Understanding Service Types

zyip發表於2015-03-16

 

Last update: June 2014. I have partially rewritten this article to provide more technical details and also to show their differences more clearly.

Angular comes with different types of services. Each one with its own use cases.

Something important that you have to keep in mind is that the services are always singleton, it doesn’t matter which type you use. This is the desired behavior.

NOTE: A singleton is a design pattern that restricts the instantiation of a class to just one object. Every place where we inject our service, will use the same instance.

Provider

Provider is the parent of almost all the other services (all but constant) and it is also the most complex but more configurable one.

Let’s see a basic example:

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app.provider('foo', function() {

  return {

    $get: function() {
      var thisIsPrivate = "Private";
      function getPrivate() {
        return thisIsPrivate;
      }

      return {
        variable: "This is public",
        getPrivate: getPrivate
      };
    }

  };

});

A provider on its simplest form, just needs to return a function called $get which is what we inject on the other components. So if we have a controller and we want to inject this foo provider, what we inject is the $get function of it.

Why should we use a provider when a factory is much simple? Because we can configure a provider in the config function. We can do something like this:

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app.provider('foo', function() {

  var thisIsPrivate = "Private";

  return {

    setPrivate: function(newVal) {
      thisIsPrivate = newVal;
    },

    $get: function() {
      function getPrivate() {
        return thisIsPrivate;
      }

      return {
        variable: "This is public",
        getPrivate: getPrivate
      };
    }

  };

});

app.config(function(fooProvider) {
  fooProvider.setPrivate('New value from config');
});

Here we moved the thisIsPrivate outside our $get function and then we created a setPrivate function to be able to change thisIsPrivate in a config function. Why do we need to do this? Won’t it be easier to just add the setter in the $get? This has a different purpose.

Imagine we want to create a generic library to manage our models and make some REST petitions. If we hardcode the endpoints URLs, we are not making it any generic, so the idea is to be able to configure those URLs and to do so, we create a provider and we allow those URLs to be configured on a config function.

Notice that we have to put nameProvider instead of just name in our config function. To consume it, we just need to use name.

Seeing this we realize that we already configured some services in our applications, like $routeProvider and $locationProvider, to configure our routes and html5mode respectively.

Providers have two different places to make injections, on the provider constructor and on the $get function. On the provider constructor we can only inject other providers and constants (is the same limitation as the config function). On the $get function we can inject all but other providers (but we can inject other provider’s $get function).

Remember: To inject a provider you use: name + ‘Provider’ and to inject its $get function you just use name

Try it

Factory

Provider are good, they are quite flexible and complex. But what if we only want its $getfunction? I mean, no configuration at all. Well, in that cases we have the factory. Let’s see an example:

Example:

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app.factory('foo', function() {
  var thisIsPrivate = "Private";
  function getPrivate() {
    return thisIsPrivate;
  }

  return {
    variable: "This is public",
    getPrivate: getPrivate
  };
});

// or..

app.factory('bar', function(a) {
  return a * 2;
});

As you see, we moved our provider $get function into a factory so we have what we had on the first provider example but with a much simpler syntax. In fact, internally a factory is a provider with only the $get function.

As I said before, all types are singleton, so if we modify foo.variable in one place, the other places will have that change too.

We can inject everything but providers on a factory and we can inject it everywhere except on the provider constructor and config functions.

Try it

Value

Factory is good, but what if I just want to store a simple value? I mean, no injections, just a simple value or object. Well angular has you covered with the value service:

Example:

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app.value('foo', 'A simple value');

Internally a value is just a factory. And since it is a factory the same injection rules applies, AKA can’t be injected into provider constructor or config functions.

Try it

Service

So having the complex provider, the more simple factory and the value services, what is the service service? Let’s see an example first:

Example:

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app.service('foo', function() {
  var thisIsPrivate = "Private";
  this.variable = "This is public";
  this.getPrivate = function() {
    return thisIsPrivate;
  };
});

The service service works much the same as the factory one. The difference is simple: The factory receives a function that gets called when we create it and the servicereceives a constructor function where we do a new on it (actually internally is uses Object.create instead of new).

In fact, it is the same thing as doing this:

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app.factory('foo2', function() {
  return new Foobar();
});


function Foobar() {
  var thisIsPrivate = "Private";
  this.variable = "This is public";
  this.getPrivate = function() {
    return thisIsPrivate;
  };
}

Foobar is a constructor function and we instantiate it in our factory when angular processes it for the first time and then it returns it. Like the service, Foobar will be instantiated only once and the next time we use the factory it will return the same instance again.

If we already have the class and we want to use it in our service we can do that like the following:

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app.service('foo3', Foobar);

If you’re wondering, what we did on foo2 is actually what angular does with services internally. That means that service is actually a factory and because of that, same injection rules applies.

Try it

Constant

Then, you’re expecting me to say that a constant is another subtype of provider like the others, but this one is not. A constant works much the same as a value as we can see here:

Example:

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app.constant('fooConfig', {
  config1: true,
  config2: "Default config2"
});

So… what’s the difference then? A constant can be injected everywhere and that includes provider constructor and config functions. That is why we use constant services to create default configuration for directives, because we can modify those configuration on our config functions.

You are wondering why it is called constant if we can modify it and well that was a design decision and I have no idea about the reasons behind it.

Try it

Bonus 1: Decorator

So you decided that the foo service I sent to you lacks a greet function and you want it. Will you modify the factory? No! You can decorate it:

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app.config(function($provide) {
  $provide.decorator('foo', function($delegate) {
    $delegate.greet = function() {
      return "Hello, I am a new function of 'foo'";
    };

    return $delegate;
  });
});

$provide is what Angular uses internally to create all the services. We can use it to create new services if we want but also to decorate existing services. $provide has a method called decorator that allows us to do that. decorator receives the name of the service and a callback function that receives a $delegate parameter. That $delegate parameter is our original service instance.

Here we can do what we want to decorate our service. In our case, we added a greetfunction to our original service. Then we return the new modified service.

Now when we consume it, it will have the new greet function as you will see in the Try it.

The ability to decorate services comes in handy when we are consuming 3rd party services and we want to decorate it without having to copy it in our project and then doing there the modifications.

Note: The constant service cannot be decorated.

Try it

Bonus 2: Creating new instances

Our services are singleton but we can create a singleton factory that creates new instances. Before you dive deeper, keep in mind that having singleton services is the way to go and we don’t want to change that. Said that, in the rare cases you need to generate new instances, you can do that like this:

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// Our class
function Person( json ) {
  angular.extend(this, json);
}

Person.prototype = {
  update: function() {
    // Update it (With real code :P)
    this.name = "Dave";
    this.country = "Canada";
  }
};

Person.getById = function( id ) {
  // Do something to fetch a Person by the id
  return new Person({
    name: "Jesus",
    country: "Spain"
  });
};

// Our factory
app.factory('personService', function() {
  return {
    getById: Person.getById
  };
});

Here we create a Person object which receives some json to initialize the object. Then we created a function in our prototype (functions in the prototype are for the instances of the Person) and a function directly in Person (which is like a class function).

So we have a class function that will create a new Person object based on the id that we provide (well, it will in real code) and every instance is able to update itself. Now we just need to create a service that will use it.

Every time we call personService.getById we are creating a new Person object, so you can use this service in different controllers and even when the factory in a singleton, it generates new objects.

Kudos to Josh David Miller for his example.

Try it

Bonus 3: Coffeescript

Coffeescript can be handy with services since they provide a prettier way to create classes. Let’s see an example of the Bonus 2 using Coffeescript:

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app.controller 'MainCtrl', ($scope, personService) ->
  $scope.aPerson = personService.getById(1)

app.controller 'SecondCtrl', ($scope, personService) ->
  $scope.aPerson = personService.getById(2)
  $scope.updateIt = () ->
    $scope.aPerson.update()

class Person

  constructor: (json) ->
    angular.extend @, json

  update: () ->
    @name = "Dave"
    @country = "Canada"

  @getById: (id) ->
    new Person
      name: "Jesus"
      country: "Spain"

app.factory 'personService', () ->
  {
    getById: Person.getById
  }

It is prettier now in my humble opinion.

Try it

NOTE: This last one, being Coffeescript seems to fail a little bit with JSbin. Go to the Javascript tab and select Coffeescript to make it work.

Conclusion

Services are one of the coolest features of Angular. We have a lot of ways to create them, we just need to pick the correct one for our use cases and implement it.

If you found any issue or you think that this can be improved, please leave an issue or pull request at Github. In any case, a comment will be appreciated :).

 

http://angular-tips.com/blog/2013/08/understanding-service-types/

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