Event-driven architecture patterns in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this event handler code snippet?

Consider a blockchain smart contract that emits an event when a token is transferred. What will be the output of the following event listener code when a transfer event is emitted?

Blockchain / Solidity

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

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

// Event listener pseudocode
Token.on('Transfer', (from, to, amount) => {
  console.log(`Transfer from ${from} to ${to} of ${amount} tokens`);
});

// Simulate transfer
Token.transfer('0xABC', 100);

A Transfer from 0xSender to 0xABC of 100 tokens

B Transfer from 0xABC to 0xSender of 100 tokens

C Transfer from 0xSender to 0xABC of 0 tokens

D No output, event listener not triggered

💡 Hint

Remember that the 'from' address is the sender of the tokens, usually msg.sender.

The event emits the sender's address as 'from', the recipient as 'to', and the amount transferred. The listener logs these values accordingly.

🧠 Conceptual

intermediate

Which pattern best describes decoupling components in blockchain event-driven systems?

In blockchain event-driven architecture, which pattern helps to decouple the smart contract logic from off-chain services by using events?

A Factory pattern

B Singleton pattern

C Observer pattern

D Decorator pattern

💡 Hint

Think about how listeners subscribe to events emitted by contracts.

The Observer pattern allows off-chain services to listen to events emitted by smart contracts, enabling decoupling.

🔧 Debug

advanced

Identify the error in this event subscription code

This code subscribes to a blockchain event but does not receive any events. What is the error?

Blockchain / Solidity

contract Token {
  event Approval(address indexed owner, address indexed spender, uint value);
}

// Subscription code
Token.on('approval', (owner, spender, value) => {
  console.log(`Approved ${value} tokens from ${owner} to ${spender}`);
});

A Event name is case-sensitive; should be 'Approval' not 'approval'

B Callback function parameters are in wrong order

C Missing await keyword before Token.on

D Event cannot have indexed parameters

💡 Hint

Check the exact event name spelling and casing.

Event names in blockchain are case-sensitive. Using 'approval' instead of 'Approval' means no events are caught.

📝 Syntax

advanced

Which option correctly defines an event with multiple indexed parameters in Solidity?

Select the correct Solidity event declaration with two indexed parameters.

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

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

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

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

💡 Hint

Indexed keyword comes immediately after the type.

In Solidity, 'indexed' must come after the type and before the parameter name.

🚀 Application

expert

How many events will be processed by the listener in this scenario?

A smart contract emits an event 'DataUpdated' every time data changes. The listener subscribes once and processes events from block 100 to 105. The contract emitted 'DataUpdated' events at blocks 101, 103, 103, 104, and 106. How many events will the listener process?

A 6

B 5

C 3

D 4

💡 Hint

Count only events emitted between blocks 100 and 105 inclusive.

Events at blocks 101, 103 (two events), and 104 are within the range, totaling 4 events. The event at block 106 is outside the range.