Emitting events in Blockchain / Solidity - Mini Project: Build & Apply

Emitting Events in a Smart Contract

📖 Scenario: You are creating a simple smart contract on the blockchain that tracks when a user sends a greeting message. You want to record each greeting by emitting an event so that external apps can listen and react to these greetings.

🎯 Goal: Build a Solidity smart contract that emits an event every time a greeting is sent. This event should include the sender's address and the greeting message.

📋 What You'll Learn

Create an event called Greeted with parameters address sender and string message.

Create a public function called sendGreeting that takes a string memory message as input.

Emit the Greeted event inside the sendGreeting function with the sender's address and the message.

Test the contract by calling sendGreeting and ensure the event is emitted.

💡 Why This Matters

🌍 Real World

Events are used in blockchain apps to notify wallets, user interfaces, and other contracts about important actions like transfers, votes, or messages.

💼 Career

Blockchain developers must know how to emit and listen to events to build responsive decentralized applications (dApps) and smart contracts.

1

Create the smart contract and define the Greeted event

Write a Solidity contract named GreetingContract. Inside it, declare an event called Greeted that has two parameters: address sender and string message.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract GreetingContract {
    // Declare the Greeted event here
    
    // Your code here
}

Hint

Use the event keyword followed by the event name and parameters inside parentheses.

2

Add the sendGreeting function with a string memory message parameter

Inside the GreetingContract, add a public function called sendGreeting that takes one parameter: string memory message. The function should be public and payable is not needed.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract GreetingContract {
    event Greeted(address sender, string message);

    // Add the sendGreeting function here
    
    // Your code here
}

Hint

Define the function with the correct name, parameter type, and visibility.

3

Emit the Greeted event inside sendGreeting

Inside the sendGreeting function, emit the Greeted event. Pass msg.sender as the sender and the message parameter as the message.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract GreetingContract {
    event Greeted(address sender, string message);

    function sendGreeting(string memory message) public {
        // Emit the Greeted event here
        
        // Your code here
    }
}

Hint

Use the emit keyword followed by the event name and parameters.

4

Test by calling sendGreeting and observe the event

Add a comment line that shows how to call sendGreeting with the message "Hello, blockchain!". Then, print a confirmation string "Greeting sent!" using console.log (assume a testing environment that supports it).

Blockchain / Solidity

pragma solidity ^0.8.0;

contract GreetingContract {
    event Greeted(address sender, string message);

    function sendGreeting(string memory message) public {
        emit Greeted(msg.sender, message);
    }
}

// Call sendGreeting with "Hello, blockchain!"
// Print "Greeting sent!" below

// Your code here

Hint

Solidity contracts do not print output. Use comments to show the call and a confirmation message.

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