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5 Media/8 Courses/Design Patterns by Construx.md

@@ -83,10 +83,53 @@ The programmer needs to focus much more on semantics, because the complier will
 > 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 i
+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.
+
+![[Pasted image 20250116171230.png]]
+
+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 ))
 
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 ## Exercises