Installation

def deps do
  [{:exw3, "~> 0.3.0"}]
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 of this library 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}

Asynchronous

ExW3 provides async versions of call and send. They both return a Task that can be awaited on.

t = ExW3.Contract.call_async(:SimpleStorage, :get)
{:ok, data} = Task.await(t)

Events

ExW3 allows the retrieval of event logs using filters or transaction receipts. In this example we will demonstrate a filter. Assume we have already deployed and registered a contract called EventTester.

# We can optionally specify extra parameters like `:fromBlock`, and `:toBlock`
{:ok, filter_id} = ExW3.Contract.filter(:EventTester, "Simple", %{fromBlock: 42, toBlock: "latest"})

# After some point that we think there are some new changes
{:ok, changes} = ExW3.Contract.get_filter_changes(filter_id)

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

Indexed Events

Ethereum allows a user to add topics to filters. This means the filter will only return events with the specific index parameters. For all of the extra options see here

If you have written your event in Solidity like this:

event SimpleIndex(uint256 indexed num, bytes32 indexed data, uint256 otherNum);

You can add a filter on which logs will be returned back to the RPC client based on the indexed fields.

ExW3 allows for 2 ways of specifying these parameters (:topics) in two ways. The first, and probably more preferred way, is with a map:

indexed_filter_id = ExW3.Contract.filter(
  :EventTester,
  "SimpleIndex",
  %{
    topics: %{num: 46, data: "Hello, World!"},
  }
)

The other option is a list (mapped version is an abstraction over this). The downside here is this is order dependent. Any values you don't want to specify must be represented with a nil. This approach has been included because it is the implementation of the JSON RPC spec.

indexed_filter_id = ExW3.Contract.filter(
  :EventTester,
  "SimpleIndex",
  %{
    topics: [nil, "Hello, World!"]
  }
)

Here we are skipping the num topic, and only filtering on the data parameter.

NOTE!!! These two approaches are mutually exclusive, and for almost all cases you should prefer the map.

Continuous Event Handling

In many cases, you will want some process to continuously listen for events. We can implement this functionality using a recursive function. Since Elixir uses tail call optimization, we won't have to worry about blowing up the stack.

def listen_for_event do
  {:ok, changes} = ExW3.Contract.get_filter_changes(filter_id) # Get our changes from the blockchain
  handle_changes(changes) # Some function to deal with the data. Good place to use pattern matching.
  :timer.sleep(1000) # Some delay in milliseconds. Recommended to save bandwidth, and not spam.
  listen_for_event() # Recurse
end

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

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