6.3 KiB
title, created_date, updated_date, aliases, tags, type, course_name, author, source, status
| title | created_date | updated_date | aliases | tags | type | course_name | author | source | status | |
|---|---|---|---|---|---|---|---|---|---|---|
| Design Patterns by Construx | 2025-01-16 | 2025-01-16 |
|
course | Design Patterns | Construx | https://construx.vueocity.com/portal/design-patterns | not_started |
Design Patterns by Construx
- 🏷️Tags : #01-2025 #book
Summary
[!summary] Summary 3 Sentences only!
- What are the main ideas?
- If I implemented one idea from this book right now, which one would it be?
- How would I describe the course to someone else?
Ideas and Thoughts
[!info]+ Inspiring Questions
- Did you think about other concepts from other resources?
- Do the concepts fit to your past, to your memories?
- Can you relive them and reflect them from a different angle?
Chapters
Getting Started
[!definition] UML Notation Contents
[!Definition] What is a Design Pattern A standard solution to a recurring problem.
We should not reinvent the wheel. Patterns are much more reusable than code.
A bridge is a design pattern because it is a standard solution to a recurring problem: we need to get things across a not passable obstacle. For every problem you have design criteria to choose the best type of bridge (e.g. span, clearance, loads, cost, etc.). In design patterns design criteria are called Goal forces and constraint forces. As a beginner: focus on the problem, not the solutions!
UML Notations:
Note
!
Class, Attribute and Operation
!
Operations are viewed from extern Methods are viewed from intern
Minimize Overall Complexity
Measures of complexity
- Cyclomatic complexity: the number of decisions that are being made --> local complexity
- Depth of decision making: embedding of decision within decision and decision --> local complexity
- number of parameters: --> global complexity
- fan out: number of functions that are called by the function --> global complexity
!
Local complexity is a measure on how complicated each function is, whereas the global complexity is the complexity in between functions. Remember to get an appropriate balance.
Use Abstractions
Syntax: Is all about structure Semantics: Is all about meaning
- Colorless green dreams sleep furiously: correct syntax, completely wrong semantics --> defective or buggy code, because it is semantically meaningless
The compiler is a master of syntax but cannot know anything about semantics. The programmer needs to focus much more on semantics, because the complier will take care of the syntax.
[!NOTE] Abstraction the principle of ignoring those aspects of a subject that are not relevant to the current purpose in order to concentrate solely on those that are.
It is a tool to reduce and manage complexity.
what is the role of one function / subroutine? The more abstraction you have, the less performant the code becomes. Abstraction is not learnable. Good abstractions lead to clean code, which is maintainable and leads to less defects.
Encapsulate Design Decisions
Abstraction: permission to ignore Encapsulation: I prevent you from knowing Meaning they are not the same thing.
Goal:
- hide design decision (data representation, algorithms, etc.)
- Modules should be black boxes: programm to an interface, not through an interface
Idea: Design by Contract Requires vs Guarantees A function promises to give certain outputs depending on the inputs This is important for documentation of code if the code is used by different people. Docstrings for example. We only worry about semantic things. Explain exactly what the input values should be and what the return values will be. Explain exactly what errors and exceptions. Include Constraints on state (function may open file), include performance limits (use no more than 200 bytes per instance, etc). Defensive contract: requires less and guarantees more (check input for example). Contracts are great, because you can treat the function as a blackbox. When you modify the contract you need to inform all the users of the function about the updates of the contract
Desing by Contract is the means to encapsulation Use: Purpose, Requires and Guarantees
Maximize Cohesion, Minimize Coupling
Cohesion: Indivisibility of a given part: Does this function do exactly one thing? It should also not split functionality into two separate functions Coupling: Dependence between parts: Do not connect things that don't need to be connected. If you must connect things, connect them such that they can be disconnected as easily as possible.
Goal: Highly cohesive, very loosely coupled
Generally software solves problems by Decomposition, solution, recomposition. There are an infinite amount of different solutions, which have different quality. Cohesion and Coupling characterise the quality of the solution.
!
Always try to Decouple code:
| Avoid | Do |
|---|---|
| Global variables (form of coupling) | Design by Contract |
| Friend (C++) | Dependency inversion: if you depend on things, depend on the most generic thing (type of car, not model) |
| Self-modifying code | Publish-subscribe (observer pattern) |
| Reflection | Law of Demeter |
Law of Demeter
Principle of least knowledge. a.b().c() breaks the law --> violates coupling a.b() doesn't break the law
Design to Invariants, Design
Product Families: attack different target markets with different value propositions with the same base platform: (pickup truck and suv example. For ford it's the same car (slight difference in build but 95% the s))
Exercises
Design Patterns Construx
-file: "Design Patterns by Construx.md"