Listening to events using EventWatcher

Question

I am trying to listen to events emitted from a contract using eth-brownie. More specifically I am attempting to use EventWatcher. My contract emit has an event eventI in it. My run.py file looks like,

from brownie.network.event import EventWatcher
from brownie import Contract, emit
def run():
    # get contract
    emit = Contract.from_abi( emit,
                              [contract adress], 
                              [contract abi])

    # start watcher
    EventWatcher._start_watch 

    # define callback function
    def watched():
        print('watched')

    # use add_event_callback
    EventWatcher.add_event_callback(
        event=emit.events["eventI"], 
        callback=watched, 
        delay=2.0, 
        repeat=True)
        
def main():
    run()

I have two questions; 1) am I implementing the above arguments correctly? 2) when I run it I get the error,

EventWatcher.add_event_callback() missing 
1 required positional argument: 'self'

From running inspect.signature(EventWatcher.add_event_callback) I know self should be the first input, but I don't know what the form of it should be. I tried things like emit.self if that makes any sense...

Answer 1

Following asnwers to this question and also getting some clues from print(dir(EventWatcher)) I resolved the above error as follows,

start watcher:

    watch = EventWatcher()
    print(f'has started? {watch._has_started}')          # OUT: False
    watch._start_watch()
    print(f'has started? {watch._has_started}')          # OUT: True

add callback function:

    def watched(number):              # callback function
        print(f'{number} was watched.')

    print(f'targets:  {watch.target_events_watch_data}')        # OUT: {}
    addCallBack = watch.add_event_callback(event=emit.events["eventI"],
                                              callback=watched, 
                                              delay=2.0, 
                                              repeat=True)
    print(f'targets:  {watch.target_events_watch_data}')  
              # OUT: {'[emit address]+eventI': <brownie.network.event.
                      _EventWatchData object at 0x000001A45B67EA10>}

Note that my event is defined as,

    event eventI(uint256 input);

notice that the callback function has a uint256 input as well (*clarify needeed; the two must match).

Really wish they'd give more detailed documentation on this function. EventWatcher seems to be some well-knonw function in javascript or other language, and its manifestation in brownie seems to presume everyone using it is familiar with its use in some other context.