poodinis/TUTORIAL.md

Poodinis Tutorial

This tutorial will give you an overview of all functionality offered by Poodinis and how to use them.

The container

To register a class, a new dependency container must be instantiated:

// Create a shared container
auto dependencies = new shared DependencyContainer();

A shared dependency container is thread-safe and resolves the same dependencies across all threads. ###Registering dependencies To make dependencies available, they have to be registered:

// Register concrete class
dependencies.register!ExampleClass;
// Register by super type
dependencies.register!(ExampleInterface, ExampleClass);

In the above example, dependencies on the concrete class and interface will resolve an instance of class ExampleClass. A dependency registered by super type will automatically be registered by concrete type.

If you want to prevent registrations from being both registered by interface and concrete type, use the doNotAddConcreteTypeRegistration option when registering:

dependencies.register!(ExampleInterface, ExampleClass)([RegistrationOption.doNotAddConcreteTypeRegistration]);

Resolving dependencies

To manually resolve a dependency, all you have to do is resolve the dependency's type using the container in which it is registered:

auto exampleClassInstance = dependencies.resolve!ExampleClass;

If the class is registered by interface and not by concrete type, you can still resolve the class by concrete type (unless doNotAddConcreteTypeRegistration is applied):

auto exampleClassInstance = dependencies.resolve!ExampleInterface;
auto exampleClassInstance2 = dependencies.resolve!ExampleClass;
assert(exampleClassInstance is exampleClassInstance2);

It is also possible to register a type while resolving it. Doing so means you don't need to explicitly register it beforehand. To do this, use the resolve option registerBeforeResolving:

dependencies.resolve!ExampleClass([ResolveOption.registerBeforeResolving]);

Naturally this can only be done when you are resolving a concrete type or an interface type by qualifier.

Dependency creation behaviour

You can control how a dependency is resolved by specifying a creation scope during registration. The scope determines which instance is returned, be it the same each time or a new one. The following scopes are available:

• Resolve a dependency using a single instance (default):

dependencies.register!ExampleClass.singleInstance();

• Resolve a dependency with a new instance each time it is resolved:

dependencies.register!ExampleClass.newInstance();

• Resolve a dependency using a pre-existing instance

auto preExistingInstance = new ExampleClass();
dependencies.register!ExampleClass.existingInstance(preExistingInstance);

Automatic Injection

The real value of any dependency injection framework comes from its ability to automatically inject dependencies. Poodinis supports automatic injection either through autowiring members annotated with the @Autowire UDA or through constructor injection.

UDA-based autowiring

UDA-based autowiring can be achieved by annotating members of a class with the @Autowire UDA:

class ExampleClassA {}

class ExampleClassB {
	@Autowire
	private ExampleClassA dependency;
}

dependencies.register!ExampleClassA;
auto exampleInstance = new ExampleClassB();
dependencies.autowire(exampleInstance);
assert(exampleInstance.dependency !is null);

It is possible to autowire public as well as protected and private members.

Dependencies are automatically autowired when a class is resolved. So when you resolve ExampleClassB, its member dependency is automatically autowired:

dependencies.register!ExampleClassA;
dependencies.register!ExampleClassB;
auto instance = dependencies.resolve!ExampleClassB;
assert(instance.dependency !is null);

If an interface is to be autowired, you must register a concrete class by interface. A class registered only by concrete type can only be injected into members of that type, not its supertypes.

Using the UDA OptionalDependency you can mark an autowired member as being optional. When a member is optional, no ResolveException will be thrown when the type of the member is not registered and ResolveOption.registerBeforeResolving is not set on the container. The member will remain null or an empty array in case of array dependencies.

class ExampleClass {
	@Autowire
	@OptionalDependency
	private AnotherExampleClass dependency;
}

Constructor injection

Poodinis also supports automatic injection of dependencies through constructors:

class ExampleClassA {}

class ExampleClassB {
	private ExampleClassA dependency;

	this(ExampleClassA dependency) {
		this.dependency = dependency;
	}
}

dependencies.register!ExampleClassA;
dependencies.register!ExampleClassB;

auto instance = dependencies.resolve!ExampleClassB;
 

ExampleClassA is automatically resolved and passed to ExampleClassB's constructor. Classes with multiple constructors can be injected. The following rules apply to constructor injection:

• Injection is attempted at the order of declaration. However, this is compiler dependant and may not always be the case.
• Injection is attempted for the first constructor which has non-builtin types only in its parameter list.
• When a constructor with an empty parameter list is found, no other constructors are attempted (and nothing is injected). This can be used to explicitly prevent constructor injection.
• When no injectable constructor is found an InstanceCreationException will be thrown on resolve.

If the constructors of a class are not suitable for injection, you could manually configure its creation using Application Contexts (see chapter further down).
Constructor injection has the advantage of not having to import Poodinis throughout your application.

Value injection

Besides injecting class instances, Poodinis can also inject values:

class ExampleClass {
	@Value("a.key.for.this.value")
	private int someNumber;
}

The value will automatically be injected during the autowiring process. In order for Poodinis to be able to inject values, ValueInjectors must be available and registered with the dependency container:

class MyIntInjector : ValueInjector!int {
	int get(string key) {
		// read from some value source, such as a configuration file.
	}
}

dependencies.register!(ValueInjector!int, MyIntInjector);

Each injector injects a value of a specific type. You are required to register value injectors by interface; when injecting values, the autowiring process will try to resolve an injector of the member type, such as ValueInjector!int.

You can only register one value injector per type, a resolve exception will be thrown otherwise (unless you suppress them via resolve options). Naturally a resolve exception will also be thrown when there is no injector for a certain value type.

Besides injecting primitive types, it is also possible to inject structs. While it is possible to inject class instances this way, this mechanism isn't really meant for that.

Circular dependencies

Poodinis can autowire circular dependencies when they are registered with singleInstance or existingInstance registration scopes. Circular dependencies in registrations with newInstance scopes will not be autowired, as this would cause an endless loop. Circular dependencies are only supported when autowiring members through the @Autowire UDA; circular dependencies in constructors are not supported and will result in an InstanceCreationException.

Registering and resolving using qualifiers

You can register multiple concrete types to a super type. When doing so, you will need to specify a qualifier when resolving that type:

// Color is an interface, Blue and Red are classes implementing that interface
dependencies.register!(Color, Blue);
dependencies.register!(Color, Red);
auto blueInstance = dependencies.resolve!(Color, Blue);

If you want to autowire a type registered to multiple concrete types, specify a qualified type as template argument:

class BluePaint {
	@Autowire!Blue
	private Color color;
}

If you registered multiple concrete types to the same supertype and you do not resolve using a qualifier, a ResolveException is thrown stating that there are multiple candidates for the type to be resolved.

Autowiring all registered instances to an array

If you have registered multiple concrete types to a super type, you can autowire them all to an array, in which case you can easily operate on them all:

// Color is an interface, Blue and Red are classes implementing that interface

class ColorMixer {
	@Autowire
	private Color[] colors;
}

dependencies.register!(Color, Blue);
dependencies.register!(Color, Red);
auto mixer = dependencies.resolve!ColorMixer;

Member mixer.colors will now contain instances of Blue and Red. The order in which instances are resolved is not guaranteed to be that of the order in which they were registered.

Application Contexts

You can fine-tune dependency configuration using application contexts. Application contexts allow you to centralize all dependency configuration as well as define how instances of certain classes should be constructed using factory methods.

###Defining and using application contexts An application context is defined as follows:

class Context : ApplicationContext {
	public override void registerDependencies(shared(DependencyContainer) container) {
		container.register!SomeClass;
		container.register!(SomeInterface, SomeOtherClass).newInstance();
	}
	
	@Component
	public SomeLibraryClass libraryClass() {
		return new SomeLibraryClass("This class uses a constructor parameter of a built-in type so I have to register it through an application context");
	}
}

In the override registerDependencies() you can register all dependencies which do not need complex set-up, just like you would do when directly using the dependency container.

This override is optional. You can still register simple dependencies outside of the context (or in another context).
Complex dependencies are registered through member methods of the context. These member methods serve as factory methods which will be called when a dependency is resolved.

They are annotated with the @Component UDA to let the container know that these methods should be registered as dependencies. The type of the registration is the same as the return type of the method.

Factory methods are useful when you have to deal with dependencies which require constructor arguments or elaborate set-up after instantiation.

Application contexts have to be registered with a dependency container. They are registered as follows:

container.registerContext!Context;

All registered dependencies can now be resolved by the same dependency container. Registering a context will also register it as a dependency, meaning you can autowire the application context in other classes. You can register as many types of application contexts as you like.

###Autowiring application contexts Application contexts can make use of autowired dependencies like any other dependency. When registering an application context, all its components are registered first after which the application context is autowired. This means that after the registration of an application context some dependencies will already be resolved and instantiated. The following example illustrates how autowired members can be used in a context:

class Context : ApplicationContext {

	@Autowire
	private SomeClass someClass;
	
	@Autowire
	private SomeOtherClass someOtherClass;

	public override void registerDependencies(shared(DependencyContainer) container) {
		container.register!SomeClass;
	}
	
	@Component
	public SomeLibraryClass libraryClass() {
		return new SomeLibraryClass(someClass, someOtherClass);
	}
}

As you can see, autowired dependencies can be used within factory methods. When SomeLibraryClass is resolved, it will be created with a resolved instance of SomeClass and SomeOtherClass. As shown, autowired dependencies can be registered within the same application context, but don't neccesarily have to be. You can even autowire dependencies which are created within a factory method within the same application context.

Application contexts are directly autowired after they have been registered. This means that all autowired dependencies which are not registered in the application context itself need to be registered before registering the application context.

###Controlling component registration You can further influence how components are registered and created with additional UDAs:

class Context : ApplicationContext {
	@Component
	@Prototype // Will create a new instance every time the dependency is resolved.
	@RegisterByType!SomeInterface // Registers the dependency by the specified super type instead of the return type
	public SomeClass someClass() {
		return new SomeClass();
	}
}

Persistent Registration and Resolve Options

If you want registration options to be persistent (applicable for every call to register()), you can use the container method setPersistentRegistrationOptions():

dependencies.setPersistentRegistrationOptions(RegistrationOption.doNotAddConcreteTypeRegistration); // Sets the options
dependencies.unsetPersistentRegistrationOptions(); // Clears the persistentent options

Likewise, the same is possible for resolve options:

dependencies.setPersistentResolveOptions(ResolveOption.registerBeforeResolving); // Sets the options
dependencies.unsetPersistentResolveOptions(); // Clears the persistentent options

Please note that setting options will unset previously set options; the options specified will be the only ones in effect.