Get rid of static object access

[schlessera]

This is just one possible way of getting rid of the static access.

• Adds a factory to share an object cache instance. This replaces the getInstance() call.
• Uses the global $wp_object_cache. This already exists in WordPress, and external code might expect it to be populated.

[felixarntz]

I just noticed this PR, and had already worked on this: https://github.com/felixarntz/wp-psr-cache/blob/master/src/ObjectCacheService.php

What do you think? It's not amazing, but doesn't introduce a global variable.

[schlessera]

The global variable already exists in WordPress, and other code will use it. If you don't fill that global variable with an object instance, some plugins might break.

[schlessera]

See https://schlessera.github.io/wceu-2017/#/custom-cache-implementation-1

[schlessera]

Here you can see that WordPress checks this: https://github.com/WordPress/WordPress/blob/aaf99e691391cfceb004d848450dbbf3344b1bee/wp-activate.php#L21-L23

[felixarntz]

That generally makes sense, I'll look into it further soon and how it is used.

[felixarntz]

I merged the PR, with a few tweaks:

• The factory class is only used to create instances, not to share or store.
• The service class which I had implemented before still exists and is used for static access to the main object cache instance.
• The main instance is stored in the global $wp_object_cache as you suggested, for compatibility with some core edge-cases.
• The service class simply uses that global and is an alternate access point. It is an abstraction that allows to possibly get rid of the global in the future, and the developer tweaking the object-cache.php file can simply continue to use the service class and set their object cache instance to use there, after creating it with the factory.