In this blog I’ll show you how to guard routes in Angular 2 Router.

Let’s consider some scenarios that require a certain validation to be performed to decide if the user (or a program) is allowed to navigate to or leave the route:

* Allow to open the route only if the user is authenticated and authorized to do so.

* Implement a multi-part form that consists of several components, and the user is allowed to navigate to the next form section only if the data entered in the current one is valid.

* Remind the user about the unsaved changes if he or she tries to navigate from the route.

The router has the hooks that give you more control over the navigation to/from a route, and you can use these hooks to implement the any of above scenarios to guard the routes.

Angular includes a number of component lifecycle hooks that allow you to handle important events in the life of a component. First, it worth noting that the route configuration is done outside of the components. You configure routes in an object of type RouterConfig, then give it to the provideRouter() function, which in turn is given to the function bootstrap() that loads your app.
The type RouterConfig is a collection of items that conforms to the Route interface shown below:

export interface Route {
 path?: string;
 pathMatch?: 'full' | 'prefix';
 component?: Type | string;
 redirectTo?: string;
 outlet?: string;
 canActivate?: any[];
 canDeactivate?: any[];
 data?: Data;
 resolve?: ResolveData;
 children?: Route[];
}

While configuring routes you’ll typically use two properties from this interface: path and component. For example, an app that has two links Home and Product Details can specify and bootstrap the routes as follows:

bootstrap(RootComponent, [
    provideRouter([
      {path: '',        component: HomeComponent},
      {path: 'product', component: ProductDetailComponent}]),
    {provide: LocationStrategy, useClass: HashLocationStrategy}
]);

But in this blog I’d like to you to get familiar with the properties canActivate and canDeactivate that allow you to hook up the routes with the guards. Basically you need to write a function(s) implementing the validating logic that will return either true or false and assign it to one of these properties. If canActivate() of the guard returns true, the user can navigate to the route. If canDeactivate() returns true, the user can navigate from the route. Since both canActivate and canDeactivate properties of Route accept an array as a value, you can assign multiple functions (the guards) if you need to check more than one condition to allow or forbid the navigation.

Let’s create a simple app with Home and Product Details links to illustrate how you can protect the product route from the users who are not logged in. To keep the example simple, we won’t use an actual login service, but will generate the login status randomly.

We’ll create a guard class that implements the interface CanActivate, which declares only one function to implement: canActivate(). This function should contain the application logic that returns true or false. If the function returns false (the user is not logged in) the application will not navigate to the route and will print the error message on the console.

import {CanActivate} from "@angular/router";
import {Injectable} from "@angular/core";

@Injectable()
export class LoginGuard implements CanActivate{

  canActivate() {
      return this.checkIfLoggedIn();
  }

  private checkIfLoggedIn(): boolean{

      // A call to the actual login service would go here
      // For now we'll just randomly return true or false

      let loggedIn:boolean = Math.random() < 0.5;

      if(!loggedIn){
          console.log("LoginGuard: The user is not logged in and can't navigate product details");
      }

      return loggedIn;
  }
}

As you see from the code, my implementation of the function canActivate() will randomly return true or false emulating the user’s logged in status.

The next step is to update the router configuration so it uses our guard. The code snippet below shows how the function provideRouter() can look like for the app that has Home and Product Detail routes and the latter is protected by our LoginGuard:

provideRouter([
  {path: '',        component: HomeComponent},
  {path: 'product', component: ProductDetailComponent,
                    canActivate:[LoginGuard]}
])

Adding one or more guards to the array given to the canActivate property will automatically invoke all guards one after the other. If any of the guards returns false, the navigation to the route will be prohibited.

But who will instantiate the class LoginGuard? Angular will do it for us using its dependency injection mechanism, but you have to mention this class in the list of providers which are needed for injection to work. We’ll just add the name LoginGuard to the list of providers in the bootstrap() function of our app:

bootstrap(RootComponent, [
    provideRouter([
      {path: '',        component: HomeComponent},
      {path: 'product', component: ProductDetailComponent,
                        canActivate:[LoginGuard]}]),
    LoginGuard,
    {provide: LocationStrategy, useClass: HashLocationStrategy}
]);

The complete code of the main app script is shown next:

import {bootstrap} from '@angular/platform-browser-dynamic';
import {Component} from '@angular/core';
import {LocationStrategy, HashLocationStrategy} from '@angular/common';
import {provideRouter, ROUTER_DIRECTIVES} from '@angular/router';

import {HomeComponent} from './components/home';
import {ProductDetailComponent} from './components/product';

import {LoginGuard} from './guards/login.guard';

@Component({
    selector: 'basic-routing',
    directives: [ROUTER_DIRECTIVES],
    template: `
        <a [routerLink]="['/']">Home</a>
        <a [routerLink]="['/product']">Product Details</a>
        <router-outlet></router-outlet>
    `
})
class RootComponent {}

bootstrap(RootComponent, [
    provideRouter([
      {path: '',        component: HomeComponent},
      {path: 'product', component: ProductDetailComponent,
                        canActivate:[LoginGuard]}]),
    LoginGuard,
    {provide: LocationStrategy, useClass: HashLocationStrategy}
]);

If you run this app and will try to lick on the Product Details link, it’ll either navigate to this route or print the error message on the browser console depending on the randomly generated value in the LoginGuard. The snapshot below was taken after the user tried to click on the Product Details link, but the LoginGuard “decided” that the user is not logged in.

But if our unpredictable LoginGuard “decided” that the user is logged in, the screen will look as follows after clicking on the Product Details link:

Dependency Injection Benefit. Since a guard is injected into your app, you can use it as any other object. For example, you can inject it in your RootComponent:

 constructor (private _loginGuard:LoginGuard){}

Then invoke any methods defined on the guard class, e.g.:

 this._loginGuard.changeLoginStatus(true);

Our LoginGuard implements the method canActivate() without providing any arguments to it. But this method can be used with the following signature:

canActivate(destination: ActivatedRouteSnapshot,
            state: RouterStateSnapshot)

The values of the ActivatedRouteSnapshot and RouterStateSnapshot will be injected by Angular automatically and may become quite handy if you want to analyze the current state of the router. For example, if you’d like to know the name of the route the user tried to navigate to, this is how to do it:

canActivate(destination: ActivatedRouteSnapshot,
            state: RouterStateSnapshot) {

      console.log(destination.component.name);
   ...
}

Implementing the CanDeactivate interface that would control the process of navigating from a route works similarly. Just create a guard class that implements the method canDeactivate(), for example:

import {CanDeactivate, Router} from "@angular/router";
import {Injectable} from "@angular/core";
import {ProductDetailComponent} from "../product.component";

@Injectable()
export class UnsavedChangesGuard implements CanDeactivate<ProductDetailComponent>{

    constructor(private _router:Router){}

    canDeactivate(component: ProductDetailComponent){
      return window.confirm("You have unsaved changes. Still want to leave?");
    }
}

Don’t forget to add the canDeactivate property to the route configuration and inject the new guard in bootstrap(), e.g.:

bootstrap(RootComponent, [
    provideRouter([
      {path: '',        component: HomeComponent},
      {path: 'product', component: ProductDetailComponent,
          canActivate:[LoginGuard], canDeactivate:[UnsavedChangesGuard]}
    ]),
    LoginGuard, UnsavedChangesGuard,
    {provide: LocationStrategy, useClass: HashLocationStrategy}
]);

For a fancier way of displaying confirmation dialogs use the MdDialog component from the Material Design 2 library (see https://github.com/angular/material2). This component will be released in the upcoming Alpha 8.

NOTE: If you want to delay the navigation to a route until certain data structures have been populated, use the property resolve from the Route interface. I’ll write a separate blog showing how to do it.

In this blog I didn’t show you the code of HomeComponent and ProductDetail component, but you can find them in the Github repo with the code samples from Chapter 3 from our book Angular 2 Development with TypeScript.

If you’re interested in learning Angular 2 in depth, enroll into one of our workshops. The next one we’ll run online starting from September 11, 2016.