How to deal with global information without creating a Singleton class

Question

I am trying to walk away from the Singleton pattern when it comes to develop plugins for WordPress.
Why?
Because I have been reading in some sources (here, here, here, here ...) and came up with the conclusion that they are a bad thing. Or at least, not something one should use just because.
Why would I need a Singleton-like behavior?
Because I want to have an object with properties where multiple classes along different plugins can write and read those properties.
And I sincerely do not know how to approach this.
Can you please let me know how you would do it?
There may be different cases with different solutions, so I will suppose a real example.
Example:
A plugin called "Example Form" creates a custom form on the WordPress site.
The form is processed via a method added to the init hook by the plugin class.
public static function setup_actions_and_filters(){
   $this_class = new self();
   add_action('init', [$this_class, 'process_form']);
....

And while processing the form I may want to add some error or success messages to the active instance, so I can then display those messages in a theme doing somethig like:
$instance_of_PLUGINCLASS->show_errors();

But I don't have access to the "original" instance because I called a satic setup method in the main plugin file.
PLUGINCLASS::setup_actions_and_filters();

And that is why a common solution is to make the PLUGINCLASS use the Singleton pattern, which I want to avoid.
Did I come up with my own ideas?
Yes.
I thought about:

Using the database - Horrible idea
Using cookies - Terrible idea
Using sessions - Not so nice
Going from one request to the other sending GET or POST info - Vulnerable and dirty
Using globals - I have read many times that it's not a good idea to use them
Using WP Object Cache (see Singleton VS Object Caching)

mozboz · Answer

Thought it might be useful to summarise some of the points from the comments here. (Note: 'Singleton' is a name given to the pattern of storing an instance of a class in a static property of a class, usually the same one, so here 'singleton' and 'static property' are used more or less interchangeably)
Singleton Dogma
Heed the good advice about singletons and the warnings about bad habits they can get you in, but don't let that stop you using singletons or static properties. Like using global variables, or not structuring your code well into functions and files, or not commenting your code or naming your variables well; or a thousand other bad programming habits, if you're not aware of the strucutres you're using and taking care of good programming habits your life (and maybe other peoples' lives) will get harder later.
There are only 2 options
Without going outside of the PHP process and its memory, you simply only have two options to share variables between scopes:

Global variables
Static properties on classes.

There are no other options. Within the Wordpress codebase, where you can't change the structure of swathes of code just to pass your variable around neatly, you therefore have to use one of these two options. The question is not if but how you use them.
Using appropriate complexity
Here are my thoughts on the basic options available. It's worth considering which of these makes sense for you. Good development without premature optimisation would suggest that you should use the simplest one that supports what you know about your requirements now, with flexibility to change your code if/when you need to.
Get Creative, Use Wordpress Ways
Get creative. Access control on top of a configuration class? Use an array or a queue data structure instead of a single variable? Employ Wordpress's filters to pass values around and expose import interfaces in your code to other users?
Some little neat features might save you a ton of heartache to catch situations where, for example:

You need to set a value exactly once and read it exactly once
You want to allow any number of values to be set from many different places and then read them all in one place
You want to debug by seeing who set/get a variable in what order
You want to add caching between PHP page loads / sessions
You want to make the internals pf your plugin accessible to other people who may want to extend its functionality / content

How to deal with global information without creating a Singleton class

One Answer

Singleton Dogma

There are only 2 options

Using appropriate complexity

Get Creative, Use Wordpress Ways

Add your own answers!

Ask a Question