GRASP (Object Oriented Design)
Encyclopedia
General Responsibility Assignment Software Patterns (or Principles), abbreviated GRASP, consists of guidelines for assigning responsibility to classes and objects in object-oriented design.
The different patterns and principles used in GRASP are: Information Expert, Creator, Controller, Low Coupling
, High Cohesion
, Polymorphism, Pure Fabrication, Indirection, Protected Variations. All these patterns answer some software problem, and in almost every case these problems are common to almost every software development
project. These techniques have not been invented to create new ways of working but to better document and standardize old, tried-and-tested programming principles in object oriented design.
It has been said that "the critical design tool for software development is a mind well educated in design principles. It is not the UML
or any other technology". Thus, GRASP is really a mental toolset, a learning aid to help in the design of object oriented software.
Simply "Creator pattern is responsible for creating an object of class".
In general, a class B should be responsible for creating instances of class A if one, or preferably more, of the following apply:
These responsibilities include methods, computed fields and so on.
Using the principle of Information Expert a general approach to assigning responsibilities is to look at a given responsibility, determine the information needed to fulfill it, and then determine where that information is stored.
Information Expert will lead to placing the responsibility on the class with the most information required to fulfill it.
class that represent the overall system or a use case
scenario. A Controller object is a non-user interface object responsible for receiving or handling a system event.
A use case controller should be used to deal with all system events of a use case, and may be used for more than one use case (for instance, for use cases Create User and Delete User, one can have one UserController, instead of two separate use case controllers).
It is defined as the first object beyond the UI layer that receives and coordinates ("controls") a system operation. The controller should delegate to other objects the work that needs to be done; it coordinates or controls the activity. It should not do much work itself.
The GRASP Controller can be thought of as being a part of the Application/Service layer (assuming that the application has made an explicit distinction between the Application/Service layer and the Domain layer) in an object-oriented system with common layers
.
This kind of class is called "Service" in Domain-driven design
.
pattern.
and using polymorphism to create various implementations of this interface.
The different patterns and principles used in GRASP are: Information Expert, Creator, Controller, Low Coupling
Coupling (computer science)
In computer science, coupling or dependency is the degree to which each program module relies on each one of the other modules.Coupling is usually contrasted with cohesion. Low coupling often correlates with high cohesion, and vice versa...
, High Cohesion
Cohesion (computer science)
In computer programming, cohesion is a measure of how strongly-related each piece of functionality expressed by the source code of a software module is...
, Polymorphism, Pure Fabrication, Indirection, Protected Variations. All these patterns answer some software problem, and in almost every case these problems are common to almost every software development
Software development
Software development is the development of a software product...
project. These techniques have not been invented to create new ways of working but to better document and standardize old, tried-and-tested programming principles in object oriented design.
It has been said that "the critical design tool for software development is a mind well educated in design principles. It is not the UML
Unified Modeling Language
Unified Modeling Language is a standardized general-purpose modeling language in the field of object-oriented software engineering. The standard is managed, and was created, by the Object Management Group...
or any other technology". Thus, GRASP is really a mental toolset, a learning aid to help in the design of object oriented software.
Creator
Creation of objects is one of the most common activities in an object-oriented system. Which class is responsible for creating objects is a fundamental property of the relationship between objects of particular classes.Simply "Creator pattern is responsible for creating an object of class".
In general, a class B should be responsible for creating instances of class A if one, or preferably more, of the following apply:
- Instances of B contains or compositely aggregates instances of A
- Instances of B record instances of A
- Instances of B closely use instances of A
- Instances of B have the initializing information for instances of A and pass it on creation.
Information Expert
Information Expert is a principle used to determine where to delegate responsibilities.These responsibilities include methods, computed fields and so on.
Using the principle of Information Expert a general approach to assigning responsibilities is to look at a given responsibility, determine the information needed to fulfill it, and then determine where that information is stored.
Information Expert will lead to placing the responsibility on the class with the most information required to fulfill it.
Controller
The Controller pattern assigns the responsibility of dealing with system events to a non-UIUser interface
The user interface, in the industrial design field of human–machine interaction, is the space where interaction between humans and machines occurs. The goal of interaction between a human and a machine at the user interface is effective operation and control of the machine, and feedback from the...
class that represent the overall system or a use case
Use case
In software engineering and systems engineering, a use case is a description of steps or actions between a user and a software system which leads the user towards something useful...
scenario. A Controller object is a non-user interface object responsible for receiving or handling a system event.
A use case controller should be used to deal with all system events of a use case, and may be used for more than one use case (for instance, for use cases Create User and Delete User, one can have one UserController, instead of two separate use case controllers).
It is defined as the first object beyond the UI layer that receives and coordinates ("controls") a system operation. The controller should delegate to other objects the work that needs to be done; it coordinates or controls the activity. It should not do much work itself.
The GRASP Controller can be thought of as being a part of the Application/Service layer (assuming that the application has made an explicit distinction between the Application/Service layer and the Domain layer) in an object-oriented system with common layers
Common layers in an information system logical architecture
The following four layers are the most common layers in a logical multilayered architecture for an information system with an object-oriented design:* User Interface Layer...
.
Low Coupling
Low Coupling is an evaluative pattern, which dictates how to assign responsibilities to support:- low dependency between classes;
- low impact in a class of changes in other classes;
- high reuse potential;
High Cohesion
High Cohesion is an evaluative pattern that attempts to keep objects appropriately focused, manageable and understandable. High cohesion is generally used in support of Low Coupling. High cohesion means that the responsibilities of a given element are strongly related and highly focused. Breaking programs into classes and subsystems is an example of activities that increase the cohesive properties of a system. Alternatively, low cohesion is a situation in which a given element has too many unrelated responsibilities. Elements with low cohesion often suffer from being hard to comprehend, hard to reuse, hard to maintain and adverse to change.Polymorphism
According to Polymorphism, responsibility of defining the variation of behaviors based on type is assigned to the types for which this variation happens. This is achieved using polymorphic operations.Pure Fabrication
A Pure Fabrication is a class that does not represent a concept in the problem domain, specially made up to achieve low coupling, high cohesion, and the reuse potential thereof derived (when a solution presented by the Information Expert pattern does not).This kind of class is called "Service" in Domain-driven design
Domain-driven design
Domain-driven design is an approach to developing software for complex needs by deeply connecting the implementation to an evolving model of the core business concepts...
.
Indirection
The Indirection pattern supports low coupling (and reuse potential) between two elements by assigning the responsibility of mediation between them to an intermediate object. An example of this is the introduction of a controller component for mediation between data (model) and its representation (view) in the Model-view-controllerModel-view-controller
Model–view–controller is a software architecture, currently considered an architectural pattern used in software engineering. The pattern isolates "domain logic" from the user interface , permitting independent development, testing and maintenance of each .Model View Controller...
pattern.
Protected Variations
The Protected Variations pattern protects elements from the variations on other elements (objects, systems, subsystems) by wrapping the focus of instability with an interfaceInterface (computer science)
In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...
and using polymorphism to create various implementations of this interface.
See also
- Anemic Domain ModelAnemic Domain ModelAnemic domain model is a term used to describe the use of a software domain model where the business logic is implemented outside the domain objects.-Overview:...
- Design pattern (computer science)Design pattern (computer science)In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...
- Design Patterns (book)
- Solid (object-oriented design)