. My preferred method uses the async pipe in the template alleviating the responsibility to explicitly unsubscribe in the component.

What is the class selectPodcastAction?

The class SelectPodcastAction includes a type property of [alsoa.ui.podcast.component] SELECT_PODCAST', along with payload property of type string. The payload represents the action data associated with the action necessary to complete the action.

How does a reducer work?

The reducer generates a new state based on the action dispatched and any payload information contained within the action. These files contain a switch statement for any action that changes and returns the new state. Get familiar with the spread syntax as a mechanism to preserve immutability.

What Exactly Is State in Ngrx?Link to This Section

What Do Selectors Actually Do?Link to This Section

• Selectors are pure, preferably simple functions that map our entire state to some part of it, essentially providing us with a slice of the state (and sometimes making minor modifications to it). Selectors are basically the NgRx change detection mechanism. When a state is updated, NgRx doesn't do any comparison between old state and the new state to...

See more on indepth.dev

Benefits of Using Selectorslink to This Section

Writing Good Selectorslink to This Section

Reducing Code Complexity Using Selectorslink to This Section

in Conclusionlink to This Section