Event-driven architecture patterns in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to emit an event named 'Transfer' in a smart contract.

Blockchain / Solidity

emit [1](msg.sender, recipient, amount);

Drag options to blanks, or click blank then click option'

ATransfer

BSend

CNotify

DEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong event name causes a compilation error.

The event name must match the declared event 'Transfer' to emit it correctly.

2fill in blank

medium

Complete the code to listen for the 'Approval' event in a blockchain client.

Blockchain / Solidity

contractInstance.events.[1]({}, (error, event) => { console.log(event); });

Drag options to blanks, or click blank then click option'

ADeposit

BTransfer

CApproval

DWithdraw

Attempts:

3 left

💡 Hint

Common Mistakes

Listening to a wrong event name results in no events captured.

The event listener must use the exact event name 'Approval' to capture those events.

3fill in blank

hard

Fix the error in the event filter to only get events where 'from' equals the sender address.

Blockchain / Solidity

contractInstance.getPastEvents('Transfer', { filter: { [1]: senderAddress } });

Drag options to blanks, or click blank then click option'

Aamount

Bfrom

Cto

Drecipient

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'to' or 'recipient' filters the wrong address field.

The filter key must be 'from' to filter events by sender address.

4fill in blank

hard

Fill both blanks to create a subscription that listens for 'Deposit' events and logs the amount.

Blockchain / Solidity

subscription = contract.events.[1]().on('[2]', (event) => { console.log(event.returnValues.amount); });

Drag options to blanks, or click blank then click option'

ADeposit

BTransfer

Cdata

Dchanged

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong event name or event type causes no logs.

The subscription listens for 'Deposit' events and the event type to listen for is 'data' to receive event data.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps event names to their handler functions if the event name starts with 'On'.

Blockchain / Solidity

handlers = { [1]: [2] for [3] in eventList if [1].startswith('On') }

Drag options to blanks, or click blank then click option'

Aevent

Bhandle_event

Dhandler

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing variable names or using wrong function names.

The comprehension uses 'event' as key and 'handle_event' as value for each 'event' in eventList starting with 'On'.

Practice

1. What is the main purpose of event-driven architecture in blockchain applications?

easy

A. To allow smart contracts to emit events that trigger automatic responses

B. To store large amounts of data on the blockchain

C. To speed up the mining process

D. To encrypt transactions for security

Solution

1. Step 1: Understand event-driven architecture basics

   Event-driven architecture means systems react automatically to events or actions.

2. Step 2: Apply this to blockchain context

   In blockchain, smart contracts emit events that apps listen to and respond automatically.

3. Final Answer:

   To allow smart contracts to emit events that trigger automatic responses -> Option A

4. Quick Check:

   Event-driven architecture = automatic reactions to events [OK]

Hint: Event-driven means automatic reaction to smart contract events [OK]

Common Mistakes:

• Confusing event-driven with data storage
• Thinking it speeds up mining
• Mixing encryption with event handling

2. Which of the following is the correct syntax to listen for an event named Transfer in a blockchain app using JavaScript?

easy

A. contract.listen('Transfer', function(from, to, amount) {});

B. contract.emit('Transfer', (from, to, amount) => {});

C. contract.on('Transfer', (from, to, amount) => { /* handle event */ });

D. contract.catch('Transfer', (from, to, amount) => {});

Solution

1. Step 1: Identify correct event listening method

   In JavaScript blockchain libraries, on is used to listen for events.

2. Step 2: Check syntax for event callback

   The callback receives event parameters like from, to, and amount.

3. Final Answer:

   contract.on('Transfer', (from, to, amount) => { /* handle event */ }); -> Option C

4. Quick Check:

   Use on to listen for events [OK]

Hint: Use .on() method to listen for blockchain events in JS [OK]

Common Mistakes:

• Using .emit() instead of .on() to listen
• Using .listen() which is invalid
• Using .catch() which is for errors

3. Given this Solidity event and JavaScript listener:

event Deposit(address indexed user, uint amount);
contract.on('Deposit', (user, amount) => { console.log(amount.toString()); });

What will be printed if the event is emitted with amount = 500?

medium

A. 0x1f4

B. 500

C. undefined

D. Error: amount.toString is not a function

Solution

1. Step 1: Understand event parameters

   The event emits amount as a number (uint) which is received in JS as a BigNumber or number.

2. Step 2: Check the listener code

   The listener calls amount.toString() which converts the number 500 to string "500".

3. Final Answer:

   500 -> Option B

4. Quick Check:

   amount.toString() converts number 500 to string '500' [OK]

Hint: toString() converts number to string for console output [OK]

Common Mistakes:

• Thinking amount prints as hex
• Assuming amount is undefined
• Confusing toString() usage

4. Identify the error in this JavaScript event listener code for a blockchain contract:

contract.on('Approval', (owner, spender, value) => {
  console.log('Approved:', owner, spender, value.toString);
});

medium

A. contract.on is not a function

B. Wrong event name 'Approval'

C. Callback parameters are incorrect

D. Missing parentheses after value.toString

Solution

1. Step 1: Check usage of toString method

   The code uses value.toString without parentheses, so it logs the function reference, not the string.

2. Step 2: Correct usage requires parentheses

   It should be value.toString() to convert the value to a string for logging.

3. Final Answer:

   Missing parentheses after value.toString -> Option D

4. Quick Check:

   toString() needs () to call method [OK]

Hint: Call toString() with parentheses to convert value [OK]

Common Mistakes:

• Forgetting () after toString
• Assuming event name is wrong
• Misunderstanding callback parameters

5. You want to build a blockchain app that updates user balances automatically when a BalanceUpdated event is emitted by a smart contract. Which event-driven pattern best fits this use case?

hard

A. Event Listener pattern to react and update balances on event emission

B. Command Query Responsibility Segregation (CQRS) to separate reads and writes

C. Event Sourcing to store all balance changes as events

D. Batch Processing to update balances periodically

Solution

1. Step 1: Understand the requirement

   The app must update balances automatically when an event occurs.

2. Step 2: Match the pattern to the need

   The Event Listener pattern listens for events and triggers immediate updates, fitting this use case perfectly.

3. Final Answer:

   Event Listener pattern to react and update balances on event emission -> Option A

4. Quick Check:

   Use Event Listener for automatic reaction to events [OK]

Hint: Use Event Listener to react instantly to blockchain events [OK]

Common Mistakes:

• Confusing Event Sourcing with immediate updates
• Choosing CQRS which separates reads/writes but not event reaction
• Using Batch Processing which delays updates