Repository files navigation

❤️ Sponsor

Check out my new packagewatch_it the state management addon to get_itThere is now a discord support channel for my packageshttps://discord.gg/ZHYHYCM38h

This is a simpleService Locator for Dart and Flutter projects with some additional goodies highly inspired bySplat. It can be used instead ofInheritedWidget orProvider to access objects e.g. from your UI.

Typical usage:

• Accessing service objects like REST API clients or databases so that they easily can be mocked.
• Accessing View/AppModels/Managers/BLoCs from Flutter Views

As your App grows, at some point you will need to put your app's logic in classes that are separated from your Widgets. Keeping your widgets from having direct dependencies makes your code better organized and easier to test and maintain.But now you need a way to access these objects from your UI code. When I came to Flutter from the .Net world, the only way to do this was the use of InheritedWidgets. I found the way to use them by wrapping them in a StatefulWidget; quite cumbersome and have problems working consistently. Also:

• I missed the ability to easily switch the implementation for a mocked version without changing the UI.
• The fact that you need aBuildContext to access your objects made it inaccessible from the Business layer.

Accessing an object from anywhere in an App can be done in other ways, but:

• If you use a Singleton you can't easily switch the implementation out for a mock version in tests
• IoC containers for Dependency Injections offer similar functionality, but with the cost of slow start-up time and less readability because you don't know where the magically injected object comes from. Most IoC libs rely on reflection they cannot be ported to Flutter.

As I was used to using the Service LocatorSplat from .Net, I decided to port it to Dart. Since then, more features have been added.

If you are not familiar with the concept of Service Locators, it's a way to decouple the interface (abstract base class) from a concrete implementation, and at the same time allows to access the concrete implementation from everywhere in your App over the interface.I can only highly recommend reading this classic article by Martin FowlerInversion of Control Containers and the Dependency Injection pattern.

GetIt is:

• Extremely fast (O(1))
• Easy to learn/use
• Doesn't clutter your UI tree with special Widgets to access your data like, Provider or Redux does.

GetIt isn't a state management solution! It's a locator for your objects so you need some other way to notify your UI about changes likeStreams orValueNotifiers. But together with my packagewatch_it, it gets a full-featured easy to use state management solution that integrates with the Objects registered in get_it.

At your start-up you register all the objects you want to access later like this:

final getIt=GetIt.instance;voidsetup() {  getIt.registerSingleton<AppModel>(AppModel());// Alternatively you could write it if you don't like global variablesGetIt.I.registerSingleton<AppModel>(AppModel());}

After that you can access yourAppModel class from anywhere like this:

MaterialButton(  child:Text("Update"),  onPressed:getIt<AppModel>().update// given that your AppModel has a method update),

You can find here adetailed blog post on how to use GetIt

As Dart supports global (or euphemistic ambient) variables I often assign my GetIt instance to a global variable to make access to it as easy as possible.

Although the approach with a global variable worked well, it has its limitations if you want to useGetIt across multiple packages. Therefore GetIt itself is a singleton and the default way to access an instance ofGetIt is to call:

GetIt getIt=GetIt.instance;//There is also a shortcut (if you don't like it just ignore it):GetIt getIt=GetIt.I;

Through this, any call toinstance in any package of a project will get the same instance ofGetIt. I still recommend just assigning the instance to a global variable in your project as it is more convenient and doesn't harm (Also it allows you to give your service locator your own name).

GetIt getIt=GetIt.instance;

You can use any name you want which makes Brian 😃 happy like (sl, backend, services...) ;-)

Before you can access your objects you have to register them withinGetIt typically direct in your start-up code.

getIt.registerSingleton<AppModel>(AppModelImplementation());getIt.registerLazySingleton<RESTAPI>(()=>RestAPIImplementation());// if you want to work just with the singleton:GetIt.instance.registerSingleton<AppModel>(AppModelImplementation());GetIt.I.registerLazySingleton<RESTAPI>(()=>RestAPIImplementation());///`AppModel` and`RESTAPI` are both abstract base classes in this example

To access the registered objects callget<Type>() on yourGetIt instance

var myAppModel= getIt.get<AppModel>();

Alternatively, asGetIt is acallable class depending on the name you choose for yourGetIt instance you can use the shorter version:

var myAppModel=getIt<AppModel>();// as Singleton:var myAppModel=GetIt.instance<AppModel>();var myAppModel=GetIt.I<AppModel>();

GetIt offers different ways how objects are registered that affect the lifetime of these objects.

voidregisterFactory<T>(FactoryFunc<T> func)

You have to pass a factory functionfunc that returns a NEW instance of an implementation ofT. Each time you callget<T>() you will get a new instance returned. How to pass parameters to a factory you can findhere.

Although I always would recommend using an abstract base class as a registration type so that you can vary the implementations you don't have to do this. You can also register concrete types.

TregisterSingleton<T>(T instance)

You have to pass an instance ofT or a derived class ofT that you will always get returned on a call toget<T>(). The newly registered instance is also returned which can be sometimes convenient.

As creating this instance can be time-consuming at app start-up you can shift the creation to the time the object is the first time requested with:

voidregisterLazySingleton<T>(FactoryFunc<T> func)

You have to pass a factory functionfunc that returns an instance of an implementation ofT. Only the first time you callget<T>() this factory function will be called to create a new instance. After that, you will always get the same instance returned.

There are certain circumstances where you might wish to register multiple implementations of the same interface and then get a list of all of the relevant implementations later on. For instance, you might have a modular design where each module registers an interface defining a page and then all of these get injected into your navigation bar in your main layout without your layout needing to know about each module.

getIt.enableRegisteringMultipleInstancesOfOneType();

Then, you just register your classes as you normally would:

getIt.registerLazySingleton<MyBase>(  ()=>ImplA(),);getIt.registerLazySingleton<MyBase>(  ()=>ImplB(),);

Then, later on you can fetch all instances of this interface by calling:

finalIterable<MyBase> instances= getIt.getAll<MyBase>();

The returnedIterable will then contain all registered instances of the requested interface with or without an instance name.There is also anasync implementation available for this:

finalIterable<MyBase> instances=await getIt.getAllAsync<MyBase>();

If you try to register a type more than once you will fail with an assertion in debug mode because normally this is not needed and probably a bug.If you really have to overwrite a registration, then you can by setting the propertyallowReassignment = true.

If you try to register a type more than once and whenallowReassignment = false you will fail with an assertion in debug mode.If you want to just skip this double registration silently without an error, then you can by setting the propertyskipDoubleRegistration = true.This is only available inside tests where is can be handy.

You can check if a certain Type or instance is already registered in GetIt with:

/// Tests if an[instance] of an object or aType[T] or a name[instanceName] /// is registered inside GetItboolisRegistered<T>({Object instance,String instanceName});

If you need to you can also unregister your registered singletons and factories and pass an optionaldisposingFunction for clean-up.

/// Unregister an[instance] of an object or a factory/singleton by Type[T] or by name[instanceName]/// if you need to dispose some resources before the reset, you can/// provide a[disposingFunction]. This function overrides the disposing/// you might have provided when registering.voidunregister<T>({Object instance,String instanceName,voidFunction(T) disposingFunction})

In some cases, you might not want to unregister a LazySingleton but instead, reset its instance so that it gets newly created on the next access to it.

/// Clears the instance of a lazy singleton,  /// being able to call the factory function on the next call  /// of[get] on that type again.  /// you select the lazy Singleton you want to reset by either providing  /// an[instance], its registered type[T] or its registration name.  /// if you need to dispose some resources before the reset, you can  /// provide a[disposingFunction]. This function overrides the disposing  /// you might have provided when registering.voidresetLazySingleton<T>({Object instance,String instanceName,voidFunction(T) disposingFunction})

/// Clears all registered types in the reverse order in which they were registered./// Handy when writing unit tests or before quitting your application./// If you provided dispose function when registering they will be called///[dispose] if`false` it only resets without calling any dispose/// functions/// As dispose funcions can be async, you should await this function.Future<void>reset({bool dispose=true});

With V5.0 of GetIt, it now supports hierarchical scoping of registration. What does this mean?You can push a new registration scope like you push a new page on the Navigator. Any registration after that will be registered in this new scope. When accessing an object withget GetIt first checks the topmost scope for registration and then the ones below. This means you can register the same type that was already registered in a lower scope again in the scope above and you will always get the latest registered object.

Imagine an app that can be used with or without a login. On App start-up, aDefaultUser object is registered with the abstract typeUser as a singleton. As soon as the user logs in, a new scope is pushed and a newLoggedInUser object again with theUser type is registered that allows more functions. For the rest of the App, nothing has changed as it still accessesUser objects through GetIt.As soon as the user Logs off all you have to do is pop the Scope and automatically theDefaultUser is used again.

Another example could be a shopping basket where you want to ensure that not a cart from a previous session is used again. So at the beginning of a new session, you push a new scope and register a new cart object. At the end of the session, you pop this scope again.

/// Creates a new registration scope. If you register types after creating  /// a new scope they will hide any previous registration of the same type.  /// Scopes allow you to manage different live times of your Objects.  ///[scopeName] if you name a scope you can pop all scopes above the named one  /// by using the name.  ///[dispose] function that will be called when you pop this scope. The scope  /// is still valid while it is executed  ///[init] optional function to register Objects immediately after the new scope is  /// pushed. This ensures that[onScopeChanged] will be called after their registration  /// if[isFinal] is set to true, you can't register any new objects in this scope after  /// this call. In Other words you have to register the objects for this scope inside  ///[init] if you set[isFinal] to true. This is useful if you want to ensure that  /// no new objects are registered in this scope by accident which could lead to race conditionsvoidpushNewScope({voidFunction(GetIt getIt)? init,String scopeName,ScopeDisposeFunc dispose});/// Disposes all factories/Singletons that have been registered in this scope  /// and pops (destroys) the scope so that the previous scope gets active again.  /// if you provided dispose functions on registration, they will be called.  /// if you passed a dispose function when you pushed this scope it will be  /// called before the scope is popped.  /// As dispose functions can be async, you should await this function.Future<void>popScope();/// if you have a lot of scopes with names you can pop (see[popScope]) all  /// scopes above the scope with[name] including that scope unless[inclusive]= false  /// Scopes are popped in order from the top  /// As dispose functions can be async, you should await this function.  /// If no scope with[name] exists, nothing is popped and`false` is returnedFuture<bool>popScopesTill(String name, {bool inclusive=true});/// Clears all registered factories and singletons in the provided scope,  /// then destroys (drops) the scope. If the dropped scope was the last one,  /// the previous scope becomes active again.  /// if you provided dispose functions on registration, they will be called.  /// if you passed a dispose function when you pushed this scope it will be  /// called before the scope is dropped.  /// As dispose functions can be async, you should await this function.Future<void>dropScope(String scopeName);/// Tests if the scope by name[scopeName] is registered in GetItboolhasScope(String scopeName);/// Clears all registered types for the current scope in the reverse order in which they were registered.  /// If you provided dispose function when registering they will be called  ///[dispose] if`false` it only resets without calling any dispose  /// functions  /// As dispose funcions can be async, you should await this function.Future<void>resetScope({bool dispose=true});

In some cases, it might be helpful to know if an Object gets shadowed by another one e.g. if it has some Stream subscriptions that it wants to cancel before the shadowing object creates a new subscription. Also, the other way round so that a shadowed Object gets notified when it's "active" again meaning when a shadowing object is removed.

For this a class had to implement theShadowChangeHandlers interface:

abstractclassShadowChangeHandlers {voidonGetShadowed(Object shadowing);voidonLeaveShadow(Object shadowing);}

When the Object is shadowed itsonGetShadowed() method is called with the object that is shadowing it. When this object is removed from GetItonLeaveShadow() will be called.

When using scopes with objects that shadow other objects it's important to give the UI a chance to rebuild and acquire references to the now active objects. For this, you can register a call-back function in GetIt.The getit_mixin has a matchingrebuiltOnScopeChange method.

/// Optional call-back that will get called whenever a change in the current scope happens  /// This can be very helpful to update the UI in such a case to make sure it uses  /// the correct Objects after a scope changevoidFunction(bool pushed)? onScopeChanged;

From V5.0 on you can pass adispose function when registering any Singletons. For this the registration functions have an optional parameter:

DisposingFunc<T> dispose

whereDisposingFunc is defined as

typedefDisposingFunc<T>=FutureOrFunction(T param);

So you can pass simple or async functions as this parameter. This function is called when you pop or reset the scope or when you reset GetIt completely.

When you push a new scope you can also pass adispose function that is called when a scope is popped or reset but before the dispose functions of the registered objects is called which means it can still access the objects that were registered in that scope.

Instead of passing a disposing function on registration or when pushing a Scope from V7.0 on your objectsonDispose() method will be calledif the object that you register implements theDisposable interface:

abstractclassDisposable {FutureOronDispose();}

If a factory needs to call an async function you can useregisterFactoryAsync()

///[T] type to register///[func] factory function for this type///[instanceName] if you provide a value here your factory gets registered with that/// name instead of a type. This should only be necessary if you need to register more/// than one instance of one type.voidregisterFactoryAsync<T>(FactoryFuncAsync<T> func, {String instanceName});

To access instances created by such a factory you can't useget() but you have to usegetAsync() so thatyou can await the creation of the requested new instance.

/// Returns a Future of an instance that is created by an async factory or a Singleton that is/// not ready with its initialization.Future<T>getAsync<T>([String instanceName]);

Additionally, you can register asynchronous Singletons which means Singletons that have an initialization that requires async function calls. To be able to control such asynchronous start-up behaviour GetIt supports mechanisms to ensure the correct initialization sequence.

You create a Singleton with an asynchronous creation function

voidregisterSingletonAsync<T>(FactoryFuncAsync<T> factoryfunc,      {String instanceName,Iterable<Type> dependsOn,bool signalsReady=false});

The difference to a normal Singleton is that you don't pass an existing instance but provide a factory functionthat returns aFuture that completes at the end offactoryFunc and signals that the Singleton is ready to use unlesstrue is passed forsignalsReady. (see next chapter)To synchronize with other "async Singletons" you can pass a list ofTypes independsOn that have to be ready before the passed factory is executed.

There are two ways to signal the system that an instance is ready.

Often your registered services need to do asynchronous initialization work before they can be used from the rest of the app. As this is such a common task, and it's closely related to registration/initialization GetIt supports you here too.

GetIt has the functionallReady which returnsFuture<void> that can be used e.g. with a Flutter FutureBuilder to await that all asynchronous initialization is finished.

Future<void>allReady({Duration timeout,bool ignorePendingAsyncCreation=false});

There are different approaches to how the returned Future can be completed:

If you register any async SingletonsallReady will complete only after all of them have completed their factory functions. Like:

classRestService {Future<RestService>init()async {Future.delayed(Duration(seconds:1));returnthis;    }  }final getIt=GetIt.instance;/// in your setup function:  getIt.registerSingletonAsync<ConfigService>(()async {final configService=ConfigService();await configService.init();return configService;  });  getIt.registerSingletonAsync<RestService>(()async=>RestService().init());// here we asume an async factory function `createDbServiceAsync`  getIt.registerSingletonAsync<DbService>(createDbServiceAsync);/// ... in your startup page:returnFutureBuilder(      future: getIt.allReady(),      builder: (BuildContext context,AsyncSnapshot snapshot) {if (snapshot.hasData) {returnScaffold(            body:Center(              child:Text('The first real Page of your App'),            ),          );        }else {returnCircularProgressIndicator();        }      });

The above example shows you different ways to register async Singletons. The start-up page will display aCircularProgressIndicator until all services have been created.

In a case, these services have to be initialized in a certain order because they depend on that other services are already ready to be used you can use thedependsOn parameter ofregisterFactoryAsync. If you have a non-async Singleton that depends on other Singletons, there isregisterSingletonWithDependencies. In the following example,DbService depends onConfigService, andAppModel depends onConfigService andRestService

  getIt.registerSingletonAsync<ConfigService>(()async {final configService=ConfigService();await configService.init();return configService;  });  getIt.registerSingletonAsync<RestService>(()async=>RestService().init());  getIt.registerSingletonAsync<DbService>(createDbServiceAsync,      dependsOn: [ConfigService]);  getIt.registerSingletonWithDependencies<AppModel>(      ()=>AppModelImplmentation(),      dependsOn: [ConfigService,DbService,RestService]);

When usingdependsOn you ensure that the registration waits with creating its singleton on the completion of the type defined independsOn.

ThedependsOn field also acceptsInitDependency classes that allow specifying the dependency by type andinstanceName.

  getIt.registerSingletonAsync<RestService>(()async=>RestService().init(), instanceName:"rest1");  getIt.registerSingletonWithDependencies<AppModel>(      ()=>AppModelImplmentation(),      dependsOn: [InitDependency(RestService, instanceName:"rest1")]);

Sometimes the mechanism ofdependsOn might not give you enough control. For this case you can useisReady to wait for a certain singleton:

/// Returns a Future that completes if the instance of a Singleton, defined by Type[T] or  /// by name[instanceName] or passing the an existing[instance], is ready  /// If you pass a[timeout], a[WaitingTimeOutException] will be thrown if the instance  /// is not ready in the given time. The Exception contains details on which Singletons are  /// not ready at that time.  ///[callee] optional parameter which makes debugging easier. Pass`this` in here.Future<void>isReady<T>({Object instance,String instanceName,Duration timeout,Object callee,  });

To signal that a singleton is ready it can usesignalReady, provided you have set the optionalsignalsReady parameter when registering it OR make your registration type implement the empty abstract classWillSignalReady. Otherwise,allReady will wait on a call to signalsReady. No automatic signaling will happen in that case.

/// Typically this is used in this way inside the registered objects init/// method`GetIt.instance.signalReady(this);`voidsignalReady(Object instance);

You can use this to initialize your Singletons without async registration by using fire and forget async function from your constructors like so:

classConfigService {ConfigService()  {init();  }Futureinit()async {// do your async initialisation...GetIt.instance.signalReady(this);  }}

Even if you already have awaitedallReady, the moment you register new async singletons or singletons with dependencies you can useallReady again. This makes especially sense if you use scopes where every scope needs to get initialized.

By callingsignalReady(null) on yourGetIt instance theFuture you can get fromallReady will be completed.This is the most basic way to synchronize your start-up. If you want to do thatdon't usesignalsReady or async Singletons!!!I recommend using one of the other ways because they are more flexible and express your intention more clear.

You can find here adetailed blog post on async factories and startup synchronization

In some cases, it's handy if you could pass changing values to factories when callingget(). For that there are two variants for registering factories:

/// registers a type so that a new instance will be created on each call of[get] on that type based on/// up to two parameters provided to[get()]///[T] type to register///[P1] type of param1///[P2] type of param2/// if you use only one parameter pass void here///[factoryfunc] factory function for this type that accepts two parameters///[instanceName] if you provide a value here your factory gets registered with that/// name instead of a type. This should only be necessary if you need to register more/// than one instance of one type.////// example:///getIt.registerFactoryParam<TestClassParam,String,int>((s,i)///=> TestClassParam(param1:s, param2: i));////// if you only use one parameter://////getIt.registerFactoryParam<TestClassParam,String,void>((s,_)///=> TestClassParam(param1:s);voidregisterFactoryParam<T,P1,P2>(FactoryFuncParam<T,P1,P2> factoryfunc, {String instanceName});

and

voidregisterFactoryParamAsync<T,P1,P2>(FactoryFuncParamAsync<T,P1,P2> factoryfunc, {String instanceName});

The reason why I settled to use two parameters is that I can imagine some scenarios where you might want to register a builder function for Flutter Widgets that need to get aBuildContext and some data object.

When accessing these factories you pass the parameters a optional arguments toget():

var instance=getIt<TestClassParam>(param1:'abc',param2:3);

These parameters are passed asdynamics (otherwise I would have had to add more generic parameters toget()), but they are checked at runtime to be the correct types.

When you are writing unit tests with GetIt in your App you have two possibilities:

• Register all the Objects you need inside your unit Tests so that GetIt can provide its objects to the objects that you are testing.
• Pass your dependent objects into the constructor of your test objects like:

GetIt getIt=GetIt.instance;classUserManager {AppModel appModel;DbService dbService;UserManager({AppModel? appModel,DbService? dbService}) {this.appModel= appModel?? getIt.get<AppModel>();this.dbService= dbService?? getIt.get<DbService>();  }}

This way you don't need to pass them in theAppModel anddbService inside your App but you can pass them (or a mocked version) in your Unit tests.

If you have a mocked version of a Service, you can easily switch between that and the real one based on a flag:

if (testing) {    getIt.registerSingleton<AppModel>(AppModelMock());  }else {    getIt.registerSingleton<AppModel>(AppModelImplementation());  }

Ok, you have been warned! All registration functions have an optional named parameterinstanceName. Providing a name with factory/singleton here registers that instance with that name and a type. Consequentlyget() has also an optional parameterinstanceName to accessfactories/singletons that were registered by name.

IMPORTANT: Each name must be unique per type.

abstractclassRestService {}classRestService1implementsRestService{Future<RestService1>init()async {Future.delayed(Duration(seconds:1));returnthis;    }  }classRestService2implementsRestService{Future<RestService2>init()async {Future.delayed(Duration(seconds:1));returnthis;    }  }  getIt.registerSingletonAsync<RestService>(()async=>RestService1().init(), instanceName:"restService1");  getIt.registerSingletonAsync<RestService>(()async=>RestService2().init(), instanceName:"restService2");  getIt.registerSingletonWithDependencies<AppModel>(      () {RestService restService1=GetIt.I.get<RestService>(instanceName:"restService1");returnAppModelImplmentation(restService1);      },      dependsOn: [InitDependency(RestService, instanceName:"restService1")],  );

In rare occasions you might be faced with the problem that you don't know the type that you want to retrieve from GetIt at compile time which means you can't pass it as a generic parameter. For this theget functions have an optionaltype parameter

    getIt.registerSingleton(TestClass());final instance1= getIt.get(type:TestClass);expect(instance1isTestClass,true);

Be careful that the receiving variable has the correct type and don't passtype and a generic parameter.

While not recommended, you can create your own independent instance ofGetIt if you don't want to share your locator with someother package or because the physics of your planet demands it :-)

/// To make sure you really know what you are doing/// you have to first enable this feature:GetIt myOwnInstance=GetIt.asNewInstance();

This new instance does not share any registrations with the singleton instance.

Many thanks to the insightful discussions on the API withBrian Egan andSimon Lightfoot