Responsibility-driven design

Objects are described as things that have machine-like behaviors that can be plugged together to work in concert. These objects play well-defined roles and encapsulate scripted responses and information.[5]

• Object Neighborhoods: Another term for subsystem.[9] It is a logical grouping of collaborators.[9]
• Responsibilities: A responsibility is an obligation to perform a task or know information.[5] These are further categorized according to their usage scenario.
  • Public Responsibilities: Public responsibilities are the responsibilities an object offers as services to others and the information it provides to others.[10]
  • Private Responsibilities: Private responsibilities are the actions an object takes in support of public responsibilities.[10]
  • Subresponsibilities: Sometimes, a large or complicated responsibility is split up into smaller ones called subresponsibilities.[11] They are further categorized based on what they do.
    • Subordinate Responsibilities: These include the major steps in each subresponsibility.[11]
    • Sequencing Responsibilities: These refer to the sequencing of the execution of subordinate responsibilities.[11]

Object role refers to an exterior view of what general service is offered by the object. It is a set of related responsibilities.[5] It can be implemented as a class or an interface. Interface, however, is the preferred implementation as it increases flexibility by hiding the concrete class which ultimately does the work.[12]

Role Stereotypes: Role stereotypes are simplified roles that come with predefined responsibilities.[13] There are several categories.

• Controller: Object implementing this role makes decisions and closely directs the action of other objects.[13]
• Coordinator: This role reacts to events by delegating tasks to others.[13]
• Information Holder: Information holder knows and provides information.[13]
  • Information Provider: A slight variation of an information holder is the information provider, which takes a more active role in managing and maintaining information. This distinction can be used if a designer needs to get more specific.[14]
• Interfacer: This role transforms information and requests between distinct parts of an application.[13] It is further divided into more specific roles.
  • External Interfacer: External interfacer communicates with other applications rather than its own.[14] It is mainly used for encapsulating non-object-oriented APIs and does not collaborate a lot.[15]
  • Internal Interfacer: Also called intersystem interfacer.[14] It act as a bridge between object neighborhoods.[15]
  • User Interfacer: User interfacer communicates with users by responding to events generated in the UI and then passing them on to more appropriate objects.[14][15][16]
• Service Provider: This role performs work and offers computing services.[14]
• Structurer: This role maintains relationships between objects and information about those relationships.[14]

This control style inflicts a procedural paradigm on the structure of the application and places major-decision making responsibilities in only a few objects or a single object.

Types

• Call-return model : The control of the objects in the application is in hierarchical way. Control starts at root and moves downwards. It is used in a sequential model.
• Manager model : The control of the objects in the application is in with only one object. Generally, it is implemented in concurrent models. It can also be implemented in sequential model using case statement.

Advantages

• Application logic is in one place.

Disadvantages

• Control logic can get overly complex
• Controllers can become dependent on information holders' contents
• Objects can become coupled indirectly through the actions of their controller
• The only interesting work is done in the controller

When to use

When decisions to be made are few, simple, and related to a single task.

A delegated control style lies in between a centralized and dispersed control style. It passes some of the decision making and much of the action to objects surrounding a control center. Each neighboring object has a significant role to play. It can also be called as event driven model, where the control is delegated to the object requesting it to process the event.

Types[reference]

• Broadcast model : An event is broadcast to all objects in the application. The object which can handle the event can acquire the control.
• Interrupt-driven model : There will be the interrupt handler to process the interrupt and passes to some object to process it.

Advantages

• It is easy to understand.
• Though there is an external coordinator, Objects can be made smarter to know what they are supposed to do and can be reused in other applications.
• Delegating coordinators tend to know about fewer objects than dominating controllers.
• Dialogs are higher-level.
• It is easy to change as changes typically affect fewer objects.
• It is easier to divide design work among team members.

Disadvantages

• Too much distribution of responsibility can lead to weak objects and weak collaborations

When to use

When one wants to delegate work to objects that are more specialized.

This control style is a variation of the centralized control style wherein control is factored among a group of objects whose actions are coordinated.[19] The main difference between a clustered and delegated control style is that in a clustered control style, the decision making objects are located within a control center whereas in a delegated control style they are mostly outside.[20]

A dispersed control style does not contain any control centers. The logic is spread across the entire population of objects, keeping each object small and building in as few dependencies among them as possible.[21]

Advantages

• None

Disadvantages

• When you want to find out how something works, you must trace the sequence of requests for services across many objects
• Not very reusable because no single object contributes much

When to use

Never.

After extensive results of experiments conducted, only the senior management has the necessary skills to make use of delegated control style and centralized control style benefits programmers. There is no context mentioned about the mid-level employees.[17]