Modules Definitions

Writing definitions in Koin is done via Dart functions, that describe ishow is built your instance. Once you have configured your Koin application, let's write some modules and definitions.

Module & definitions

Given some classes that we need to inject:

class HttpClient {
final String url;
class DataRepository {
final HttpClient httpClient;
class Bloc {
final DataRepository dataRepository;

Here how we could define some components:

final myModule = Module()
// Define a singleton for type HttpClient
..single<HttpClient>(((s) => HttpClient('server_url')))
// Define a singleton for type DataRepository
// Resolve the HttpClient dependency with s.get()
..single<DataRepository>(((s) => DataRepository(s.get())))
// Define a factory (create a new instance each time) for type Bloc (infered parameter in <>)
// Resolve the DataRepository dependency with s.get()
..factory<Bloc>(((s) => Bloc(s.get())));


You can give a qualifier to a component. This qualifier can be a string or a type, and is setup with the named() function:

final myModule = Module()
// Define a single Bloc named 'single'
..single<Bloc>(((s) => Bloc(s.get())), qualifier: named('single'))
// Define a factory for type Bloc
..factory<Bloc>(((s) => Bloc(s.get())));

This qualifier can be associated to a class or directly to a type:

class MyClass {
// Type qualifier
..single<DataRepository>(((s) => DataRepository(s.get())), qualifier: named<MyClass>())
// String qualifier
..single<DataRepository>(((s) => DataRepository(s.get())), qualifier: named('single'))

Additional types

In the module API, for a definition, you can give some extra type to bind, with the bind operator (binds for list of Class):

class Component1Interface {}
class Component1 implements Component1Interface {}
final myModule = Module()
..single<Component1>(((s) => Component1())).bind<Component1Interface>();

Then you can request your instance with get<Component1>() or get<ComponentInterface1>().

You can bind multiple definitions with the same type:

final myModule = Module()
..single<Component1>(((s) => Component1())).bind<Component1Interface>()
..single<Component2>(((s) => Component2())).bind<Component1Interface>();

But here you won't be able to request an instance with get<ComponentInterface1>(). You will have to use koin.bind<Component1,ComponentInterface1>() to retrieve an instance of ComponentInterface1, with the Component1 implementation.

Note that you cal also look for all components binding a given type: getAll<ComponentInterface1>() will request all instances binding ComponentInterface1 type.

Combining several modules

There is no concept of import in Koin. Just combine several complementary modules definitions.

Let's dispatch definitions in 2 modules:

final myModule1 = Module()
..single<HttpClient>(((s) => HttpClient('server_url')))
..single<DataRepository>(((s) => DataRepository(s.get())));
final myModule2 = Module()..factory<Bloc>(((s) => Bloc(s.get())));

We just need to list them for Koin:

app.modules([myModule1, myModule2]);

Loading after start

After Koin has been started with startKoin { } function, it is possible to load extra definitions modules with the following function: loadKoinModules(modules...)

Dropping definitions

Once a modules has been loaded into Koin, we can unload it and then drop definitions and instances, related to those definitions. for this we use the unloadKoinModules(modules...)

void main() {
var koin = startKoin((app) {
koin.getWithParam<MySingle>(45); // -> id is 45
koin.getWithParam<MySingle>(40); // -> id is 40

Declare on the fly

Allows you to declare an instance on the fly with Koin already initialized or in a scope already created.

void main() {
// no def
var koin = startKoin((app) {}).koin;
// Create an instance
var component = Component1();
// declare it
// retrieve it
expect(component, koin.get<Component1>());

You can also use a qualifier or secondary types to help create your definition:

koin.declare(component, qualifier: named('qualifier'), secondaryTypes: []);


A quick recap of the Koin keywords:

  • module() - create a Koin Module or a submodule (inside a module)
  • factory() - provide a factory bean definition
  • single() - provide a single bean definition
  • get() - resolve a component dependency
  • named() - define a qualifier with type, enum or string
  • bind() - additional Dart type binding for given bean definition
  • binds() - list of additional Dart types binding for given bean definition