Depin Core - Dependency injection framework core module for Ceylon

Logging

This module uses standard Ceylon logging defined via module ceylon.logging. Configuration of logging may be altered using log value. Whole module uses log for logging purpose. Reference to module ceylon.logging documentation for more information.

Introduction

Whole concept of this framework, is based on Dependency and Injection abstract classes. Both are tightly coupled together. The Dependency class is different view on Ceylon declaration. Dependencies are identified by name and declaration type. Functional dependency type identification contains:

• return type of function
• parameter types (parameter names doesn't matter). To inject dependency both type and name must match in lookup phase.
  

From point of view of Injection, two declaration (and after provisioning Dependency) having same name, (for example from different packages), with different open types are not colliding.

Depin instantiation

To perform dependency injection Depin class object needs to be instantiated. It takes a stream of ceylon declarations as an arguments. Using those declarations dependencies will be provided. It is possible to provide stream manually but it is cumbersome in most cases.

Declaration scanning

For this purpose scanner object is available. Currently (0.2.0) scanning of declarations may be performed in various ways. scanner.dependencies method gives ability to scan provided Scopes in search of all declaration annotated with dependency annotation.

scanner.subtypeDependencies method give ability to scan provided Scopes in search of all declaration, which are subtype of provided type. Only concrete classes are taken in consideration.

Scanning visibility

Scanner will scan all classes and they members it doesn't matter either they are shared or not and if the packages condained in Scope are shared.

Dependency provisioning

When declaration are provided to the Depin default constructor provisioning process occures. Declaration are turned into Dependencyies. Validation of duplications is executed (no validation for circullar dependencies is in place right now). Then Dependency.Decorator are applied. At the end of provisioning Depin will post an event Depin.ready for all Dependency.Decorators so they can furtherly setup if needed.

Dependency lookup

Whenver injection using Depin.inject or extraction Depin.extract are performed the lookup phase of Dependency must occur. Dependecies stored inside Depin are taken in consideration. There is no possibility to change what Dependency are stored in Depin. Whenever it is required new instance of Depin should be created. Lookup phase takes in consideration name of dependency and it's type identifier.

Dependency resolution

Resolution process is executed via Dependency.resolve attribute which provides a value or function that is beign used later in process, of injection or extraction. This is done every time dependency has been looked up and being injected or extracted. To cache resolution there are Dependency.Decorators which can be applied, further described in this guide. By default dependency resolution is lazy and not cached in any way.

Dependency injection

Executed via Depin.inject, requires Injectable parameter which is alias for class, function or value model to which injection will happen. This process subjects:

• Dependency lookup
• Dependency resolution
• Calling function, or getting a value as a result of appling parameters to Injectable provided as parameter.

Example:

dependency String topLevelValue="some value";
dependency Integer topLevelFunction(String someString) => someString.size;

Integer topLevelInjection(Integer topLevelFunction(String someString), String topLevelValue){
        return topLevelFunction(topLevelValue);
}

shared void run() {
        value depedencencyDeclarations=scanner.dependencies({`package`});
        value result=Depin(depedencencyDeclarations).inject(`topLevelInjection`);
        print(result);
}

Dependency extraction (since 0.1.0)

To provide easier interoperation with frameworks where programmer has no control, over creating objects such as Android SDK, Depin.extract functionality has been introduced. It allows to provide resolved dependencies into the caller. So going with example of Android SDK, in Activity.onCreate, dependencies can be obtained by using correct naming and typing. Then they can be bounded to late or variable fields and used in life-cycle of Activity. Be aware that Depin does not provide any ability for disposing of these dependencies. Although this can be achieved using dependency decorators and event handlers, notified through Depin.notify method. It would vary by use-case, as each framework uses different interface for disposing.

Example:

class UnaccesibleDependencyContainer(){
  suppressWarnings("unusedDeclaration")
  dependency String name="abc";
}



class Activity(){
  late String name;

  shared void onCreate(){
    //Used because can't obtain metamodel reference to local declaration: name value
    //top level values are obtainable
    assert(exists nameReference = `class Activity`.getDeclaredMemberDeclaration<ValueDeclaration>("name"));
    value dependencies = scanner.dependencies({`package`});
    name = Depin(dependencies).extract<String>(nameReference);
    assert(name=="abc");
    print(name);
  } 
}
shared void run(){
  Activity().onCreate();
}

Dependency visibility and encapsulation

In this release Depin, does not honor Ceylon encapsulation in any way. Whatever is scanned, can be injected. This will be modified in further release.

Naming

For some cases it is required to rename given Dependency, for such requirements named annotation has been introduced. It takes String name as argument. This hints Depin that Dependency created from this named declaration will have name as given in [[named.name]].

Example:

dependency Integer[] summable =[1,2,3];
class DependencyHolder(named("summable") Integer[] numbers){
 named("integerSum") dependency 
 Integer? sum = numbers.reduce((Integer partial, Integer element) => partial+element);
}

void printInjection(Integer? integerSum){
       print("Sum of summable is: ``integerSum else "null"``");
}


shared void run(){
 Depin{
    scanner.dependencies({`package`});
 }.inject(`printInjection`);
}

Warning !

It is important to remember that to identify a dependency, it's type, must exactly match with declaration of type in injection. So in given example sum is declared with Integer? type, printInjection first parameter has exactly the same type! All intersection types, interfaces and unions must match exactly!

Warning 2 !

Because of https://github.com/eclipse/ceylon/issues/7448 it is not possible to rename (using named annotation) constructor parameters, for Dependency containers or injection constructor parameters.

Targeting

In some cases it is required to declare more than one constructor in a class. Depin won't be able to gues which constructor to use. In this case target annotation can be used. This is applicable for injections and dependencies.

Example:

class TargetedInjection {

 String constructorName;

 shared new(){
       constructorName="default";
 }

 shared target new targetedConstructor(){
       constructorName="targeted";
 }


 shared void printInjection(){
       print("Selected construcotr was: ``constructorName``");
 }
}


shared void run(){
 Depin{
     scanner.dependencies({`package`});
 }.inject(`TargetedInjection.printInjection`);
} 

Decorators

This framework uses concept of decorators defined via Dependency.Decorator interface. Each decorator is an annotation, allowing to change way of dependency resolution. Example usage is to provide ability to define singletons or eager dependency resolvers. Dependency.Decorators can be defined outside of this module, they are recognized during dependency provisioning from declarations. This feature in frameworks like Spring is called scopes. Build in decorators:

• Singleton - represented by singleton annotation,
• Eager - represented by eager annotation,
• Fallback - represented by fallback annotation.

More information can be found in specific annotation documentation.

Handlers

Each decorator can be notified, from outside of framework, it needs just to implement Handler interface. This feature provides ability to change way decorators works. For example It allows to free up resources. To notify decorator Depin.notify method needs to be called.

Collectors

Collector class is used for collecting of dependencies with specific open type. In this case naming doesn't matter. Depin.inject will always inject whole known set of dependencies for given type declared in [[Collector.Collected]] type parameter.

Example:

dependency Integer one=1;
dependency Integer two=2;

void assertCollectorInjection(Collector<Integer> namingDoesntMatters){
       assert(namingDoesntMatters.collected.containsEvery({one,two}));
}

shared void run(){
 Depin{
   scanner.dependencies({`package`});
   }.inject(`assertCollectorInjection`);
  }
} 

Subtype collector (since 0.2.0)

It is sometimes usefull to retreive whole set of object beign subtype of given type. subtype annotation has been introduced for this case. Whenver declaring injection with Collector parameter, annotated with subtype annotation, Depin will know that You want all dependencies subtyping type parameter [[Collector.Collected]] and try to inject them. Union types and Intersection types are also taken in consideration. Results includes exact types of Collector type parameter.

Example:

dependency Integer one=1;
dependency Integer two=2;
dependency String str="abc";
dependency Float float=1.3;
void assertSubtypeCollectorInjection(subtype Collector<Object?> namingDoesntMatters){
    assert(namingDoesntMatters.collected.containsEvery({one,two,str,float}));
}

shared void run(){
    Depin{
        scanner.dependencies({`package`});
    }.inject(`assertSubtypeCollectorInjection`);
}

Interoperation with java

Because of Java type-system definition, where generics are not part of the type declaration, Depin usage can be a bit of pain. For cases, where there is no type parameters dependency injection should function without issues but whenever generics are in place <out Anything>, type parameter declaration must be used.

Example:

java class in native jvm module

public class Generic<T>{
 private T data;

 public Generic(T data){
       this.data=data;
 }

 public T getData(){
     return data;
 }
 @Override
 public String toString(){
       return data.toString();
 }
}

dependencies declaration and usage in Ceylon

dependency Generic<out Anything> data= Generic("data");

suppressWarnings("uncheckedTypeArguments")
shared Generic<String> injection(Generic<out Anything> data){
    assert( is Generic<String> data  );
return data;
}

shared void run(){
    value dependencies=scanner.dependencies({`package`});
 value result=Depin(dependencies).inject(`injection`);
 assert(result==data);
 print(result);
}