Advanced features

Methods in events

For some use cases it could be helpful to be able to call a method when a certain event occurs. One use case is recording all erc20 Transfer events, and wanting to parse the value using the decimals. We don't know the decimals in advance, so we can call decimals() at the time the event occurs. This is the syntax:

query eth_transfers {
  chain = "ethereum"

  event Transfer {
    abi = "erc20.abi.json"

    outputs = ["from", "to", "value"]

    // Will automatically know to call the method on the
    // event address
    method decimals {
      outputs = ["decimals"]
    }

    transform {
      value = parse_decimals(value, decimals)
    }
  }

  save {
    ...
  }

note

Since decimals stay the same per token, the result is cached per token, resulting in a lot less calls for schema's like this.

Loops

Sometimes, you will want to execute a query for a lot of different addresses or chains. One example could be to listen for Transfer events on a couple stablecoins, like USDT, USDC and DAI. Another example could be cross-chain queries, where you want to run the same query on a different chain with a different smart contract address. For this, apollo has loops. The loop block is a top-level element, and always requires an items property. This property will contain the elements you want to loop over. Finally you define your query, with access to each item element. Let's look at an example.

Imagine you want to see the mid prices of both the Sushiswap USDC-ETH pool on Arbitrum, and the Uniswap USDC-ETH pool on Ethereum. The variables are thus the chain and the address, which we can store in the loop items. We can then define our query, and the loop elements will be found in item. Make sure to save item.chain, otherwise you won't be able to differentiate the output.

loop {
  items = [
    {chain = "arbitrum", address = "0x905dfCD5649217c42684f23958568e533C711Aa3"},
    {chain = "ethereum", address = "0xB4e16d0168e52d35CaCD2c6185b44281Ec28C9Dc"},
  ]

  query usdc_eth_reserves {
    // Get the element chain
    chain = item.chain

    contract {
      // Get the element address
      address = item.address
      abi = "unipair.abi.json"

      // Call getReserves
      method getReserves {
        outputs = ["_reserve0", "_reserve1"]
      }


      transform {
        eth_reserve = parse_decimals(_reserve0, 18)
        usdc_reserve = parse_decimals(_reserve1, 6)
      }
    }

    save {
      block = blocknumber
      timestamp = timestamp
      // Save chain to differentiate outputs
      chain = item.chain
      usdc_reserve = usdc_reserve
      eth_reserve = eth_reserve
      // Calculate mid price
      mid_price = usdc_reserve / eth_reserve
    }
  }
}

Custom Helper Functions

For some very common operations, apollo has helper functions. For now, these are limited to balance, for getting native balances, and token_balance, for getting ERC20 token balances. See some examples here