Emitting events in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Emitting events

Start Transaction

↓

Call Function

↓

Emit Event

↓

Log Event Data

↓

Transaction Ends

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Event Available for Listeners

When a function runs, it can emit an event that logs data. This event is stored in the blockchain logs and can be read by apps.

Execution Sample

Blockchain / Solidity

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

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

This code defines an event and emits it inside a function to log a transfer.

Execution Table

Step	Action	Evaluation	Result
1	Start transaction	N/A	Transaction begins
2	Call send() with to=0xABC, amount=100	Check inputs	Inputs valid
3	Emit Sent event	Create event log with from=caller, to=0xABC, amount=100	Event logged on blockchain
4	End transaction	N/A	Transaction confirmed
5	Event available	Listeners can read event data	Event data accessible

💡 Transaction ends after event is emitted and logged

Variable Tracker

Variable	Start	After emit	Final
msg.sender	caller address	caller address	caller address
to	input address	input address	input address
amount	input uint	input uint	input uint
Event Sent	not emitted	emitted with data	emitted with data

Key Moments - 3 Insights

Why do we use 'emit' before the event name?

The 'emit' keyword tells the blockchain to create a log entry for the event. Without 'emit', the event won't be recorded. See execution_table step 3.

Does emitting an event change contract state?

No, emitting an event only logs data. It does not modify variables or contract storage. This is shown in variable_tracker where variables stay the same.

Can external apps read emitted events?

Yes, events are stored in transaction logs and external apps can listen and react to them. See execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 3?

A The event is emitted and logged on the blockchain

B The transaction ends without logging

C The function inputs are checked

D The event is removed

💡 Hint

Check execution_table row with Step 3 describing event emission

According to variable_tracker, what is the state of 'amount' after emitting the event?

A Changed to zero

B Remains the input value

C Undefined

D Incremented by one

💡 Hint

Look at 'amount' row in variable_tracker after emit column

If the 'emit' keyword is removed, what changes in the execution flow?

A Event logs twice

B Transaction will fail immediately

C Event will not be logged, so no event data is available

D Function inputs become invalid

💡 Hint

Refer to key_moments about the role of 'emit' and execution_table step 3

Concept Snapshot

Syntax: event EventName(params);
Use 'emit EventName(args);' inside functions.
Emitting events logs data on blockchain.
Events do not change contract state.
External apps listen to events for updates.

Full Transcript

Emitting events in blockchain means creating a log entry during a transaction. When a function runs, it can emit an event using the 'emit' keyword followed by the event name and data. This event is stored in the blockchain logs and can be read by external apps or listeners. Emitting events does not change the contract's variables or state; it only records information. The transaction continues and ends after the event is logged. This helps apps track what happened during transactions without changing the blockchain state.