# EthEvent
[](https://travis-ci.org/etherharvest/eth_event) [](https://hex.pm/packages/eth_event) [](https://hex.pm/packages/eth_event)
`EthEvent` is a library to declare and request Solidity events easily.
## Queries
This project provides a simple way of querying events and other information
from the Ethereum blockchain.
### Query Solidity events
Solidity events are no more than encoded block logs. They are usually used to
record that something changed in the contract's state. The following contract
declares and `emit`s an event every time someone sends some tokens to other
address:
```solidity
contract SomeToken is ERC20 {
Transfer(address indexed from, address indexed to, uint value);
...
function transfer(address to, uint value) {
...
emit Transfer(msg.sender, to, value)
}
}
```
Using the `EthEvent.Schema` behaviour you can query the events in the form of
Elixir structs. The `Transfer` event defined in the previous contract, could be
defined as follows:
```
defmodule Transfer do
use EthEvent.Schema
event "Transfer" do
address :from, indexed: true
address :to, indexed: true
uint :value
end
end
```
The previously defined module includes the functions `Transfer.query/1`,
`Transfer.query/2` and a struct definition for the `Transfer` event e.g:
Let's say we want to look for:
- The `Transfer` event in the contract of address
`0xd09de8b6b510aecd508a22811398f468e75c8c4d`.
- Where the tokens where transferred `from` the wallet address
`0x93ecb3962981e1ba2928297cb09c1932aa2c9c51`.
- And only between the blocks `0` and `100`.
We would do the following:
```
> contract_address = "0xd09de8b6b510aecd508a22811398f468e75c8c4d"
> from = "0x93ecb3962981e1ba2928297cb09c1932aa2c9c51"
> query = %Transfer{address: contract_address, from: from}
> options = [from_block: 0, to_block: 100]
> Transfer.query(query, options)
{:ok,
[
%Transfer{
...
address: "0xd09de8b6b510aecd508a22811398f468e75c8c4d",
block_number: 42,
from: "0x93ecb3962981e1ba2928297cb09c1932aa2c9c51",
to: "0x1e529de18f95ad5a4f41ac5e159fa307d5a85967",
value: 100
}
]
}
```
The result is the list of events that match the query.
### Query block number
It is possible to query the block number as well using the function
`Block.query/1` e.g:
```
> use EthEvent
> Block.query(%Block{block_number: "latest"})
{:ok,
%Block{
...
block_number: 42,
timestamp: #DateTime<...>
}
}
```
### Query balance
Similarly to the two previous sections, balances are available as well using
the function `query/1` e.g:
```
> use EthEvent
> address = "0xd09de8b6b510aecd508a22811398f468e75c8c4d"
> Balance.query(%Balance{address: address, block_number: "latest"})
{:ok,
%Balance{
address: "0xd09de8b6b510aecd508a22811398f468e75c8c4d"
balance: 100
}
}
```
The `balance` field is in _wei_.
### Query composability
One important thing is that events are composable, though they only preserve
some fields when composed e.g:
To query the balance of the wallet address
`0xd09de8b6b510aecd508a22811398f468e75c8c4d` for the block 42 there are two
ways of doing it:
Without block hash or block_number:
```
> address = "0xd09de8b6b510aecd508a22811398f468e75c8c4d"
> Balance.query(%Balance{address: address})
{:ok,
%Balance{
address: "0xd09de8b6b510aecd508a22811398f468e75c8c4d"
block_number: nil,
block_hash: nil,
balance: 100
}
}
```
or with block hash:
```
> address = "0xd09de8b6b510aecd508a22811398f468e75c8c4d"
> Block.query!(block_number: 42) |> Balance.query(address: address)
{:ok,
%Balance{
address: "0xd09de8b6b510aecd508a22811398f468e75c8c4d"
block_number: 42,
block_hash: "0x15feeab052b4bd65c8e3a2e3efab391debb9d8b5def6ced89ea772...",
balance: 100
}
}
```
## Installation
To install just add the following to your dependencies:
```elixir
def deps do
[
{:eth_event, "~> 0.1"}
]
end
```
