In AngularJS, the resolve property is a powerful tool that allows you to preload data before a route is activated. It helps ensure that the necessary data is available and ready to be used by the view or controller once the user navigates to a specific route. This is particularly useful when the data is required for the page to function correctly, and you don’t want to show the view until the data is ready.
In this guide, we’ll go over how to use the resolve property in AngularJS, understand its benefits, and explore a few examples to demonstrate its use.
1. Understanding resolve in AngularJS Routing
The resolve property in AngularJS routing allows you to define a set of values or data that should be fetched or computed before the controller for a route is executed. Once the data is resolved, the view or controller for the route is loaded. This ensures that your application is not rendered with incomplete or empty data.
How resolve Works:
- Before Route Activation: When a route is about to be activated, AngularJS will check if there’s any data defined in the
resolveproperty. - Fetch Data: It will then execute the promises in the
resolveproperty. If the promise resolves successfully, the data will be passed to the route’s controller. If any of the promises fail, the route will not be activated, and you can handle error cases as necessary. - Controller Access: The resolved data is available as a property in the
$routeParamsor$route.current.localsobject and can be injected directly into the route’s controller.
2. Basic Usage of resolve
To use the resolve property, you’ll define it within the route configuration using $routeProvider. Each key in the resolve object represents a data dependency that must be resolved before the route is activated.
Here’s a basic example:
app.config(function($routeProvider) {
$routeProvider
.when('/profile', {
templateUrl: 'profile.html',
controller: 'ProfileController',
resolve: {
userData: function(UserService) {
// The UserService returns a promise to get user data
return UserService.getUserData();
}
}
});
});
In this example:
- The
resolveproperty is used to preload theuserDatabefore theProfileControlleris invoked. - The
UserService.getUserData()function returns a promise. Once the promise is resolved, theuserDatawill be available in the controller.
3. Accessing Resolved Data in the Controller
Once the data is resolved, it’s available to the controller as part of the $route.current.locals. AngularJS will inject each resolved property into the controller, and you can access it via dependency injection.
Here’s how the controller can access the resolved data:
app.controller('ProfileController', function($scope, userData) {
// userData is automatically injected from the resolve property
$scope.user = userData;
});
In this controller:
- The
userDatais passed directly from theresolveproperty and assigned to$scope.user, which will then be available in the view.
4. Handling Multiple Data Dependencies with resolve
In some cases, you may need to preload multiple pieces of data before activating the route. You can do this by adding multiple key-value pairs in the resolve object.
Here’s an example where we preload user data and a list of posts:
app.config(function($routeProvider) {
$routeProvider
.when('/dashboard', {
templateUrl: 'dashboard.html',
controller: 'DashboardController',
resolve: {
userData: function(UserService) {
return UserService.getUserData();
},
posts: function(PostService) {
return PostService.getUserPosts();
}
}
});
});
In this case:
userDataandpostsare resolved before the controller is instantiated. Both pieces of data are fetched from different services.
In the controller, both userData and posts will be injected:
app.controller('DashboardController', function($scope, userData, posts) {
$scope.user = userData;
$scope.posts = posts;
});
5. Handling Errors in Resolved Data
What if one of the promises in resolve fails? For example, if the user data request fails or the posts cannot be fetched? You can handle errors gracefully by using a route error handler or by checking if a promise was resolved or rejected.
Using resolve with Error Handling
app.config(function($routeProvider) {
$routeProvider
.when('/dashboard', {
templateUrl: 'dashboard.html',
controller: 'DashboardController',
resolve: {
userData: function(UserService, $q) {
var deferred = $q.defer();
UserService.getUserData().then(function(response) {
deferred.resolve(response);
}, function(error) {
deferred.reject('Error loading user data');
});
return deferred.promise;
}
}
});
});
In this example:
- The
UserService.getUserData()promise is wrapped in a$q.defer()object to handle the resolution or rejection manually. - If the promise is rejected, an error message (
'Error loading user data') will be passed to the route.
Error Handling in Controller
To catch errors and handle them appropriately, you can use $route.reload() or manually trigger error notifications.
6. Using resolve for Route Guards or Authentication
Another common use case for the resolve property is handling authentication or authorization before route activation. For example, you might want to make sure a user is authenticated before allowing them to access a certain route.
Example: Authentication Guard
app.config(function($routeProvider) {
$routeProvider
.when('/admin', {
templateUrl: 'admin.html',
controller: 'AdminController',
resolve: {
auth: function(AuthService, $q, $location) {
var deferred = $q.defer();
AuthService.isAuthenticated().then(function(response) {
if (response.isAuthenticated) {
deferred.resolve();
} else {
$location.path('/login');
deferred.reject('User not authenticated');
}
});
return deferred.promise;
}
}
});
});
In this scenario:
- Before navigating to the
/adminroute, theauthkey inresolvechecks if the user is authenticated via theAuthService.isAuthenticated()function. - If the user is not authenticated, they are redirected to the login page (
$location.path('/login')), and the route will not be activated.
7. Using resolve with ngRoute or UI-Router
resolve is primarily used with the $routeProvider in ngRoute. If you’re using UI-Router (a more flexible and advanced routing library), it also supports a similar mechanism for preloading data before route activation.
UI-Router Example with resolve
app.config(function($stateProvider) {
$stateProvider
.state('dashboard', {
url: '/dashboard',
templateUrl: 'dashboard.html',
controller: 'DashboardController',
resolve: {
userData: function(UserService) {
return UserService.getUserData();
},
posts: function(PostService) {
return PostService.getUserPosts();
}
}
});
});
With UI-Router, the resolved data is injected into the controller in a similar way to $routeProvider in ngRoute.
8. Conclusion
Using resolve in AngularJS routing is an effective way to ensure that data is preloaded before a route is activated. It allows for better user experiences by ensuring that all necessary data is available before rendering the view, preventing issues such as empty views or slow loading times. By combining resolve with promises, you can load data from external APIs, authenticate users, and manage dependencies in your AngularJS application effectively.
Key takeaways:
- Preloading Data: Use
resolveto preload data before a route is activated. - Error Handling: Handle errors by rejecting promises in
resolveand providing appropriate feedback. - Authentication: Use
resolvefor route guards to ensure that routes are only accessible by authorized users. - UI-Router Support:
resolvecan also be used with UI-Router in a similar manner.