General Responsibility Assignment Software Patterns
General Responsibility Assignment Software Patterns (or Principles), abbreviated GRASP, is a set of "nine fundamental principles in object design and responsibility assignment" first published by Craig Larman in his 1997 book Applying UML and Patterns. The different patterns and principles used in GRASP are controller, creator, indirection, information expert, low coupling, high cohesion, polymorphism, protected variations, and pure fabrication. All these patterns solve some software problem common to many software development projects. 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. Larman states that "the critical design tool for software development is a mind well educated in design principles. It is not UML or any other technology." Thus, the GRASP principles are really a mental toolset, a learning aid to help in the design of object-oriented software. In object-oriented design, a pattern is a named description of a problem and solution that can be applied in new contexts; ideally, a pattern advises us on how to apply its solution in varying circumstances and considers the forces and trade-offs. Many patterns, given a specific category of problem, guide the assignment of responsibilities to objects.
Creator See also: Factory pattern The 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. Controller The controller pattern assigns the responsibility of dealing with system events to a non-UI class that represents 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. Indirection The indirection pattern supports low coupling and reuses 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-controller pattern. This ensures that coupling between them remains low.
Low coupling Coupling is a measure of how strongly one element is connected to, has knowledge of, or relies on other elements. Polymorphism According to the polymorphism principle, responsibility for defining the variation of behaviors based on type is assigned to the type for which this variation happens. This is achieved using polymorphic operations. The user of the type should use polymorphic operations instead of explicit branching based on type. 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 a "service" in domain-driven design.