Event filtering in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is event filtering in blockchain?

Event filtering means selecting only specific events from the blockchain that match certain rules, so you don't have to look at all events.

beginner

Why do we use event filtering in blockchain applications?

We use event filtering to find only the events we care about, which saves time and computing power because we don't process unnecessary data.

intermediate

How does an event filter work with topics in Ethereum logs?

An event filter uses topics, which are like labels or tags on events, to match and find events that have specific values in those topics.

intermediate

What is the difference between filtering by address and filtering by topics?

Filtering by address means selecting events from a specific contract, while filtering by topics means selecting events based on event parameters or types.

beginner

Give an example of a simple event filter in Ethereum using web3.js.

Example: <br>const filter = {<br>  address: '0x123...',<br>  topics: ['0xabc...']<br>};<br>web3.eth.getPastLogs(filter).then(console.log);<br>This finds past events from the contract at '0x123...' with the first topic '0xabc...'.

What does event filtering help you do in blockchain?

A Delete old blockchain events

B Select only events that match certain criteria

C Create new blockchain events

D Encrypt blockchain events

Event filtering is used to select only the events that match specific rules or criteria.

In Ethereum, what are 'topics' used for in event filtering?

A They label events to help filter by event parameters

B They store the event's timestamp

C They encrypt event data

D They identify the miner of the block

Topics are used to label events and help filter events by their parameters or types.

Which filter option would you use to get events only from a specific smart contract?

A transactionHash

B topics

C blockNumber

D address

The 'address' filter selects events from a specific contract address.

What happens if you do not use event filtering when searching blockchain logs?

A You get all events, which can be slow and large

B You get no events at all

C You only get events from the latest block

D You get encrypted events

Without filtering, you receive all events, which can be very slow and use a lot of resources.

Which method in web3.js is commonly used to get past events with filters?

A subscribe

B sendTransaction

C getPastLogs

D getBlock

The getPastLogs method retrieves past events using filters.

Explain how event filtering improves blockchain application performance.

Think about why you wouldn't want to look at every event.

Describe how you would create a filter to find events from a specific contract with a certain event type.

Remember filters can have multiple parts like address and topics.

Practice

1.

What is the main purpose of event filtering in blockchain applications?

easy

A. To encrypt blockchain data

B. To create new blockchain events

C. To mine new blocks faster

D. To listen only to specific events that match certain criteria

Solution

1. Step 1: Understand event filtering concept

   Event filtering allows you to listen only to events that match specific parameters or conditions.

2. Step 2: Identify the main purpose

   This helps reduce unnecessary data processing and improves app performance by focusing on relevant events.

3. Final Answer:

   To listen only to specific events that match certain criteria -> Option D

4. Quick Check:

   Event filtering = selective listening [OK]

Hint: Event filtering means listening only to events you want [OK]

Common Mistakes:

• Confusing event filtering with event creation
• Thinking event filtering speeds up mining
• Believing event filtering encrypts data

2.

Which of the following is the correct syntax to filter events by a parameter named user in a blockchain event listener?

contract.events.Transfer({ filter: { ??? } })

easy

A. { user: '0x123...' }

B. { 'user' }

C. [ user ]

D. user = '0x123...'

Solution

1. Step 1: Understand filter syntax

   Filters use an object with key-value pairs where keys are parameter names and values are filter values.

2. Step 2: Match correct syntax

   The correct syntax is an object with the parameter name as key and the desired value, like { user: '0x123...' }.

3. Final Answer:

   { user: '0x123...' } -> Option A

4. Quick Check:

   Filter = object with key-value pairs [OK]

Hint: Filters use {paramName: value} object syntax [OK]

Common Mistakes:

• Using array or string instead of object for filters
• Writing filter as assignment instead of object
• Omitting quotes around string values

3.

Given the following code snippet, what events will be received?

contract.events.Transfer({ filter: { from: '0xabc...' }, fromBlock: 100, toBlock: 200 })

Assuming events with from address '0xabc...' occurred at blocks 90, 150, and 210.

medium

A. Only event at block 150

B. Events at blocks 90, 150, and 210

C. Events at blocks 90 and 150

D. No events

Solution

1. Step 1: Analyze filter parameters

   The filter listens for events where 'from' equals '0xabc...'. The block range is from 100 to 200.

2. Step 2: Check event blocks against filter

   Events at blocks 90 and 210 are outside the block range, so only the event at block 150 matches both filter and block range.

3. Final Answer:

   Only event at block 150 -> Option A

4. Quick Check:

   Filter + block range = event at block 150 only [OK]

Hint: Events must match filter and block range [OK]

Common Mistakes:

• Ignoring block range and including all matching events
• Including events outside the block range
• Confusing filter parameter with block number

4.

Identify the error in this event filter code snippet:

contract.events.Approval({ filter: { owner: 0xabc123 }, fromBlock: 'latest' })

medium

A. Filter object should be an array

B. The filter value 0xabc123 should be a string

C. fromBlock cannot be 'latest'

D. The event name 'Approval' is incorrect

Solution

1. Step 1: Check filter parameter types

   Filter values must be strings for addresses, but 0xabc123 is missing quotes, so it's treated as a variable or number.

2. Step 2: Validate other parts

   The event name and fromBlock usage are valid; fromBlock can be 'latest'. Filter should be an object, not an array.

3. Final Answer:

   The filter value 0xabc123 should be a string -> Option B

4. Quick Check:

   Address filter values must be strings [OK]

Hint: Address filters need quotes around them [OK]

Common Mistakes:

• Omitting quotes around address strings
• Thinking fromBlock 'latest' is invalid
• Using array instead of object for filter

5.

You want to listen to Transfer events where the value is greater than 1000 tokens. Which approach correctly filters these events?

// Assume event signature: Transfer(address from, address to, uint256 value)

hard

A. Use filter: { value: '>1000' }

B. Use filter: { value: val => val > 1000 }

C. Filter events after receiving all by checking value > 1000 in code

D. Set fromBlock and toBlock to limit event range

Solution

1. Step 1: Understand filter limitations

   Event filters only support exact matches or arrays of values, not comparison operators like greater than.

2. Step 2: Choose correct approach

   To filter by value > 1000, you must receive events first and then check the value in your code logic.

3. Final Answer:

   Filter events after receiving all by checking value > 1000 in code -> Option C

4. Quick Check:

   Filters do exact matches; use code for range checks [OK]

Hint: Filters match exact values; use code for greater than checks [OK]

Common Mistakes:

• Trying to use comparison operators in filter
• Assuming string '>1000' works as filter
• Confusing block range with value filtering