Chainlink – How to Return an Array or List of Strings from Functions-request-source.js

byteschainlinkfunctionstring

Hope someone can help.
I am stuck trying to work out how best to extract the different string values from the returned bytes data.
Chainlink Function request source requires the return value to be a buffer so I am including all the values required as per an example I found below from https://www.usechainlinkfunctions.com/ :

// Encode and return the distance (in meters), standard duration (in seconds), and duration in traffic (in seconds) as a string which can be parsed and split
return Functions.encodeString(`${distance},${stdDuration},${duration_in_traffic}`)

I am stuck trying to split the return values in the fulfillRequest function in the FunctionsConsumer contract. Below is the standard function before implementing anything to split out a number of string variables from the response:

function fulfillRequest(
    bytes32 requestId,
    bytes memory response,
    bytes memory err
) internal override {
    latestResponse = response;
    latestError = err;
    emit OCRResponse(requestId, response, err);
}

Here is the github repo link for chainlink functions:
https://github.com/smartcontractkit/functions-hardhat-starter-kit/tree/main#functions-library

Any help is greatly appreciated. Thank you.

Best Answer

since you're returning a string, parsing that string and splitting it would be very expensive. Instead you could return with a buffer.concat to concat your distance, std duration and duration in traffice as uint256 values (assuming they can ONLY be positive integers) and int256 if they can be negative.

Chainlink Functions code with Buffer.Concat example

Then in your Functions Consumer Smart Contract you use abi.decode with the exact int types. See this code example from the same repo

This way you get individual values and dont have to split them on chain.

Hope this helps!

Related Solutions

[Ethereum] How to return multiple strings from a contract function

There are a few types of string values that you may be referring to.

bytes1 - bytes32: fixed size.
bytes or string: dynamically sized.

Solidity supports functions with multiple return values.

contract MultiReturner {
    function getData() constant returns (bytes32, bytes32) {
        bytes32 a = "abcd";
        bytes32 b = "wxyz";
        return (a, b);
    }

    function getDynamicData() constant returns (bytes, bytes) {
        bytes a;
        a.push('a');
        a.push('b');
        a.push('c');
        bytes b;
        b.push('x');
        b.push('y');
        b.push('z');
        return (a, b);
    }
}

You can do the same with bytes or string but with the limitation that solidity does not support returning dynamically sized values to other functions. This means that you would be able to use the getData function from within another contract and retrieve the return values but you would not be able to retrieve the return values from getDynamicData from within a contract.

contract UsesMultiReturner {
    function doIt() {
        mr = MultiReturner(0x1234);

        // this will work
        var (a, b) = mr.getData();

        // this won't work
        var (a, b) = mr.getDynamicData();
    }
}

You can however retrieve the return values from both getData and getDynamicData when call from outside of the blockchain.

[Ethereum] Array of Strings in Solidity

Life is easier if you can assume 32 bytes. If you are able to restrict the multihash formats you accept or assume sha2-256, which is common now, you could just drop some meta-data so that they fit in 32 bytes. To convert to a bytes32, base-58-decode the data, then strip what should be 0x1220 off the front. The 12 indicates the format (sha2-256), and the 20 is the length of the data, ie 32 bytes.

If you want to handle all the legal multihash formats properly, you could concatenate them together as a single byte array, then manage a separate array holding the start offset of each entry.

Best Answer

Related Solutions

[Ethereum] How to return multiple strings from a contract function

[Ethereum] Array of Strings in Solidity

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