软件设计原则
SOLID
Single responsibility principle 单一职责
The single responsibility principle is a computer programming principle that states that every module, class, or function should have responsibility over a single part of the functionality provided by the software, and that responsibility should be entirely encapsulated by the class, module or function. All its services should be narrowly aligned with that responsibility. Robert C. Martin expresses the principle as, “A class should have only one reason to change,” although, because of confusion around the word “reason” he more recently stated “This principle is about people.(Actor)”
Martin defines a responsibility as a reason to change, and concludes that a class or module should have one, and only one, reason to be changed (i.e. rewritten). As an example, consider a module that compiles and prints a report. Imagine such a module can be changed for two reasons. First, the content of the report could change. Second, the format of the report could change. These two things change for very different causes; one substantive, and one cosmetic. The single responsibility principle says that these two aspects of the problem are really two separate responsibilities, and should therefore be in separate classes or modules. It would be a bad design to couple two things that change for different reasons at different times.
The reason it is important to keep a class focused on a single concern is that it makes the class more robust. Continuing with the foregoing example, if there is a change to the report compilation process, there is greater danger that the printing code will break if it is part of the same class.
Open–closed principle 开闭原则
Open/closed principle states “software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification”; that is, such an entity can allow its behaviour to be extended without modifying its source code.
The name open/closed principle has been used in two ways. Both ways use generalizations (for instance, inheritance or delegate functions) to resolve the apparent dilemma, but the goals, techniques, and results are different.
Liskov substitution principle 里式替换
Interface segregation principle (ISP)
ISP states that no client should be forced to depend on methods it does not use. ISP splits interfaces that are very large into smaller and more specific ones so that clients will only have to know about the methods that are of interest to them. Such shrunken interfaces are also called role interfaces. ISP is intended to keep a system decoupled and thus easier to refactor, change, and redeploy.
Dependency inversion principle
In object-oriented design, the dependency inversion principle is a specific form of decoupling software modules. When following this principle, the conventional dependency relationships established from high-level, policy-setting modules to low-level, dependency modules are reversed, thus rendering high-level modules independent of the low-level module implementation details. The principle states:
- High-level modules should not depend on low-level modules. Both should depend on abstractions (e.g. interfaces).
- Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions.
By dictating that both high-level and low-level objects must depend on the same abstraction, this design principle inverts the way some people may think about object-oriented programming.
Depend upon abstractions. Do not depend upon concrete classes. P177
In conventional application architecture, lower-level components (e.g. Utility Layer) are designed to be consumed by higher-level components (e.g. Policy Layer) which enable increasingly complex systems to be built. In this composition, higher-level components depend directly upon lower-level components to achieve some task. This dependency upon lower-level components limits the reuse opportunities of the higher-level components.The goal of the dependency inversion pattern is to avoid this highly coupled distribution with the mediation of an abstract layer, and to increase the re-usability of higher/policy layers.
With the addition of an abstract layer, both high- and lower-level layers reduce the traditional dependencies from top to bottom. Nevertheless, the “inversion” concept does not mean that lower-level layers depend on higher-level layers. Both layers should depend on abstractions that draw the behavior needed by higher-level layers.
In a direct application of dependency inversion, the abstracts are owned by the upper/policy layers. This architecture groups the higher/policy components and the abstractions that define lower services together in the same package. The lower-level layers are created by inheritance/implementation of these abstract classes or interfaces.[1]
The inversion of the dependencies and ownership encourages the re-usability of the higher/policy layers. Upper layers could use other implementations of the lower services. When the lower-level layer components are closed or when the application requires the reuse of existing services, it is common that an Adapter mediates between the services and the abstractions.
GRASP
General Responsibility Assignment Software Patterns (or Principles), abbreviated GRASP, consist of guidelines for assigning responsibility to classes and objects in object-oriented design. It is not related to the SOLID design principle.
The different patterns and principles used in GRASP are controller, creator, indirection, information expert, high cohesion, low coupling, polymorphism, protected variations, and pure fabrication. All these patterns answer some software problems, and 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.
Separation of Concerns 关注点分离
Don’t Repeat Yourself
Inversion of Control
Inversion of control is sometimes facetiously referred to as the “Hollywood Principle: Don’t call us, we’ll call you”.
(The Hollywood Principle: Don’t call us, we’ll call you. 《Head First design patterns - ora 2004》 P334)
Inversion of Control by Martin Fowler