Add a "listen for event" function

[PatrickAlphaC]

Overview

This is something I would have liked to see some additional functionality in brownie from.

Specification

Maybe the API looks something like:

from ape import networks

event = networks.listen_for_event(contract_address, event_name, timeout=200)

I made a custom edition of this in my chainlink-mix would be great for ape to have this built-in.

[PatrickAlphaC]

Maybe contract_address is optional. This would be a blocking call, it might make sense to integrate with asyncio in the distant future. For now, having it be blocking is perfect I think.

[fubuloubu]

Perhaps not well documented, but we do have Contract Event polling for this purpose

[PatrickAlphaC]

It doesn't look like it actually listens for events tho? I don't see a way to get events from that.

I wrote a method in python for almost having a general enough func (takes a contracts and no event stuff).

Seems like this could be pretty easy to implement.

def wait_for_randomness(vrf_consumer, timeout=200, poll_interval=2):
    print("Waiting for random response...")
    start_time = time.time()
    current_time = time.time()
    while current_time - start_time < timeout:
        # Would swap this out for some event stuff
        response = vrf_consumer.random_words(0)
        if response > 0:
            return response
        time.sleep(poll_interval)
        current_time = time.time()
    print("Done waiting!")
    return None

[fubuloubu]

It doesn't look like it actually listens for events tho? I don't see a way to get events from that.

I wrote a method in python for almost having a general enough func (takes a contracts and no event stuff).

Seems like this could be pretty easy to implement.

def wait_for_randomness(vrf_consumer, timeout=200, poll_interval=2):
    print("Waiting for random response...")
    start_time = time.time()
    current_time = time.time()
    while current_time - start_time < timeout:
        # Would swap this out for some event stuff
        response = vrf_consumer.random_words(0)
        if response > 0:
            return response
        time.sleep(poll_interval)
        current_time = time.time()
    print("Done waiting!")
    return None

It calls out to BlockManager.poll_blocks which does a cyclical polling action just like that example does

EDIT: although looking further it's likely to be extremely inefficient and should use the query manager instead

[antazoey]

@PatrickAlphaC It does listen for events, that is exactly what it does!

For example the script:

import click
from ape import config, networks, project


# TODO: Parametrize and make a better example
def main():
    instance = project.TestContractVy.at("0x274b028b03A250cA03644E6c578D81f019eE1323")
    event_type = instance.NumberChange
    click.echo(f"Polling for new '{event_type.name}' logs...")

    for new_log in event_type.poll_logs():
        click.echo(new_log.newNum)

It will wait for new events as they come in and then log them. The process never ends.

@fubuloubu How would the query manager help here? These are all new logs coming in and that are not yet entered into an any DB

The one missing piece I see is a way to exit the poll upon some condition... Perhaps you are only waiting for a single event or something.

[antazoey]

OK yeah reading more closely, you are trying to basically have an async type deal that waits for a log and then carries on with something else once it arrives, right?

[fubuloubu]

@fubuloubu How would the query manager help here? These are all new logs coming in and that are not yet entered into an any DB

To get the historicial logs from query manager, and then wait on any new ones stays as it is today

[wwignes88]

If the docs would give an example of how to implement EventWatcher this would be very helpful. After importing with from brownie.network.event import EventWatcher I see I can print out print the directory print(f' {dir(EventWatcher)}') but it is unclear how I can make use of these commands.

I have posted a question on StackExchange

[fubuloubu]

If the docs would give an example of how to implement EventWatcher this would be very helpful. After importing with from brownie.network.event import EventWatcher I see I can print out print the directory print(f' {dir(EventWatcher)}') but it is unclear how I can make use of these commands.

I have posted a question on StackExchange

In ape, we added special methods to create exactly this type of event handling loop: https://docs.apeworx.io/ape/stable/methoddocs/contracts.html#ape.contracts.base.ContractEvent.poll_logs

you would use it like this:

for log in my_contract.MyEvent.poll_logs(...):
    # Do something with log e.g. `assert log.arg1 == 1`
    ...

You are correct that we don't have this very well documented at present, so I'll make this issue into a documentation update to add both ContractEvent.poll_logs and it's companion chain.blocks.poll_blocks to the userguides.

To give us some insight, where in our docs would you expect this to live?

[fubuloubu]

Overview

This is something I would have liked to see some additional functionality in brownie from.

Specification

Maybe the API looks something like:

from ape import networks

event = networks.listen_for_event(contract_address, event_name, timeout=200)

I made a custom edition of this in my chainlink-mix would be great for ape to have this built-in.

To follow up on this issue, we've actually made a whole platform from this idea which allows provisioning event handlers using a message queue processing architecture, made super simple using our SDK: https://github.com/SilverBackLtd/silverback

---

A snapshot from the example:

from ape_tokens import tokens
from silverback import SilverBackApp

app = SilverBackApp()
YFI = tokens["YFI"]


@app.on_(YFI.Approval)
async def exec_event2(log):
    return log.value

[wwignes88]

ohh maybe I am out of place here. I just now saw this is the apeWorx page. Beginners mistake but I did not even check what page I was posting too. But I think ANY page which a basic google search might turn up on how to use EventWatcher in the context of eth-brownie would be helpful. Since commenting I've managed to figure it out though, so maybe it can be said that if someone is to the point they are using EventWatcher they probably can figure it out as well. Still, some more detailed examples would have been helpful. To this end I answered my own question on stack exchange.