Learn the specific technical learning experience of designing reusable software from the three levels of class, API and framework

One ． Software reuse
Software reuse refers to the development of new software systems , Reuse existing software or software modules , The software can be pre-existing software , It can also be a dedicated reusable component . The software reusability affects the production efficiency 、 Software quality and system maintainability .
Mainly introduces several aspects of reuse ：
（ One ） Software reuse based on program library
Program library is the most basic part of software reuse 、 The most common form . Libraries are often used 、 A collection of verified normalized programs or subroutines . The program in the library is a frequently used function . Libraries are usually compiled binary executables or virtual machine executables , Put them in the catalog , Set the environment variable , After being introduced into the program, it can be like an ordinary function 、 object 、 Class, etc . Procedural language ( Such as C) The library is mainly a variety of functions . The program libraries of object-oriented languages are collectively called class libraries , The essence is a comprehensive object-oriented reusable type collection , Including the interface 、 Abstract classes and concrete classes . Libraries can be built into the language system , It can also be independently developed by the third engine . Different language packaging and bowing | The form of using the library is slightly different . The keywords that reference the library in the program are include、using、 import、 with etc. . Except that libraries in the same language can be referenced , Current programming languages also allow bow | Into libraries in other languages , The most common is C The library of languages . Use the third-party development program library and use the system program library , There is no difference in essence . Usually understanding API Design 、 Look up API send F The way 、 Learn the demonstration code and practice constantly .
（ Two ） Framework based software reuse
Framework method is component technology 、 The combination of software architecture and application software development . Frameworks usually appear in the form of component libraries , But the component library is only an important part of the framework , The large-scale reuse of component libraries requires a framework . A framework is a partial code reuse , Part is design reuse , Sometimes analysis can be reused . Frameworks can be represented in code , It can also be directly executed or reused .
The development of framework starts from code 、 Design and analysis reuse the software at three levels :
Code reuse . It becomes very easy to build applications from existing component libraries , Because the components adopt the interfaces uniformly defined by the framework , Thus, the communication between components is simple .
Reuse design . It provides reusable Abstract algorithms and high-level design , And can decompose the large system into smaller components , And can describe the internal interface between components .
Reuse analysis . If all personnel analyze the affairs according to the idea of the framework , Then it can be divided into the same components , Adopt a similar solution , So that the analysts who adopt the same framework can communicate with each other .
（ 3、 ... and ） be based on API Software reuse
A typical example is to implement or call a function procedure ：
（1） When implementing a function , Parameter types of the functions have never been seen before , But we can learn this new through intelligent perception API And correctly get the information we expect from the parameters .
（2） When you call a function , We need to pass in parameter types we haven't seen before , Prompt through intelligent perception , We can know how to construct or get these types and pass them in correctly .
（3） After calling a function, we get the return value , We haven't seen this type before , But through intelligent perception , We can learn this new type , And know how to use this return value correctly .
Two ． class ,API, frame
（ One ） Design reusable classes
stay OOP Design reusable classes in , Encapsulation and information hiding , Inheritance and rewriting , polymorphism , Subtypes and overloads , Generic Programming , Behavior subtypes and Liskov The principle of substitution （LSP）, Combination and delegation .
（ Two ）API
API It's the application programming interface , Interface to a library or framework . well API It should have the following properties ：
Comprehensibility ： For an excellent API Come on , The concepts involved should be within the user's understanding , Even if there is a new concept, it should be progressive . Uniformity ： Maintain compatibility down . visibility ： It is best to provide an entry for users API The starting point of the . Simple tasks should have simple solutions ： therefore API It should be layered . Protect investment ： Be kind to API Users of . Try to find a way to make API Nice . Such as method name , Such as structure, etc .
Write a API The following aspects need to be considered ：API One thing should be done , And did a good job .API It should be as small as possible , But not too small .Implementation Should not affect API. Documentation is important . Consider performance consequences .API Must coexist peacefully with the platform . Class design ： Minimize variability , follow LSP principle . Method design ： Don't let the customer do what any module can do , Timely error reporting .
（ 3、 ... and ） frame
frame （Framework） It's a reusable design of the whole or part of the system , It represents a group of abstract components and methods of interaction between component instances ; Another definition is , The framework is an application framework that can be customized by application developers .
The former is from the application side and the latter is from the purpose side . In order to increase the reusability of code , You can use delegation and inheritance mechanisms . meanwhile , In the process of using these two mechanisms to increase code reuse , We also add relationships between different classes accordingly （ Delegation or inheritance relationship ）. And for a project , The dependencies between different classes can be regarded as a framework . A large-scale project may consist of many different frameworks .
Framework and design pattern ：
frame 、 Design patterns are always confusing , In fact, there are differences between them . Components are often code reuse , And design patterns are design reuse , The frame is somewhere in between , Partial code reuse , Partial design reuse , Sometimes analysis can be reused .

There are three levels of reuse in software production ： Internal reuse , That is, the abstract block that can be used in the same application ;
code reuse , That is to say, general modules are combined into libraries or toolsets , So that it can be used in many applications and fields ;
Reuse of application framework , That is to provide a common or off the shelf infrastructure for a specific domain , For the highest level of reuse .
Frame is similar to design pattern , But there is a fundamental difference .
A design pattern is a description of a recurring problem in a certain environment and a solution to the problem , It's more abstract than a framework ; Frameworks can be represented in code , It can also be executed or reused directly , For patterns, only instances can be represented in code ; Design patterns are smaller elements than frameworks , A framework often contains one or more design patterns , Frameworks are always for a specific application area , However, the first mock exam can be applied to various applications . so to speak , Frameworks are software , Design patterns are the knowledge of software .
The frame is divided into white box frame and black box frame .