Recover solidity funcion event in web3j

Question

EthFilter filter = new EthFilter(DefaultBlockParameterName.EARLIEST, DefaultBlockParameterName.LATEST, userContract.getContractAddress().substring(2));

I am trying to recover the event response that a solidity smart-contract return me. Searching in the web I find here post. And works, but the problem is that I dont now what method I have to call to return just the string that de contract send me.

Something like:

System.out.println(filter.getMessage());

Answer 1

Alright, let's start from scratch.

Let's say you have a very simple contract which increments and decrements a counter deployed to the address CONTRACT_X

pragma solidity ^0.4.20;

contract Counter {
    int counter; // Global state

    event CounterIncremented(address indexed _by, int _newValue);
    event CounterDecremented(address indexed _by, int _newValue);

    function increment(int _value) public {
        counter = counter + _value;
        emit CounterIncremented(msg.sender, counter);
    }

    function deincrement(int _value) public {
        counter = counter - _value;
        emit CounterDecremented(msg.sender, counter);
    }
}

i. Event definition hash

First you have to define each event (definition) in order to calculate the unique hash that represent this event on the network.

// Definition of event CounterIncremented(address indexed _by, int _newValue)
public static final Event INCREMENT_EVENT = new Event("CounterIncremented", 
    Arrays.<TypeReference<?>>asList(new TypeReference<Address>(true) {}, new TypeReference<Int>(false) {}}));

// Definition of event CounterDecremented(address indexed _by, int _newValue)
public static final Event DECREMENT_EVENT = new Event("CounterDecremented", 
    Arrays.<TypeReference<?>>asList(new TypeReference<Address>(true) {}, new TypeReference<Int>(false) {}}));

Then to calculate the hash of the event, you only need to do:

private static final String INCREMENT_EVENT_HASH = EventEncoder.encode(INCREMENT_EVENT);

private static final String DECREMENT_EVENT_HASH = EventEncoder.encode(DECREMENT_EVENT);

ii. Filter

Before subscribing to event, it is possible apply filters because by definition, the Ethereum events log (all events of all smart contracts of the network) can be query.

• Filter by Block range and Smart Contract

Return all the events of different types for the smart contract CONTRACT_X. In our case, we will get CounterIncremented and CounterDecremented events.

EthFilter filter = new EthFilter(
    DefaultBlockParameterName.EARLIEST, // From block 0
    DefaultBlockParameterName.LATEST,  // To last block
    CONTRACT_X); // Unique Smart Contract

EthFilter filter = new EthFilter(
    DefaultBlockParameterName.LATEST, // only the latest block  
    DefaultBlockParameterName.LATEST,  // 
    CONTRACT_X); // Unique Smart Contract

EthFilter filter = new EthFilter(
    DefaultBlockParameterName.valueOf("2222"), // From block no 2222
    DefaultBlockParameterName.LATEST,  // To latest
    Arrays.asList(address1, address2, address3)); // List of Smart Contracts

LATEST means you will pull the events coming in during the subscription.

• Filter by events

If you want to filter by event type, for example to only get the CounterIncremented event, you can use the property addSingleTopic

EthFilter filter = new EthFilter(
    DefaultBlockParameterName.EARLIEST, // From block 0 
    DefaultBlockParameterName.LATEST,  // To latest
    CONTRACT_X) // Unique Smart Contract
.addSingleTopic(INCREMENT_EVENT_HASH);

• Filter by event parameters

Finally, you might want to filter by event parameters, for instance all the CounterIncremented events triggered by a given address (filter by parameter _by of the CounterIncremented event)

EthFilter filter = new EthFilter(
    DefaultBlockParameterName.EARLIEST, // From block 0
    DefaultBlockParameterName.LATEST,  // To latest
    CONTRACT_X) // Unique Smart Contract
.addSingleTopic(INCREMENT_EVENT_HASH)
.addOptionalTopic("0xdDd6427Aaf3766DC97A9cA9deDD3e7911b085B6B");

Event parameter filter only applied for 'indexed' parameters.

iii. Subscribe to the events

Finally, the last step consists in subscribing for events to the node:

web3.ethLogFlowable(filter).subscribe(event -> { 
    event.getAddress(); // Smart contract address
    event.getBlockNumber(); // Block number
    event.getTransactionHash(); // Transaction that emitted the event
    event.getTopics().get(0); // Event hash
    event.getTopics().get(1-n); // Event parameter (1) _by, (2) _value
});