As I mentioned in the section about Menus, menu links are stored hierarchically inside a menu. This hierarchy is represented via a menu link tree. There are a number of key players here.

We have the MenuLinkTree service, which is the interface used to load and prepare the tree of a certain menu. The loading is deferred to the MenuTreeStorage service, which does so on the basis of a MenuTreeParameters object that contains metadata on certain restrictions to be applied on the menu links that are loaded. We will see some examples of this a bit later.

What comes out of the MenuLinkTree service is an array of MenuLinkTreeElement that is essentially a value object that wraps around the MenuLinkInterface plugins and that provides some extra data about its placement in the tree it finds itself in. One such important piece of information is the subtree (the array of MenuLinkTreeElement objects that are below it).