1. https://carlalexander.ca/dependency-injection-wordpress/

    "Dependency injection is this idea that there should be a system that handles these objects that your class depends on. This system would inject those objects (thus the name dependency injection!) into your class. It can do either by constructing the object and passing it its dependencies. Or it can use methods that you’ve created for that purpose like setter methods. We call those constructor injection and setter injection respectively."