What are the differences between a Service
, Provider
and Factory
in AngularJS?
From the AngularJS mailing list I got an amazing thread that explains service vs factory vs provider and their injection usage. Compiling the answers:
Syntax: module.service( 'serviceName', function );
Result: When declaring serviceName as an injectable argument you will be provided with an instance of the function. In other words new FunctionYouPassedToService()
.
Syntax: module.factory( 'factoryName', function );
Result: When declaring factoryName as an injectable argument you will be provided with the value that is returned by invoking the function reference passed to module.factory.
Syntax: module.provider( 'providerName', function );
Result: When declaring providerName as an injectable argument you will be provided with (new ProviderFunction()).$get()
. The constructor function is instantiated before the $get method is called - ProviderFunction
is the function reference passed to module.provider.
Providers have the advantage that they can be configured during the module configuration phase.
See here for the provided code.
Here's a great further explanation by Misko:
provide.value('a', 123);
function Controller(a) {
expect(a).toEqual(123);
}
In this case the injector simply returns the value as is. But what if you want to compute the value? Then use a factory
provide.factory('b', function(a) {
return a*2;
});
function Controller(b) {
expect(b).toEqual(246);
}
So factory
is a function which is responsible for creating the value. Notice that the factory function can ask for other dependencies.
But what if you want to be more OO and have a class called Greeter?
function Greeter(a) {
this.greet = function() {
return 'Hello ' + a;
}
}
Then to instantiate you would have to write
provide.factory('greeter', function(a) {
return new Greeter(a);
});
Then we could ask for 'greeter' in controller like this
function Controller(greeter) {
expect(greeter instanceof Greeter).toBe(true);
expect(greeter.greet()).toEqual('Hello 123');
}
But that is way too wordy. A shorter way to write this would be provider.service('greeter', Greeter);
But what if we wanted to configure the Greeter
class before the injection? Then we could write
provide.provider('greeter2', function() {
var salutation = 'Hello';
this.setSalutation = function(s) {
salutation = s;
}
function Greeter(a) {
this.greet = function() {
return salutation + ' ' + a;
}
}
this.$get = function(a) {
return new Greeter(a);
};
});
Then we can do this:
angular.module('abc', []).config(function(greeter2Provider) {
greeter2Provider.setSalutation('Halo');
});
function Controller(greeter2) {
expect(greeter2.greet()).toEqual('Halo 123');
}
As a side note, service
, factory
, and value
are all derived from provider.
provider.service = function(name, Class) {
provider.provide(name, function() {
this.$get = function($injector) {
return $injector.instantiate(Class);
};
});
}
provider.factory = function(name, factory) {
provider.provide(name, function() {
this.$get = function($injector) {
return $injector.invoke(factory);
};
});
}
provider.value = function(name, value) {
provider.factory(name, function() {
return value;
});
};