# Linking Scripts

These scripts contain all the logic for creating links between nodes. This involves, to a large extent, deciding which links to create.

## Hooks

The hooks in this section are organized in 3 sub-categories. The first category includes hooks that are triggered by changes in the graph. Some of them are tasked to schedule a "rescan-for-linking" event, which is the lowest priority linking event and its purpose is to scan through all the linkable session items and link them to a particular target. The "rescan-for-linking" event is always scheduled to run once for all the graph changes in a cycle. This is achieved by flagging the event as already scheduled in the module-standard-event-source; this flag is then cleared by a hook that runs on this event.

Selecting a target for each linkable and linking to it is deferred to another set of hooks by pushing a "select-target" event for each linkable. This event is the highest priority event and therefore no other changes in the graph are processed while targets are being selected.

Hooks triggered by changes in the graph
Hook name | File | Triggered by | Action
--- | --- | --- | ---
linking/rescan-trigger | rescan.lua | linkable SI added|removed or metadata-changed | schedules rescan-for-linking event
linking/linkable-removed | rescan.lua | linkable SI removed | destroys links related to the removed linkable
linking/follow | move-follow.lua | metadata-changed | schedules rescan-for-linking when the configured default sources/sinks are changed by the user
linking/move | move-follow.lua | metadata-changed | schedules rescan-for-linking when node target metadata properties are changed
linking/rescan-media-role-links | rescan-media-role-links.lua | link SI added, removed or metadata-changed | activates or deactivates role-based links based on role priorities and actions

rescan-for-linking hooks, in order of execution
Hook name | File | Description
--- | --- | ---
m-standard-event-source/rescan-done | module-standard-event-source.c | clears the rescan_scheduled flag
linking/rescan | rescan.lua | schedules select-target for each linkable session item

select-target hooks, in order of execution
Hook name | File | Description
--- | --- | ---
linking/find-defined-target | find-defined-target.lua | Select the target that has been defined explicitly by the 'target.object' property or metadata
linking/find-filter-target | find-filter-target.lua | Select the target of a filter node, if the subject is a filter node
linking/find-media-role-target | find-media-role-target.lua | Select the target based on the stream's media.role and the target's device.intended-roles
linking/find-default-target | find-default-target.lua | Select the default source/sink as target
linking/find-best-target | find-best-target.lua | Select target based on priority.session
linking/get-filter-from-target | get-filter-from-target.lua | Translate the found target to a filter target that should be linked instead
linking/prepare-link | prepare-link.lua | Break existing link if needed, check if the target is available for linking; send error to the client if needed
linking/link-target | link-target.lua | Create si-standard-link session item to create links between the subject linkable and the selected target