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Zend Framework: Zend_DI Component Proposal

Proposed Component Name Zend_DI
Developer Notes http://framework.zend.com/wiki/display/ZFDEV/Zend_DI
Proposers My E-mail Address
Revision 0.1 - 21 November 2007: Initial Proposal. (wiki revision: 45)

Table of Contents

1. Overview

Zend_DI is a dependency injector component. It minimizes coupling between groups of classes, makes unit testing much simpler, and provides an easy way to re-configure a package to use custom implementations of components. The architecture of the Zend_DI component is based on the following concepts:

  • Dependency injection is a technique that consists of passing objects via the constructor or setter methods.
  • The Container provides an easy way of re-configuring a package to use custom implementations of components.
  • Responsibility for object management is taken over by whatever container is being used to manage those objects.

Benefits of using a DI Container:

  • Easy best practice unit testing
  • Component reuse
  • Centralized configuration
  • Clean and declarative architecture
  • Maintainability and adaptability

2. References

3. Component Requirements, Constraints, and Acceptance Criteria

  • This component will use Reflection.

4. Dependencies on Other Framework Components

  • Zend_Config
  • Zend_Exception
  • Zend_Registry

5. Theory of Operation

Zend_DI provides generic factory classes that instantiate instances of classes. These instances are then configured by the container, allowing construction logic to be reused on a broader level. For example:

Once we separate configuration from use, we can easily test the Car with different Engines. It's just a matter of re-configuring the package and injecting Zend_Car_Parts_Engine_Gas instead of Zend_Car_Parts_Engine_Fuel.

6. Milestones / Tasks

  • Milestone 1: [DONE] Design interface
  • Milestone 2: [DONE] Write proposal
  • Milestone 3: [IN PROGRESS] Gather feedback and revise design as necessary
  • Milestone 4: Develop full implementation and unit tests
  • Milestone 5: Documentation
  • Milestone 6: Future enhancements

7. Class Index

  • Zend_DI_Container
  • Zend_DI_Exception
  • Zend_DI_Container_Manager
  • Zend_DI_Container_Exception
  • Zend_DI_Component_Factory
  • Zend_DI_Component_Exception

Future Proposals (other ideas):

  • Zend_DI_Statement
  • Zend_DI_Statement_Interface
  • Zend_DI_Statement_Parser

8. Use Cases

Zend_DI handles injections via the constructor or setters methods. In addition, the component allows the user to map out specifications for components and their dependencies in a configuration file and generate the objects based on that specification. The configuration object can be set using the setConfig() method.

Lets assume we have the following classes:

Constructor dependency injection

The configuration array is typically set up in a different class. For example, each package can have its own configuration file, a PHP class or an XML file. The configuration file holds the components specifications and package dependencies.

UC-01
UC-02

When a class is loaded, the constructor method (__construct) is selected by default.

UC-03
UC-04
UC-05

Users can map out specifications for components and their dependencies.

If the 'constructor' key is set and then the user tries to add a component, Zend_DI will throw an exception. For example:

Setter dependency injection

UC-06
UC-07

Zend_DI forces the user to inject constructor dependencies right after a class is loaded. Once the selectMethod() method is called, the user cannot inject additional dependencies to the constructor. The idea behind this is to keep the methods and their arguments together. For example:

UC-08

Containers

You can tell Zend_DI what classes to manage by adding them to a container (the order of registration has no significance). Containers can be retrieved using the Zend_DI_Container_Manager::getContainer() method, which returns an instance of the Zend_Registry class.

UC-09

You can register your own container as long as you pass an instance of the Zend_Registry class. New containers can be register using the registerContainer() method.

UC-10

Future Proposals

  • Zend_DI_Statement
UC-11

9. Class Skeletons

  • Zend_DI_Container
  • Zend_DI_Component_Factory
  • Zend_DI_Container_Manager

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