We can use the ViewChildren decorator to get the QueryList of elements or directives from the view DOM. In this article, we’ll examine how can we leverage this technique in order to manage our application filters in a reusable, efficient and clean way.

Here is an illustration of the final result:

Imagine that we’re on a page that displays a list of resources and the users can filter the list by resource type or by resource level .

To inform the server how to filter the resource, we’ll use the query string style where we define what the current filters are. For example:

Our goal:

Each time the user clicks on a filter we need to get the latest value from each filter and create the new query string.

Fortunately, we have an Rx operator precisely for this kind of functionality — combineLatest .

Let me know when any Observable emits but also give me the latest value from the others. ( Array )

Let’s create the filters component. This will be our final markup:

The Interfaces

The Filter interface is mostly for the UI. The ActiveFilter is for the server. The group is referring to the query string key, for example:

In the about URL, the group is referring to level .

The Filters Component

We have two Inputs , the group name and array of filters.

The combineLatest operator expects to receive a list of observables. We need to create an observable that will emit the active filter each time the user clicks on a filter.

But there is one catch, we need to get the initial data, so we also need to emit the value from each at the component initialization phase. We can tackle this problem with BehaviorSubject .

BehaviorSubject takes an initial value and emits it immediately.

We need to find the active filter from the filters Input; then we need to create our BehaviorSubject and emit the current filter id with the group .

Now let’s check the HTML and add the click event.

The code is straightforward, let’s move to the select() method that will run each time the user clicks on a filter.

We need to change the active state on every filter; then we need to emit the new active filter .

The component is ready for use. Finally the fun part.

Now we need to get the observable from each filters component. We are going to use the ViewChildren decorator to grab the filters components from the current view.

Great! Now we have the filters instances so we can get access to the changeFilter observable. The final step is to use combineLatest .

Every time one of the filters emits value, we’re going to receive an array containing each filter value, transform them to a query string and send to our server.

That’s all.

You can extract this code to a self-contained module and use it.

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👂🏻 Last but Not Least, Have you Heard of Akita?

Akita is a state management pattern that we’ve developed here in Datorama. It’s been successfully used in a big data production environment for over seven months, and we’re continually adding features to it.

Akita encourages simplicity. It saves you the hassle of creating boilerplate code and offers powerful tools with a moderate learning curve, suitable for both experienced and inexperienced developers alike.

I highly recommend checking it out.