What are Design Patterns?

Design patterns are a key concept in software engineering that can help developers write more efficient, maintainable, and scalable code. In this article, we will explore what design patterns are, how they can be used in Solidity, and some of the most common design patterns used in smart contract development.

What are Design Patterns?

A design pattern is a reusable solution to a commonly occurring problem in software design. They are not specific pieces of code, but rather general approaches to solving common problems that can be applied in different contexts.

Design patterns were first introduced in the field of software engineering in the book “Design Patterns: Elements of Reusable Object-Oriented Software” by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, also known as the “Gang of Four” (GoF). The GoF book describes 23 design patterns that can be used to solve common problems in software design, and these patterns have become widely adopted in the software industry.

There are three main types of design patterns: creational, structural, and behavioral. Creational patterns deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. Structural patterns deal with object composition, creating relationships between objects to form larger structures. Behavioral patterns focus on communication between objects, what goes on between objects and how they operate together.

Design patterns can help developers write better code by providing a standardized way of approaching common problems. They also help to improve communication between developers by providing a common vocabulary for discussing software design.

Design Patterns in Solidity

Design patterns can be applied in any programming language, including Solidity. Some common design patterns in Solidity include:

• Factory pattern: This pattern allows for the creation of multiple instances of a contract, each with its own unique state. It can be used to create token contracts or other types of contracts that need to track individual instances.
• Singleton pattern: This pattern ensures that there is only one instance of a contract. It can be used to create contracts that track global state, such as a central bank contract.
• Observer pattern: This pattern allows contracts to subscribe to events emitted by other contracts. It can be used to implement decentralized exchanges or other types of contracts that need to track events across multiple contracts.

Other common design patterns in Solidity include the state machine pattern, the mutex pattern, and the access control pattern.

Best Practices for Using Design Patterns in Solidity

When using design patterns in Solidity, it’s important to follow best practices to ensure your code is efficient, maintainable, and scalable:

• Use established patterns: There are many design patterns available, and it’s a good idea to use ones that have been well-reviewed and tested by the community.
• Document your code: Make sure to include clear documentation for your contract’s design pattern, including descriptions of what it does and how to use it.
• Test your code: As with any contract, it’s important to thoroughly test your code to ensure it’s reliable and secure.

Conclusion

Design patterns are a key concept in software engineering that can help developers write more efficient, maintainable, and scalable code. By providing a standardized way of approaching common problems, design patterns can help improve communication between developers and reduce the risk of vulnerabilities in smart contracts. By following best practices, such as using established patterns and thoroughly testing your code, you can ensure that your contracts are reliable and secure.

Exercises

To review these concepts, we will go through a series of exercises designed to test your understanding and apply what you have learned.

pragma solidity ^0.5.0;

contract TokenFactory {
  address[] public tokens;

  function createToken() public {
    address newToken = new Token();
    tokens.push(newToken);
  }
}

contract Token {
  // Contract code goes here...
}

pragma solidity ^0.5.0;

contract CentralBank {
  address private _singletonAddress;
  uint private _totalSupply;

  function CentralBank() public {
    // Ensure there is only one instance of this contract
    require(_singletonAddress == address(0));
    _singletonAddress = address(this);
  }

  function totalSupply() public view returns (uint) {
    return _totalSupply;
  }

  function issue(uint amount) public {
    require(msg.sender == _singletonAddress);
    _totalSupply += amount;
  }
}

pragma solidity ^0.5.0;

contract EventEmitter {
  event Event(uint value);

  function emitEvent(uint value) public {
    emit Event(value);
  }
}

contract EventObserver {
  EventEmitter public emitter;

  function EventObserver(address emitterAddress) public {
    emitter = EventEmitter(emitterAddress);
  }

  function subscribe() public {
    emitter.onEvent(handleEvent);
  }

  function handleEvent(uint value) public {
    // Event handling code goes here...
  }
}

pragma solidity ^0.5.0;

enum State { Created, Active, Inactive }

contract StateMachine {
  State public currentState;

  function StateMachine() public {
    currentState = State.Created;
  }

  function activate() public {
    require(currentState == State.Created || currentState == State.Inactive);
    currentState = State.Active;
  }

  function deactivate() public {
    require(currentState == State.Active);
    currentState = State.Inactive;
  }
}

pragma solidity ^0.5.0;

contract Mutex {
  bool public lock;

  function performCriticalAction() public {
    require(!lock);
    lock = true;
    // Perform critical action here...
    lock = false;
  }
}