Options Reloaded

[HaoK]

Fixes #145 (comment)

Initial shape with basic tests (want to add a few more)

Few points to consider:

• The feature is always 'on', any time Configure(IConfiguration) is called, we hook up the notifications (no way to disable)
• We could have the current OptionsManager implement both IOptions and IOptionsSnapshot (we'd register the class twice, singleton for IOptions, and Scoped for Snapshot, felt too weird so I didn't go that route, and copied the class, since its tiny)
• Support multiple listeners in OptionsMonitor.OnChange or not? Basically subsequent calls could either add additional callbacks, or override the first callback. I did the more complicated list of callbacks, but I'd be glad to kill this.
• (Found a nasty bug with CurrentValue in the old implementation where it wouldn't get updated if no one ever called OnChange)

[HaoK]

@divega @Eilon @DamianEdwards @davidfowl

[divega]

The feature is always 'on', any time Configure(IConfiguration) is called, we hook up the notifications (no way to disable)

At first glance it looks like the baseline cost (if noone hooks up to listen) is a singleton IOptionsChangeTokenSource. Are there other hidden costs?

Support multiple listeners in OptionsMonitor.OnChange or not? Basically subsequent calls could either add additional callbacks, or override the first callback. I did the more complicated list of callbacks, but I'd be glad to kill this.

I agree the code is a bit more involved this way, but I think it is fine, and I cannot see us getting away without it.

[divega]

Looks good to me, but you will probably want to get another round of votes for naming (in case sleeping on it or the 🍺 helped).

[HaoK]

Yeah I guess the overhead isn't too bad if no one asks for an IOptionsMonitor since the change token hookup in the ctor won't happen

[HaoK]

@Eilon @DamianEdwards @davidfowl last call for other name suggestions for the initial checkin...?

'

[galich]

Keep IOptionsSnapshot - it conveys the nature of the object ;)

[HaoK]

acecacf

[RehanSaeed]

I fully expected the reason for the existence of IOptions<T> being the reloading ability. Why was IOptionsSnapshot<T> created instead and why not just register the type T directly?