Live tracing of Mob events for IEx debugging.

Subscribe a process to receive every event that flows through Mob.Event. Tracing is opt-in and costs one :persistent_term read per dispatch when no tracers are registered.

Usage

# In IEx connected to the running app:
Mob.Event.Trace.subscribe()

# Now every event delivered via Mob.Event.dispatch/4 also lands in your
# mailbox tagged {:mob_trace, addr, event, payload}. Pattern-match it,
# log it, whatever.

flush()  # see what's in the mailbox

# Filter on the way out:
Mob.Event.Trace.subscribe(fn addr -> addr.widget == :list end)

# From a shell on another node, name the pid to deliver to — `:rpc`
# runs the call in a short-lived process that would receive nothing:
:rpc.call(node, Mob.Event.Trace, :subscribe, [self(), nil])

# Stop tracing:
Mob.Event.Trace.unsubscribe()   # this process
Mob.Event.Trace.stop()          # every tracer

Tracers are monitored by Mob.Diag.Subscribers, so one that exits (or whose node disconnects) stops being traced to without an unsubscribe/1.

Performance

When no tracers are registered (the default), Mob.Event.dispatch/4 reads an empty list from :persistent_term and returns. When tracers are registered, each one is sended a copy of the envelope. Tracer filter functions run in the dispatch path, so keep them cheap.