How to implement a Layout Manager Includelet

How to implement a Layout Manager Includelet

It's an easy, two- or three-step process!
  1. Write a template which implements the functionality you want to declare as an includelet (or find a template within a package that already does what you want). Implement it, if necessary.
  2. If your Includelet requires parameters other than the package instance it will be bound to, write an initialization procedure to generate the necessary values, and store them in the element the Layout Manager will associate when the includelet is added to a pageset.
  3. Call the layout manager Tcl API to declare your includelet.
If a template already exists that does what you want, and requires no parameters other than its associated package, only the last step is necessary. For instance, here's the Tcl API call that creates the Subsites includelet, which just calls the same subsite-listing template included by the standard acs-subsite index page: 
layout::includelet::new \
    -name subsites_includelet \
    -description "Display Subsites" \
    -title "Subsites" \
    -application acs-subsite \
    -template /packages/acs-subsite/lib/subsites \
    -required_privilege read
The "name" parameter is the internal name and primary key of the includelet, and must be unique throughout the system.

The "description" parameter should be a human-readable description of what the includelet does.

The "title" parameter provides a default title to use when creating an element from the includelet.

The "application" parameter is the package key that this includelet supports. When an element is created from this includelet, it will be bound to a single instance of this apm package type.

The "template" parameter is the path to the template which implements the includelet.

The "required_privilege" parameter is the privilege a user must have on the application package an element is bound to in order to see the element. If the user doesn't have the required privilege on the relevant application package instance, the element won't be displayed to them. Defaults to "read", the implementer should set it to "admin" when writing admin includelets.

Custom Initializer

It's often necessary to create an initialization procedure for an includelet. The optional parameter "initalizer" allows one to specify a procedure to be called after an element is created from an includelet and placed on a layout manager page. For an example, here's the call which defines the content includelet, which provides a way to generate simple content with version control (delete, publish, preview, edit): 
layout::includelet::new \
    -name content_includelet \
    -description "Displays the content includelet" \
    -title "Content Includelet" \
    -application content-includelet \
    -template /packages/content-includelet/lib/content-includelet \
    -initializer content_includelet_utilities::configure_content_id
The initalizer procedure will be called with a single parameter, the id of the element which is being initalized. This example is slightly simplified from the actual initalizer procedure, which is generalized a bit for use outside the Layout Manager: 
ad_proc content_includelet_utilities::configure_content_id {
    element_id
} {
    Create the includelet's item and set the content_id param.

    @param element_id The element to initialize.

} {

    # Get our application's package_id from the element.
    set package_id [layout::element::get_column_value \
                       -element_id $element_id \
                       -column package_id]

    # Create a new content item.
    set content_id [content::item::new \
                       -name "Content For $package_id's $parameter" \
                       -parent_id $package_id \
                       -context_id $package_id \
                       -content_type content_includelet_revision \
                       -storage_type text]

    # Add the content_id parameter to the element.
    layout::element::parameter::add_values \
        -element_id $element_id \
        -parameters [list content_id $content_id]
}
When an includelet is executed, the local variables package_id and element_id are always set, along with includelet-supplied parameters defined for the element. In this case, the local variable content_id will be set when the content-includelet script is executed.