How the menu system works

Basic concepts

Soft Roots

A soft root is a page that acts as the root for a menu navigation tree.

Typically, this will be a page that is the root of a significant new section on your site.

When the soft root feature is enabled, the navigation menu for any page will start at the nearest soft root, rather than at the real root of the site’s page hierarchy.

This feature is useful when your site has deep page hierarchies (and therefore multiple levels in its navigation trees). In such a case, you usually don’t want to present site visitors with deep menus of nested items.

For example, you’re on the page “Introduction to Bleeding”, so the menu might look like this:

School of Medicine
    Medical Education
    Departments
        Department of Lorem Ipsum
        Department of Donec Imperdiet
        Department of Cras Eros
        Department of Mediaeval Surgery
            Theory
            Cures
                Bleeding
                    * Introduction to Bleeding <current page>
                    Bleeding - the scientific evidence
                    Cleaning up the mess
                Cupping
                Leaches
                Maggots
            Techniques
            Instruments
        Department of Curabitur a Purus
        Department of Sed Accumsan
        Department of Etiam
    Research
    Administration
    Contact us
    Impressum

which is frankly overwhelming.

By making “Department of Mediaeval Surgery” a soft root, the menu becomes much more manageable:

Department of Mediaeval Surgery
    Theory
    Cures
        Bleeding
            * Introduction to Bleeding <current page>
            Bleeding - the scientific evidence
            Cleaning up the mess
        Cupping
        Leaches
        Maggots
    Techniques
    Instruments

Registration

The menu system isn’t monolithic. Rather, it is composed of numerous active parts, many of which can operate independently of each other.

What they operate on is a list of menu nodes, that gets passed around the menu system, until it emerges at the other end.

The main active parts of the menu system are menu generators and modifiers.

Some of these parts are supplied with the menus application. Some come from other applications (from the cms application in django CMS, for example, or some other application entirely).

All these active parts need to be registered within the menu system.

Then, when the time comes to build a menu, the system will ask all the registered menu generators and modifiers to get to work on it.

Generators and Modifiers

Menu generators and modifiers are classes.

Generators

To add nodes to a menu a generator is required.

There is one in cms for example, which examines the Pages in the database and adds them as nodes.

These classes are sub-classes of menus.base.Menu. The one in cms is cms.menu.CMSMenu.

In order to use a generator, its get_nodes() method must be called.

Modifiers

A modifier examines the nodes that have been assembled, and modifies them according to its requirements (adding or removing them, or manipulating their attributes, as it sees fit).

An important one in cms (cms.menu.SoftRootCutter) removes the nodes that are no longer required when a soft root is encountered.

These classes are sub-classes of menus.base.Modifier. Examples are cms.menu.NavExtender and cms.menu.SoftRootCutter.

In order to use a modifier, its modify() method must be called.

Note that each Modifier’s modify() method can be called twice, before and after the menu has been trimmed.

For example when using the {% show_menu %} template tag, it’s called:

• first, by menus.menu_pool.MenuPool.get_nodes(), with the argument post_cut = False

• later, by the template tag, with the argument post_cut = True

This corresponds to the state of the nodes list before and after menus.templatetags.menu_tags.cut_levels(), which removes nodes from the menu according to the arguments provided by the template tag.

This is because some modification might be required on all nodes, and some might only be required on the subset of nodes left after cutting.

Nodes

Nodes are assembled in a tree. Each node is an instance of the menus.base.NavigationNode class.

A NavigationNode has attributes such as URL, title, parent and children - as one would expect in a navigation tree.

It also has an attr attribute, a dictionary that’s provided for you to add arbitrary attributes to, rather than placing them directly on the node itself, where they might clash with something.

Warning

You can’t assume that a menus.base.NavigationNode represents a django CMS Page. Firstly, some nodes may represent objects from other applications. Secondly, you can’t expect to be able to access Page objects via NavigationNodes. To check if node represents a CMS Page, check for is_page in menus.base.NavigationNode.attr and that it is True.

Menu system logic

Let’s look at an example using the {% show_menu %} template tag. It will be different for other template tags, and your applications might have their own menu classes. But this should help explain what’s going on and what the menu system is doing.

One thing to understand is that the system passes around a list of nodes, doing various things to it.

Many of the methods below pass this list of nodes to the ones it calls, and return them to the ones that they were in turn called by.

The ShowMenu.get_context() method

When the Django template engine encounters the {% show_menu %} template tag, it calls the get_context() of the ShowMenu class. get_context():

• calls menus.menu_pool.MenuPool.get_nodes() (see The MenuPool.get_nodes() method below)

• cuts any nodes other than its descendants (if a root_id has been provided)

• calls menus.templatetags.menu_tags.cut_levels() to remove unwanted levels

• calls menus.menu_pool.MenuPool.apply_modifiers() with post_cut = True

• return the nodes to the context in the variable children

The MenuPool.get_nodes() method

menus.menu_pool.MenuPool.get_nodes() calls three other methods of MenuPool in turn:

• menus.menu_pool.MenuPool.discover_menus()

  Checks every application’s cms_menus.py, and registers:

  • Menu classes, placing them in the self.menus dict

  • Modifier classes, placing them in the self.modifiers list

• menus.menu_pool.MenuPool._build_nodes()

  • checks the cache to see if it should return cached nodes

  • loops over the Menus in self.menus (note: by default the only generator is cms.menu.CMSMenu); for each:

    • calls its menus.base.Menu.get_nodes() - the menu generator

    • menus.menu_pool._build_nodes_inner_for_one_menu()

    • adds all nodes into a big list

• menus.menu_pool.MenuPool.apply_modifiers()

  • menus.menu_pool.MenuPool._mark_selected()

  • loops over each node, comparing its URL with the request.path_info, and marks the best match as selected

  • loops over the Modifiers (see Menu Modifiers below) in self.modifiers calling each one’s modify() with post_cut=False.

Menu Modifiers

Each Modifier manipulates menu nodes and their attributes.

The default Modifiers, in the order they are called, are:

• cms.menu.NavExtender

• cms.menu.SoftRootCutter

  If post_cut is True, removes all nodes below the appropriate soft root; otherwise, returns immediately.

• menus.modifiers.Marker

  If post_cut or breadcrumb is True, returns immediately; otherwise, loops over all nodes; finds selected, marks its ancestors, siblings and children

• menus.modifiers.AuthVisibility

  Removes nodes that require authorisation to see

• menus.modifiers.Level

  Loops over all nodes; for each one that is a root node (level == 0) passes it to:

  • mark_levels() recurses over a node’s descendants marking their levels