Emitting events in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of emitting events in blockchain smart contracts?

Emitting events allows smart contracts to log information that external applications can listen to and react upon. It helps track contract activity without storing data on-chain.

beginner

How do you declare an event in Solidity?

You declare an event using the event keyword followed by the event name and parameters, for example: event Transfer(address indexed from, address indexed to, uint256 amount);

intermediate

What does the indexed keyword do in event parameters?

The indexed keyword marks parameters that can be used as filters when searching for events. Up to 3 parameters can be indexed to make event queries efficient.

beginner

How do you emit an event inside a Solidity function?

You use the emit keyword followed by the event name and arguments, like: emit Transfer(msg.sender, recipient, amount);

intermediate

Why are events cheaper than storing data on the blockchain?

Events store data in transaction logs, which are cheaper to write and do not consume as much gas as storing data in contract storage. However, event data is not accessible by contracts, only by external listeners.

Which keyword is used to declare an event in Solidity?

A emit

B event

C log

D notify

The event keyword declares an event. The emit keyword is used to trigger it.

What does the emit keyword do in Solidity?

A Declares a new event

B Filters events

C Deletes an event

D Triggers an event to be logged

The emit keyword triggers an event so it is recorded in the transaction logs.

How many parameters can be indexed in a Solidity event for filtering?

A 3

B 2

C 1

D Unlimited

Up to 3 parameters can be marked indexed to allow efficient filtering.

Where is event data stored in the blockchain?

A In transaction logs

B In contract storage

C In the blockchain header

D In the contract code

Event data is stored in transaction logs, which are cheaper and accessible off-chain.

Can smart contracts read event data after it is emitted?

A Yes, directly

B Only if indexed

C No, only external apps can read it

D Yes, but only in the same transaction

Contracts cannot access event data; only external applications can listen and read events.

Explain how emitting events helps external applications interact with smart contracts.

Think about how apps watch for changes without reading contract storage.

Describe the syntax and usage of declaring and emitting an event in Solidity.

Focus on how to define and then trigger an event.

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