I have recently faced an interesting issue in an Angular application. It was an e-commerce solution, where a user could change his currency. I was using a modified price pipe, which had a dependency on settings containing the currency symbol. Because settings could be changed by a user, I had to get them from the server. The problem occurred when angular wanted to use the pipe before it loaded the settings. I had to make the app wait for settings before it rendered the page.

Route Guards

The answer to this problem was creating a Route Guard. I did use them before - for checking if the user is logged in. It turned out they can also be used to make sure some data is fetched before you display a component.

The Router supports multiple kinds of guards:

CanActivate to mediate navigation to a route.

to mediate navigation to a route. CanActivateChild to mediate navigation to a child route.

to mediate navigation to a child route. CanDeactivate to mediate navigation away from the current route.

to mediate navigation away from the current route. Resolve to perform route data retrieval before route activation.

to perform route data retrieval before route activation. CanLoad to mediate navigation to a feature module loaded asynchronously.

Creating a Route Guard

In simple words, Guard is a class implementing one of above methods. This method can return either a boolean value (for synchronous processing) or an Observable<boolean (for asynchronous processing).

import { Injectable } from "@angular/core" ; import { CanActivate } from "@angular/router" ; import { UsersService } from "./shared/services/usersService" ; @ Injectable () export class SettingsGuard implements CanActivate { constructor ( private usersService : UsersService ) {} canActivate () { // Settings will be cached in UsersService return this . usersService . getSettings () . map (() => true ); } }

If you now add this guard to the route:

{ path : "orders" , loadChildren : "app/orders/orders.module#OrdersModule" , canActivate : [ SettingsGuard , AuthGuard ] },

Angular Router will delay rendering of the OrdersModule until the settings have been loaded. The UsersModule is caching the settings in a backing field so that the request is fired only once.

Other use cases for Route Guards

This is just a one way you can use guards in your application. They might be useful in some other situations too:

checking if user is authorised to see the target component

redirecting to login page

saving changes before leaving current component

asking user if changes should be saved

As you see it’s quite a powerful concept, yet I get the impression it’s not widely known. I hope you found this post useful. Have you used guards in your application? What was the purpose?