Add hooks for life cycle events: using .preAction(), .postAction()

[shadowspawn]

Pull Request

Problem

People ask for a way to run code before the action handler with access to the parsed options. And also some requests for a hook after the action.

Main issue: #1197
Related issues: #76 #229 #936 #996 #1158 #1457

Solution

Add .preAction() and .postAction() methods:

• supports async callbacks
• supports multiple hooks for same event on single command

Triggered by action handler for hooked command and its subcommands.

Callback passed two Command parameters:

• thisCommand: the command which the hook is added to, useful for working with global properties.
• actionCommand: the command with the action handler

ChangeLog

• add .preAction() and .postAction() hooks

[shadowspawn]

This is a rework of the signature for life cycle events from #1514:

• multiple named methods instead of a single hook with named events
• pass parameters instead of context hash

This gives a cleaner callback, especially when caller only cares about thisCommand, but increased chance of confusion between the two parameters.

So looks like:

program
   .preAction((thisCommand, actionCommand) => {});
   .postAction((thisCommand, actionCommand) => {});

instead of

program
   .hook('beforeAction', (context) => {});
   .hook('afterAction', (context) => {});

[shadowspawn]

For an example of the alternative approach of a single hook routine which is passed an event name rather than separately named methods for the hooks, see:

https://www.fastify.io/docs/latest/Hooks/

A key reason for the single hook approach is if there are going to be lots of hooks. The pattern is similar to eventEmitter and .on(event, ....). fastify describes 13 hooks.

[shadowspawn]

For interest, also noticed in the fastify docs:

Hook functions are invoked with this bound to the associated Fastify instance.

Commander does the same with the action callback, and EventEmitter does this with listeners. Because it does not work with arrow functions, I have not been treating this as the main way of passing in command context.

[shadowspawn]

I am quite liking how fastify looks, and I like including "hook" in the wording as it is how this sort of pattern is often described.

Thinking of switching back to hook! This time with parameters:

program
   .hook('preAction', (thisCommand, actionCommand) => {});
   .hook('postAction', (thisCommand, actionCommand) => {});