Verify whether a dependency is being passed into an Angular component

As mentioned in a discussion about IE caching issues, Internet Explorer tends to cache things when using ajax with AngularJS, causing potential problems. You can find a helpful solution to this problem here.

Given that we have multiple angular apps on our site, I believe it's important to proactively address the caching issue to assist developers working on these apps. By incorporating the suggested solution into all Angular apps as part of our style guide, developers are more likely to implement it and avoid running into the troublesome IE caching problem. Plus, let's face it, not many developers focus on developing for IE anymore.

var myApp = angular.module('myApp', ['ngRoute']);

myApp.config(['$routeProvider', '$httpProvider', function($routeProvider, $httpProvider) {
    $httpProvider.defaults.cache = false;
    if (!$httpProvider.defaults.headers.get) {
      $httpProvider.defaults.headers.get = {};
    }
    // disable IE ajax request caching
    $httpProvider.defaults.headers.get['If-Modified-Since'] = '0';
    // routes go here...
}]);

This code snippet essentially prevents IE from caching templates and forces server requests for $http calls instead of relying on cached data (which may not be ideal for dynamic content). However, adding this code to every Angular app raises an issue - some apps may not include ngRoute in their dependencies, leading to injection errors.

So, the question arises: Is there a way to check for the existence of injected dependencies in Angular? It would be great if we could implement something like the following:

var myapp = angular.module('ordersApp', [
    'ngTouch',
    'ngRoute',
    'ngAnimate',
    'myController'
]);

if(myapp.has_ngRoute) {
     myapp.config(['$routeProvider', '$httpProvider', function($routeProvider, $httpProvider) {
    $httpProvider.defaults.cache = false;
         if (!$httpProvider.defaults.headers.get) {
             $httpProvider.defaults.headers.get = {};
         }
         // disable IE ajax request caching
        $httpProvider.defaults.headers.get['If-Modified-Since'] = '0';
        // routes go here...
    }]);
}

Answer №1

To tackle this issue, there are two possible approaches:

1. Validate the Existence of a Service and Execute Behavior Accordingly

This solution directly addresses your query - only execute code if a service is present

You can utilize $injector.has(...) to verify if a provider or service has been registered.

You may verify if $routeProvider is registered and adjust the HTTP headers configuration accordingly.

Below is an illustration where we check for the presence of $routeProvider prior to configuring the $httpProvider.

// Cache configuration setup.
var configureCache = ['$httpProvider', function($httpProvider) {
        $httpProvider.defaults.cache = false;
        if (!$httpProvider.defaults.headers.get) {
            $httpProvider.defaults.headers.get = {};
        }
        // prevent IE from caching ajax requests
        $httpProvider.defaults.headers.get['If-Modified-Since'] = '0';
}];

angular.module('testApp', [
        // Test this by commenting this out.
        'ngRoute'
    ])
    .config(['$injector', function($injector){
        if ($injector.has('$routeProvider')){
            // If $routeProvider is registered, 
            // configure the $httpProvider.
            console.log('Has $routeProvider: setting up cache.');

            // Use `$injector.invoke(...)` to enable
            // Angular dependency injection in our
            // configuration function above.
            $injector.invoke(configureCache);
        } else {
            // Otherwise, take no action.
            console.log('Skipping cache setup.');
        }
}]);

Here is the demonstration on Plunker: http://plnkr.co/edit/orXOjMg0YZXDfwoqf9OD


2. Segregate Your Code into Modules

Avoid checking for module/service existence, instead use module dependencies to regulate this

This method provides more flexibility. You define the caching behavior in a separate module with a dependency on ngRoute.

This way, when you include your custom cache settings, you can be assured that ngRoute already exists due to its dependency!

This approach offers three ways to manage routing/caching behavior in your application solely based on included modules:

No routes defined

angular.module('testApp', [
        // Exclude the 'ngRoute' module dependency here
        // or modify caching behavior.
    ])

Routes configured with default caching behavior

angular.module('testApp', [
        'ngRoute'
    ])

Routes set with customized caching behavior

angular.module('testApp', [
        'noCacheRoutes'
    ])

In each scenario, adjustments were made only to module dependencies but resulted in altering the application's behavior in a predictable manner.

Here is the example showcasing the separation of cache configuration behavior into a distinct module:

/**
 * We determine that it is logical to use this 
 * module along with `ngRoute`, hence specifying it as
 * a dependency here.
 */
angular.module('noCacheRoutes', [
        'ngRoute'
    ])
    .config(['$httpProvider', function($httpProvider) {
        $httpProvider.defaults.cache = false;
        if (!$httpProvider.defaults.headers.get) {
            $httpProvider.defaults.headers.get = {};
        }
        // prevent IE from caching ajax requests
        $httpProvider.defaults.headers.get['If-Modified-Since'] = '0';
    }]);


/**
 * Avoid mentioning `ngRoute` here; let `noCacheRoutes`
 * bring it in as a transitive dependency.
 * 
 * If at a later stage you decide not to alter
 * the caching behavior, simply substitute `noCacheRoutes`
 * with `ngRoute` without requiring further modifications.
 */
angular.module('testApp', [
        'noCacheRoutes'
    ])
    .config(['$routeProvider', function($routeProvider) {
        // Define your application routes here.
    }]);

Answer №2

In a previous conversation found here regarding AngularJs and obtaining a list of all registered modules, it was determined that there is no built-in method for retrieving the module list. However, it was suggested to extend angular's module method as explained in the accepted solution.

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