If you don’t know the basics of AngularJS service, I recommend you read the other article I wrote on AngularJS Services Part 1: Provider.
Provider v.s. Factory
Factory is a thin wrapper on top of Provider. While Provider provides us the ability to configure the service provider before injection (creating the service object), Factory lacks that ability.
Although it is short of this functionality, most of the Angular devs use factories because they are, in my opinion, the easiest to use and really cover all the general needs that one might need from a service. This does not mean that Provider is bad - it is definitely great for certain use cases (configurability!).
That said, lets take a look at factories.
Factories are defined very similarly as Providers. They return a value, a function or an object literal. Usually developers go with returning an object, which would result in a nice interface to the function. For example:
Then, we can inject the GreetingService in a controller:
As you can see, returning an object from the service creates a nice looking interface to the method.
Factories in the wild
Usually, the practice is to use Factory (or services) when you need a reusable piece of code that you can inject into a controller or another service.
Lets take a look at, a more real-life-like, example:
Here we are using OpenWeatherMap’s API which is free to use. We will fetch London’s (UK) current temperature using a service.
As you can see, Angular’s built-in $http service is injected into our service so we can issue HTTP calls to a remote API.
The API’s url is defined as a variable and there is only one method in the service, get(). The url can also be extracted into another service i.e. constant and injected back into the TemperatureService, but lets keep it simple until we get to constants, in one of the next articles.
The rest, is simple - we issue an HTTP call, use promises to handle successful or errored requests and we return back the promise itself.
What would injecting the service into a controller look like? Lets see:
Simple, right? We inject the service in the controller, call get() on the service and handle the returned promise from the service. Then, when we get the temperature in the promise we can do whatever we want with it.
You can use these files to play with these services - it should all work well. Just make sure you have an HTML file that loads AngularJS from Google’s CDNs and make sure you include these files after Angular is loaded.