The rows a page shows, read once for the request and handed to everything that draws them: the head (MishkaCmsCore.Runtime.Seo), the frame (MishkaCmsCore.Runtime.SiteLayout) and the page's own blocks.

A page that shows a record is drawn by three readers: the plugin on Seo.event/0 for its head, the plugin on SiteLayout.event/0 for its languages and its bar, and the blocks of its template for the record itself. Each of them asked the database for the same rows on its own. A plugin on event/0 reads them once, before the other two run, and every one of them is handed what it read.

The event

MishkaCmsWeb.RuntimeLive calls assign_source/1 in every handle_params/3, once the page, its language, @query_params and the visitor are known, and before the frame and the head. It fires event/0 with the map given/2 builds and assigns the answer as @page_source:

• page, section, record, extra, params, language, site_id: as MishkaCmsCore.Runtime.SiteLayout hands them to its plugins. Read only.
• current_user: the signed-in visitor, or nil for a guest. The rows are read as them.
• held: what this LiveView's previous address read, for the same visitor, or nil. A live patch that stays on the same record (another page of a list, another version) takes from it what the new address did not change, rather than reading it again.
• source: nil.

A plugin answers for the pages it owns, matched as a frame plugin matches them, by putting its rows under source, and hands every other page back untouched, before anything is read. What it puts there is its own: core never looks inside it. It reaches, unchanged:

• the plugins on Seo.event/0, as source in the map they are handed (Seo.given/2);

• the plugins on SiteLayout.event/0, as source (SiteLayout.given/2);

• the page's template, as @page_source, which hands it to its blocks:

  <.component component_name="doc-section" page_source={assigns[:page_source]} … />

{:reply, :halt, given} keeps later plugins from answering for a page this one owns. A plugin that raises, or an event whose plugins cannot run, leaves the page with no source, and every reader reads for itself as it does without one.

The rules a source keeps

• IT IS READ AS THE VISITOR, through the public actions and the policies the blocks read with: the same locks, the same lock inherited from what a record is filed under, the same page lock. It is never broader than what that visitor may see, so a reader handed it shows nothing a read of its own would not have. A read made as the system belongs only where nothing of it reaches the visitor: the lock a record's page refuses the visitor by.
• IT IS THE VISITOR'S: it is read for one visitor (visitor_id/1) and says which, and a block handed one read for somebody else reads for itself.
• IT LIVES FOR ONE ADDRESS: it is read again in every handle_params/3, and lives in the LiveView's assigns and nowhere else. A task a block starts is handed what it needs as an argument, not the LiveView.
• A BLOCK STILL STANDS ON ITS OWN. On the builder's canvas, in a preview, on a page whose template does not hand it over, and for any read it was not given, a block reads as it always did. One handed a source takes from it only rows read for exactly what it would read itself.

page_source is handed only to a component that declares the attribute (MishkaCmsCore.Runtime.LiveViewHelpers): one an admin keeps on an older kit is drawn as if the template did not name it, rather than with the rows in its global attributes.