Design patterns

1. Design Patterns

Design patterns in software development, and increasingly relevant in complex MediaWiki installations and extension development, are reusable solutions to commonly occurring problems in software design. They represent best practices adopted by experienced developers, offering a proven blueprint for tackling recurring design challenges. Understanding and applying design patterns leads to more maintainable, flexible, and robust software – crucial for large-scale wiki projects and the extensions that power them. This article aims to provide a beginner-friendly introduction to the concept of design patterns, exploring their benefits, types, and illustrating them with examples applicable to MediaWiki development (though the principles are universal).

== What are Design Patterns?

At their core, design patterns aren’t finished pieces of code you can simply copy and paste. Instead, they are *templates* for how to solve a problem. They describe the elements of a solution, their relationships, and the trade-offs involved. Thinking of them as documented, well-considered solutions to common problems is a good starting point. They are not algorithms (step-by-step procedures), but rather descriptions of how to structure code to achieve a particular goal.

The term "design pattern" was popularized by the book *Design Patterns: Elements of Reusable Object-Oriented Software* by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (often referred to as the "Gang of Four" or GoF). This book categorized patterns into three main types:

• **Creational Patterns:** Concerned with the process of object creation. They abstract away the instantiation process, making systems more flexible.
• **Structural Patterns:** Deal with how classes and objects are composed to form larger structures. They focus on simplifying and organizing complex relationships.
• **Behavioral Patterns:** Focus on the interaction between objects. They define how objects communicate and collaborate to achieve a specific task.

== Why Use Design Patterns?

Employing design patterns offers several advantages:

• **Reusability:** Patterns provide pre-defined solutions that can be adapted and reused in different contexts, saving time and effort. This is especially valuable in Extension development where common functionalities might be required across multiple extensions.
• **Improved Communication:** Using a common vocabulary of patterns makes it easier for developers to understand each other’s code and collaborate effectively. For example, mentioning “Factory Pattern” immediately conveys a specific approach to object creation.
• **Reduced Complexity:** Patterns help to break down complex problems into smaller, more manageable parts, leading to simpler and more understandable code. This is critical for maintaining a large wiki codebase over time.
• **Increased Maintainability:** Code based on patterns is generally easier to modify and extend because the underlying structure is well-defined.
• **Proven Solutions:** Patterns represent solutions that have been tested and refined by experienced developers over time, reducing the risk of introducing bugs or inefficiencies.
• **Flexibility:** Design patterns often promote loose coupling between components, making it easier to adapt to changing requirements. This is vital in the ever-evolving landscape of web technologies.

== Common Design Patterns (with MediaWiki Relevance)

Let’s explore some commonly used design patterns and how they might be applied in a MediaWiki context.

1. 1. 1. 1. Singleton

• **Purpose:** Ensure that a class has only one instance and provide a global point of access to it.
• **MediaWiki Example:** Consider a custom logging service. You only want one instance of the logger to manage all log entries to avoid conflicts and ensure consistent logging behavior. A Singleton pattern would guarantee this.
• **Implementation:** A private constructor and a static method to return the single instance.

1. 1. 1. 2. Factory Method

• **Purpose:** Define an interface for creating an object, but let subclasses decide which class to instantiate. This promotes loose coupling and flexibility.
• **MediaWiki Example:** Imagine building a system to render different types of content (e.g., text, images, videos) within a template. A Factory Method could be used to create the appropriate renderer object based on the content type.
• **Implementation:** An interface or abstract class defines the creation method, and concrete subclasses provide specific implementations.

1. 1. 1. 3. Observer

• **Purpose:** Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
• **MediaWiki Example:** When a page is edited, you might want to notify various components, such as a caching system, a search indexer, or a notification service. The Observer pattern is ideal for this.
• **Implementation:** A subject (observable) maintains a list of observers and notifies them of changes.

1. 1. 1. 4. Strategy

• **Purpose:** Define a family of algorithms, encapsulate each one, and make them interchangeable. Allows you to vary the algorithm independently from the clients that use it.
• **MediaWiki Example:** Different methods for calculating page ranking or determining article relevance. You could have strategies for ranking based on views, edits, backlinks, or a combination of factors. The strategy can be switched dynamically. This is related to Search functionality. Consider different search algorithms.
• **Implementation:** An interface defines the algorithm, and concrete classes implement different variations.

1. 1. 1. 5. Adapter

• **Purpose:** Convert the interface of a class into another interface clients expect. Allows classes to work together that couldn't otherwise.
• **MediaWiki Example:** Integrating a third-party library that has a different API than MediaWiki’s. An Adapter pattern can wrap the library, providing a MediaWiki-compatible interface.
• **Implementation:** A wrapper class adapts the interface of the existing class.

1. 1. 1. 6. Decorator

• **Purpose:** Add responsibilities to an object dynamically. Provides a flexible alternative to subclassing for extending functionality.
• **MediaWiki Example:** Adding features to a parser without modifying its core code. You could decorate the parser to add support for new syntax or formatting options.
• **Implementation:** A wrapper class adds functionality to the original object.

1. 1. 1. 7. Command

• **Purpose:** Encapsulate a request as an object, allowing you to parameterize clients with different requests, queue or log requests, and support undoable operations.
• **MediaWiki Example:** Implementing an undo/redo functionality for complex edits. Each edit can be represented as a command object. This is related to History features.
• **Implementation:** A command class encapsulates an action and its parameters.

1. 1. 1. 8. Template Method

• **Purpose:** Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Allows subclasses to redefine certain steps of an algorithm without changing the algorithm's structure.
• **MediaWiki Example:** Creating a base class for different types of content rendering. The template method defines the overall rendering process, while subclasses implement the specific rendering logic for each content type.
• **Implementation:** An abstract class defines the algorithm's skeleton, and subclasses implement the concrete steps.

1. 1. 1. 9. Composite

• **Purpose:** Compose objects into tree structures to represent part-whole hierarchies. Allows clients to treat individual objects and compositions of objects uniformly.
• **MediaWiki Example:** Representing a wiki’s category structure. Categories can contain other categories, forming a hierarchy. This relates to Categories.
• **Implementation:** Both individual objects and compositions have a common interface.

1. 1. 1. 10. Iterator

• **Purpose:** Provide a way to access the elements of an object sequentially without exposing its underlying representation.
• **MediaWiki Example:** Iterating through the revisions of a page or the members of a category. This allows access to data without revealing the internal storage structure.
• **Implementation:** An iterator class provides methods for accessing elements one by one.

== Applying Design Patterns to MediaWiki Development

When developing extensions or customizing MediaWiki, consider the following:

• **Analyze the Problem:** Before jumping into code, carefully analyze the problem you’re trying to solve. Identify the recurring patterns and potential design challenges.
• **Identify Relevant Patterns:** Based on your analysis, identify design patterns that might be applicable.
• **Adapt and Implement:** Don’t blindly copy patterns. Adapt them to your specific context and implement them thoughtfully.
• **Document Your Design:** Clearly document the patterns you’ve used and the reasoning behind your choices. This will make your code easier to understand and maintain.

== Further Exploration & Resources

• **Gang of Four Book:** *Design Patterns: Elements of Reusable Object-Oriented Software*
• **Refactoring Guru:** [1] A comprehensive website with explanations and examples of design patterns.
• **Sourcemaking:** [2] Another excellent resource for learning about design patterns.
• **MediaWiki Developer Documentation:** Manual:Developing extensions - Understand the MediaWiki architecture and coding standards.
• **PHP Documentation:** [3] – Understanding PHP is fundamental for MediaWiki development.

== Related Concepts & Technologies

• **SOLID Principles:** A set of five design principles that promote maintainable and flexible software.
• **Dependency Injection:** A technique for reducing coupling between components.
• **Unit Testing:** Essential for verifying the correctness of your code, especially when using design patterns.
• **Object-Oriented Programming (OOP):** The foundation for many design patterns.
• **Technical Analysis:** Understanding market trends and patterns (e.g., Head and Shoulders, Double Top, Moving Average Crossover).
• **Trading Strategies:** Employing patterns to predict market behavior (e.g., Scalping, Swing Trading, Day Trading).
• **Indicators:** Using mathematical calculations to analyze price data (e.g., MACD, RSI, Bollinger Bands).
• **Trend Following:** Identifying and capitalizing on prevailing trends (e.g., Uptrend, Downtrend, Sideways Trend).
• **Candlestick Patterns:** Interpreting visual representations of price movements (e.g., Doji, Hammer, Engulfing Pattern).
• **Fibonacci Retracements:** Using Fibonacci ratios to identify potential support and resistance levels.
• **Elliott Wave Theory:** Analyzing market cycles based on wave patterns.
• **Ichimoku Cloud:** A comprehensive indicator that provides support and resistance levels, trend direction, and momentum.
• **Volume Spread Analysis (VSA):** Interpreting price and volume data to understand market sentiment.
• **Harmonic Patterns:** Identifying specific geometric patterns to predict price movements.
• **Gann Angles:** Using angles to identify support and resistance levels.
• **Market Sentiment Analysis:** Gauging the overall attitude of investors towards a particular asset.
• **Risk Management:** Strategies for minimizing potential losses (e.g., Stop-Loss Orders, Take-Profit Orders).
• **Position Sizing:** Determining the appropriate amount of capital to allocate to each trade.
• **Correlation Analysis:** Identifying relationships between different assets.
• **Volatility Analysis:** Measuring the degree of price fluctuations.
• **Backtesting:** Testing trading strategies on historical data.
• **Algorithmic Trading:** Using computer programs to execute trades automatically.
• **High-Frequency Trading (HFT):** A type of algorithmic trading characterized by high speed and high volume.
• **Order Flow Analysis:** Monitoring the flow of buy and sell orders to identify market trends.
• **Time Series Analysis:** Analyzing data points indexed in time order.
• **Statistical Arbitrage:** Exploiting temporary price discrepancies between related assets.
• **Mean Reversion:** A strategy based on the belief that prices will eventually return to their average level.
• **Momentum Trading:** A strategy based on the belief that prices will continue to move in the same direction.

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