"How can I select an entity from the store by its id?"
This question popped up several times lately and in this post, I’ll provide some suggestions to create a selector that works with parameters, so let’s not waste any time, and let’s get started!
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.
The simplest implementation is a factory selector
selectCustomer with a parameter
id that returns the selector created by using the
createSelector method. The selector uses the
id parameter to retrieve the customer by the passed id in the projector.
In the component, the selector
selectCustomer is used as a normal selector, but we must pass a parameter to the selector.
This results that the customer with id 47 is selector from the global store state.
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.
In the example below,
selectCustomerOrders is a factory selector that receives a customer id, and the selector invokes the factory selectors
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.
In contrast to a normal NgRx selector where the selector is shared across multiple components, we now have a new instance of the selector every time the selector factory is invoked. This has the effect that we lose the memoization benefits of the 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.
To accomplish this, we must bring our own memoization method to the table, for example lodash.memoize. After this, we can simply wrap the selector inside of the
memoize method. Note that this is a simple example and that you probably don't need to memoize an entity lookup.
By doing this, consuming the selector is no different than before. It's important to keep in mind that this builds up an in-memory cache, so that's why it's better that the added entry is disposed of when it's possible.
While the above examples do provide an answer to the question on how to select a slice of the state based on a parameter, for me retrieving data from the store like this feels a bit dirty and I consider it a bad practice in most cases. In my opinion, it's better to persist this property in the global store. In our example, it would mean that the
id parameter of the selected customer would be added to the global store.
This has the benefit that we're going to be fully reactive. When the property state changes, the selectors will be re-invoked (with the updated state), and our component will receive the new state that fits the criteria based on these "filter" properties.
To implement this in the global store, we first define actions to dispatch when a filter (the customer's id) changes. For example when the user clicks on a customer or when the user navigates to a customer's page, we dispatch an action.
The reducer below reacts to the
customerClicked action to update the
selectedCustomerId property in the global store state.
We also have to create a selector
selectSelectedCustomerId to access the selected id from the global state.
In the component, we consume the selector without having to worry about the customer's id. In larger components with more filters, this refactor makes the component and the selector easier to maintain.
In the future when there's a change to the filters that affect the result of the selector, we don't have to worry about having to change how we invoke the selector to show the correct data in the component. It will just work because this selector is entirely driven by the Store's state.
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.
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.
Nothing has changed to consume the selector.
In my opinion, the code we ended up doesn't only look cleaner but also is more manageable in comparison with the code we that we started with.
An additional benefit with using
@ngrx/router-store is that the parameters are persisted in the URL, and that you can share the URL.
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