On 26/04/2021 16.53, Simon Marchi wrote:
> From: Michael Weghorn <m.weghorn@posteo.de>
> 
> Previously, the observers attached to an observable were always notified
> in the order in which they had been attached.  That order is not easily
> controlled, because observers are typically attached in _initialize_*
> functions, we are called in an undefined order.

Just a nit-pick, should that read "functions, WHICH are called in an 
undefined order."?

> 
> However, an observer may require that another observer attached only
> later is called before itself is.
> 
> Therefore, extend the 'observable' class to allow explicitly specifying
> dependencies when attaching observers, by adding the possibility to
> specify tokens for observers that it depends on.
> 
> To make sure dependencies are notified before observers depending on
> them, the vector holding the observers is sorted in a way that
> dependencies come before observers depending on them.  The current
> implementation for sorting uses the depth-first search algorithm for
> topological sorting as described at [1].
> 
> Extend the observable unit tests to cover this case as well.  Check that
> this works for a few different orders in which the observers are
> attached.
> 
> This newly introduced mechanism to explicitly specify dependencies will
> be used in a follow-up commit.
> 
> [1] https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search
> 
> Tested on x86_64-linux (Debian testing).

Michael