Liferay 7.3: How to preconfigure a portlet embedded in a page fragment? - liferay

Question

We're using Liferay 7.3 (CE) and are trying to embrace the relatively new feature of "Content Pages" with "Page Fragments". We're able to develop page fragments that already include portlets (named "widgets" in the context of content pages), using the <lfr-widget-WIDGETALIAS> tag. So far, that works.
Now we're trying to prepare page fragments that embed portlets with special portlet configuration applied. For example, we want to prepare a page fragment that just shows an asset publisher portlet configured to list WebContent articles from a pre-defined category. The user should be able to just put that fragment onto the page without having to care about the configuration of the asset publisher portlet.
We did not find any direct way to achieve that -- our first guess that the configuration could be written as attributes or content of the <lfr-widget-...> tag was deterred by a hint in the liferay docs that there are no valid attributes or content to attach to that tag.
Does anybody have an inkling of an idea on how to achieve embedding portlets in page fragments with pre-defined portlet configuration applied? (including out-of-the-box Liferay portlets?)

Answer 1

I figured it out myself.
That one thing that the Fragment Editor does not tell you is that the HTML part of a fragment actually is interpreted as a Freemarker template, with the caveat that only Freemarkers alternative syntax is allowed.
That, in turn, means that Liferays taglibs are available, which means we can use the tag <liferay-portlet:runtime> (ported to freemarker alternative syntax, of course), which does accept a defaultPreferences attribute. Now we can just configure the portlet once, find its portletPreferences XML data in the DB (see table PortletPreferences), remove values we do not want to preconfigure and then just use the resulting preferences XML as a value for the defaultPreferences attribute of the <liferay-portlet:runtime> tag.
Care has to be taken for any IDs (e.g. if you want to preconfigure an AssetCategory filter). Better fetch the corresponding object from the corresponding service and get the ID from that object.
This example provides the HTML part for a page fragment that places an AssetPublisher onto the page, preconfigured to show 12 items (instead of the default 20). (CSS, JS and Configuration of the fragment is the default as given by the Page Fragment editor.)
<div class="fragment-12345">
[#assign assetPublisherPortletPreferences="<portlet-preferences>
<preference>
<name>delta</name>
<value>12</value>
</preference>
</portlet-preferences>" /]
[#liferay_portlet["runtime"]
instanceId="${fragmentEntryLinkNamespace}assets"
portletName="com_liferay_asset_publisher_web_portlet_AssetPublisherPortlet"
defaultPreferences="${assetPublisherPortletPreferences}"
/]
</div>

Answer 2

Thank you for this, #orithena. You saved me a lot of trouble.
Another option, to achieve the same result but with simpler syntax, is to use the built-in freeMarkerPortletPreferences:
[#assign assetPublisherPortletPreferences=freeMarkerPortletPreferences.getPreferences({
"delta": "12",
} /]