Installation

def deps do
  [{:exw3, "~> 0.1.6"}]
end

Overview

ExW3 is a wrapper around ethereumex to provide a high level, user friendly json rpc api. It currently only supports Http. The primary feature it provides is a handy abstraction for working with smart contracts.

Usage

Ensure you have an ethereum node to connect to at the specified url in your config. An easy local testnet to use is ganache-cli:

ganache-cli

Or you can use parity: Install Parity, then run it with

echo > passfile
parity --chain dev --unlock=0x00a329c0648769a73afac7f9381e08fb43dbea72 --reseal-min-period 0 --password passfile

If Parity complains about password or missing account, try

parity --chain dev --unlock=0x00a329c0648769a73afac7f9381e08fb43dbea72

Make sure your config includes:

config :ethereumex,
  url: "http://localhost:8545"

Currently, ExW3 supports a handful of json rpc commands. Mostly just the useful ones. If it doesn't support a specific commands you can always use the Ethereumex commands.

Check out the documentation

iex(1)> accounts = ExW3.accounts()
["0x00a329c0648769a73afac7f9381e08fb43dbea72"]
iex(2)> ExW3.balance(Enum.at(accounts, 0))
1606938044258990275541962092341162602522200978938292835291376
iex(3)> ExW3.block_number()
1252
iex(4)> simple_storage_abi = ExW3.load_abi("test/examples/build/SimpleStorage.abi")
%{
  "get" => %{
    "constant" => true,
    "inputs" => [],
    "name" => "get",
    "outputs" => [%{"name" => "", "type" => "uint256"}],
    "payable" => false,
    "stateMutability" => "view",
    "type" => "function"
  },
  "set" => %{
    "constant" => false,
    "inputs" => [%{"name" => "_data", "type" => "uint256"}],
    "name" => "set",
    "outputs" => [],
    "payable" => false,
    "stateMutability" => "nonpayable",
    "type" => "function"
  }
}
iex(5)> ExW3.Contract.start_link
{:ok, #PID<0.265.0>}
iex(6)> ExW3.Contract.register(:SimpleStorage, abi: simple_storage_abi)
:ok
iex(7)> {:ok, address, tx_hash} = ExW3.Contract.deploy(:SimpleStorage, bin: ExW3.load_bin("test/examples/build/SimpleStorage.bin"), options: %{gas: 300_000, from: Enum.at(accounts, 0)})
{:ok, "0x22018c2bb98387a39e864cf784e76cb8971889a5",
 "0x4ea539048c01194476004ef69f407a10628bed64e88ee8f8b17b4d030d0e7cb7"}
iex(8)> ExW3.Contract.at(:SimpleStorage, address)
:ok
iex(9)> ExW3.Contract.call(:SimpleStorage, :get)
{:ok, 0}
iex(10)> ExW3.Contract.send(:SimpleStorage, :set, [1], %{from: Enum.at(accounts, 0), gas: 50_000})
{:ok, "0x88838e84a401a1d6162290a1a765507c4a83f5e050658a83992a912f42149ca5"}
iex(11)> ExW3.Contract.call(:SimpleStorage, :get)
{:ok, 1}

Listening for Events

Elixir doesn't have event listeners like say JS. However, we can simulate that behavior with message passing. The way ExW3 handles event filters is with a background process that calls eth_getFilterChanges every cycle. Whenever a change is detected it will send a message to whichever process is listening.

# Start the background listener
ExW3.EventListener.start_link

# Assuming we have already registered our contract called :EventTester
# We can then add a filter for the event listener to look out for by passing in the event name, and the process we want to receive the messages when an event is triggered.
# For now we are going to use the main process, however, we could pass in a pid of a different process.

filter_id = ExW3.Contract.filter(:EventTester, "Simple", self())

# We can then wait for the event. Using the typical receive keyword we wait for the first instance of the event, and then continue with the rest of the code. This is useful for testing.
receive do
  {:event, {filter_id, data}} -> IO.inspect data
end

# We can then uninstall the filter after we are done using it
ExW3.uninstall_filter(filter_id)

# ExW3 also provides a helper method to continuously listen for events, with the `listen` method.
# One use is to combine all of our filters with pattern matching
ExW3.EventListener.listen(fn result ->
  case result do
    {filter_id, data} -> IO.inspect data
    {filter_id2, data} -> IO.inspect data
  end
end

# The listen method is a simple receive loop waiting for `{:event, _}` messages.
# It looks like this:
def listen(callback) do
  receive do
    {:event, result} -> apply callback, [result]
  end
  listen(callback)
end

# You could do something similar with your own process, whether it is a simple Task or a more involved GenServer.

Compiling Solidity

To compile the test solidity contracts after making a change run this command:

solc --abi --bin --overwrite -o test/examples/build test/examples/contracts/*.sol