Emitting events in Blockchain / Solidity

Introduction

Emitting events lets a blockchain program send messages that others can see and react to. It helps track what happens inside the program.

When you want to notify users that something important happened, like a transfer of tokens.

When you want to keep a record of actions for later review or auditing.

When you want other programs or interfaces to respond to changes in your contract.

When you want to debug or log what your contract is doing during execution.

Syntax

Blockchain / Solidity

event EventName(type indexed param1, type param2);

// To emit the event inside a function:
emit EventName(value1, value2);

Events are declared once, usually at the top of the contract.

Use emit keyword to send the event when something happens.

Examples

This example shows a Transfer event with sender, receiver, and amount. It is emitted when tokens are sent.

Blockchain / Solidity

event Transfer(address indexed from, address indexed to, uint amount);

function send(address to, uint amount) public {
    // ... transfer logic ...
    emit Transfer(msg.sender, to, amount);
}

This example emits a simple message event with a text string.

Blockchain / Solidity

event Message(string text);

function sayHello() public {
    emit Message("Hello, blockchain!");
}

Sample Program

This contract lets users deposit money. Each deposit emits a Deposit event with who sent money and how much.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleBank {
    event Deposit(address indexed accountHolder, uint amount);

    mapping(address => uint) public balances;

    function deposit() public payable {
        balances[msg.sender] += msg.value;
        emit Deposit(msg.sender, msg.value);
    }
}

Important Notes

Events are stored in the blockchain logs and do not cost much gas compared to storing data.

Indexed parameters help filter events easily when searching logs.

Events cannot be read by contracts, only by external tools or users.

Summary

Events send messages from blockchain programs to the outside world.

Use emit to trigger events inside functions.

Events help track actions and communicate with users or other programs.

Practice

1. What is the main purpose of emitting events in a blockchain smart contract?

easy

A. To send signals about important actions to external listeners

B. To store large amounts of data permanently on the blockchain

C. To execute complex calculations inside the contract

D. To create new tokens automatically

Solution

1. Step 1: Understand event purpose

   Events are used to notify external systems about contract actions without heavy storage.

2. Step 2: Compare options

   Only To send signals about important actions to external listeners describes sending signals to outside listeners, which matches event use.

3. Final Answer:

   To send signals about important actions to external listeners -> Option A

4. Quick Check:

   Events notify external listeners = D [OK]

Hint: Events notify outside world, not store big data [OK]

Common Mistakes:

• Thinking events store large data permanently
• Confusing events with token creation
• Believing events run complex calculations

2. Which of the following is the correct syntax to emit an event named Transfer with parameters from, to, and amount in Solidity?

easy

A. event Transfer(from, to, amount);

B. Transfer.emit(from, to, amount);

C. emit Transfer(from, to, amount);

D. emit.Transfer(from, to, amount);

Solution

1. Step 1: Recall Solidity event emission syntax

   In Solidity, events are emitted using the keyword emit followed by the event name and parameters in parentheses.

2. Step 2: Check each option

   emit Transfer(from, to, amount); matches the correct syntax: emit Transfer(from, to, amount);. Others misuse dot notation or keywords.

3. Final Answer:

   emit Transfer(from, to, amount); -> Option C

4. Quick Check:

   emit keyword + event name + params = C [OK]

Hint: Use 'emit EventName(params);' exactly [OK]

Common Mistakes:

• Using dot notation like emit.Transfer()
• Writing event declaration instead of emission
• Placing emit after event name

3. Given the Solidity code snippet:

event Deposit(address indexed sender, uint amount);

function deposit() public payable {
    emit Deposit(msg.sender, msg.value);
}

What happens when the deposit function is called with 1 ether?

medium

A. The function throws an error due to missing return

B. The contract balance decreases by 1 ether

C. No event is emitted because emit keyword is missing

D. An event Deposit is emitted with sender's address and amount 1 ether

Solution

1. Step 1: Analyze the deposit function

   The function emits the Deposit event with the caller's address and the amount sent (msg.value).

2. Step 2: Understand event emission effect

   Calling deposit with 1 ether triggers the event with sender and 1 ether amount; contract balance increases, no error.

3. Final Answer:

   An event Deposit is emitted with sender's address and amount 1 ether -> Option D

4. Quick Check:

   emit Deposit with sender and value = A [OK]

Hint: emit sends event with current call data [OK]

Common Mistakes:

• Thinking contract balance decreases on deposit
• Missing that emit keyword is present
• Expecting return value in void function

4. Identify the error in this Solidity code snippet:

event Approval(address owner, address spender, uint amount);

function approve(address spender, uint amount) public {
    Approval(msg.sender, spender, amount);
}

medium

A. Missing the emit keyword before the event call

B. Event parameters are in wrong order

C. Function must be declared as view

D. Event name should be lowercase

Solution

1. Step 1: Check event emission syntax

   In Solidity, events must be emitted using the emit keyword before calling the event.

2. Step 2: Identify the error in code

   The code calls Approval(...) without emit, causing a syntax error.

3. Final Answer:

   Missing the emit keyword before the event call -> Option A

4. Quick Check:

   emit keyword required before event call = A [OK]

Hint: Always prefix event calls with 'emit' [OK]

Common Mistakes:

• Omitting emit keyword
• Assuming event names are case-insensitive
• Thinking function needs view modifier

5. You want to track token transfers in your smart contract and filter them by sender address efficiently. Which event declaration and emission pattern is best to achieve this?

event Transfer(address indexed from, address indexed to, uint amount);

function transfer(address to, uint amount) public {
    // transfer logic
    emit Transfer(msg.sender, to, amount);
}

hard

A. Do not use indexed to save gas, filter by reading all events externally

B. Use indexed keyword on sender and receiver to enable filtering in event logs

C. Emit event without parameters to reduce complexity

D. Store all transfers in contract storage instead of events

Solution

1. Step 1: Understand indexed keyword usage

   Indexed parameters in events allow efficient filtering by those fields in external tools.

2. Step 2: Evaluate options for tracking transfers

   Use indexed keyword on sender and receiver to enable filtering in event logs uses indexed on sender and receiver, enabling fast filtering by sender address.

3. Final Answer:

   Use indexed keyword on sender and receiver to enable filtering in event logs -> Option B

4. Quick Check:

   indexed parameters enable filtering = B [OK]

Hint: Use indexed for filterable event parameters [OK]

Common Mistakes:

• Avoiding indexed to save gas but losing filtering ability
• Emitting events without parameters
• Storing all data on-chain instead of events