Lesson 4: Outline

Lesson 4 Introduction and outline.

Lesson 4: Error Handling and Events in Solidity

Objective: Understand how to handle errors and emit events in Solidity, which are critical for smart contract reliability and interaction with the front end.

Part 1: Error Handling in Solidity

• Error Handling Mechanisms:

  • require: Used to check for conditions and revert the transaction if the condition is not met. It reverts all changes made to the state.

  • revert: Provides a way to trigger an exception and revert the transaction, often with a custom error message.

  • assert: Used for internal checks as a way to prevent conditions that should never be possible. It consumes all gas when failed.

• When to Use Each:

  • Use require for input validation or to enforce proper conditions before execution.

  • Use revert for more complex condition checks, especially where a specific error message is helpful.

  • Use assert for invariants and to check for conditions that indicate a bug.

• Example: Error Handling

  pragma solidity ^0.8.0;
  contract ErrorHandling { mapping(address => uint) public balance;
  
  function deposit() public payable {
      require(msg.value > 0, "Deposit amount must be greater than 0");
      balance[msg.sender] += msg.value;
  }
  
  function withdraw(uint amount) public {
      require(amount <= balance[msg.sender], "Insufficient balance");
      balance[msg.sender] -= amount;
      payable(msg.sender).transfer(amount);
  }

  }

Part 2: Events in Solidity

• Understanding Events:

  • Events allow logging to the Ethereum blockchain.

  • Useful for tracking contract activity and interacting with the contract's front-end.

• Declaring and Emitting Events:

  • How to declare an event and emit it in functions.

• Example: Using Events

  pragma solidity ^0.8.0;
  contract EventExample { 
      event Deposit(address indexed sender, uint amount); 
      event Withdrawal(address indexed receiver, uint amount);
  
      mapping(address => uint) public balance;
      
      function deposit() public payable {
          emit Deposit(msg.sender, msg.value);
          balance[msg.sender] += msg.value;
      }
      
      function withdraw(uint amount) public {
          require(amount <= balance[msg.sender], "Insufficient balance");
          emit Withdrawal(msg.sender, amount);
          balance[msg.sender] -= amount;
          payable(msg.sender).transfer(amount);
      }
  }