# 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.
# We can also optionally specify extra parameters like `:fromBlock`, and `:toBlock`
# 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
# After some point that we think there are some new changes
# 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.
ExW3.Contract.uninstall_filter(filter_id)
```
## Listening for Indexed Events
## Indexed Events
Ethereum allows for filtering events specific to its parameters using indexing. For all of the options see [here](https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_newfilter)
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](https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_newfilter)
If you have written your event in Solidity like this:
You can add 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 or `topics` in two ways. The first, and probably more preferred way, is with a map:
You can add 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 or `topics` in two ways. The first, and probably more preferred way, is with a map:
```elixir
indexed_filter_id = ExW3.Contract.filter(
:EventTester,
"SimpleIndex",
self(),
%{
topics: %{num: 46, data: "Hello, World!"},
}
@ -171,7 +141,6 @@ The other option is with a list, but this is order dependent, and any values you
hswick
6 years ago