Feature request: global command observer (a BlocObserver for command_it) #22

One of the best things about Bloc is BlocObserver. With a single class you get a complete live log of your app: which event came in, which state followed, and which errors happened. For debugging and for production logging it is hard to beat.

command_it has the same building blocks:

• a command run is the equivalent of a Bloc event (name + parameter),
• the CommandResult is the equivalent of a Bloc state (running, data, error).

The only missing piece is a single global place where all of this comes together. Today Command.loggingHandler is called once per run, on completion, so the "running" state and the moment a command was started never show up in it.

Imagine one global observer that prints a timeline like this for the whole app:

AcceptTerms  started
AcceptTerms  completed
LoadCards    started, param: 5
LoadCards    error: ...

That would make command_it much easier to debug and monitor, and would be a real argument for people who are choosing between command_it and Bloc. I think it could become one of the package's killer features.

Ideally the observer would report:

• when a command is started (name and parameter),
• its result (data or error),
• optionally when a command is created and disposed.

A first small step could be an opt-in way to also call loggingHandler when an async command starts, without changing the current default behavior.