Attack Chain Migration: /obj/item/airlock_electronics/destroyed (#27538)

* Attack Chain Migration: /obj/item/airlock_electronics/destroyed

* don't register signals on global procs

code/_onclick/item_attack.dm

@@ -274,3 +274,16 @@
 		"<span class='combat danger'>You hear someone being attacked with a weapon!</span>"
 	)
 	return TRUE
+
+/// Used for signal registrars who wish to completely ignore all behavior
+/// in the attack chain from parent types. Should be used sparingly, as
+/// subtypes are meant to build on behavior from the parent type.
+/datum/proc/signal_cancel_activate_self(mob/user)
+	return COMPONENT_CANCEL_ATTACK_CHAIN
+
+/// Used for signal registrars who wish to completely ignore all behavior
+/// in the attack chain from parent types calling `attack_by`. Should be
+/// used sparingly, as subtypes are meant to build on behavior from the parent
+/// type.
+/datum/proc/signal_cancel_attack_by(datum/source, obj/item/attacking, mob/user, params)
+	return COMPONENT_SKIP_AFTERATTACK

code/game/machinery/doors/airlock_electronics.dm

@@ -21,6 +21,8 @@
 	/// Is this electronic installed in a door?
 	var/is_installed = FALSE
 
+	new_attack_chain = TRUE
+
 /obj/item/airlock_electronics/Initialize(mapload)
 	. = ..()
 	if(!length(door_accesses_list))
@@ -108,8 +110,9 @@
 	name = "burned-out airlock electronics"
 	icon_state = "door_electronics_smoked"
 
-/obj/item/airlock_electronics/destroyed/attack_self__legacy__attackchain(mob/user)
-	return
+/obj/item/airlock_electronics/destroyed/Initialize(mapload)
+	. = ..()
+	RegisterSignal(src, COMSIG_ACTIVATE_SELF, TYPE_PROC_REF(/datum, signal_cancel_activate_self))
 
 /obj/item/airlock_electronics/destroyed/decompile_act(obj/item/matter_decompiler/C, mob/user)
 	C.stored_comms["metal"] += 1

docs/references/attack_chain.md

@@ -595,6 +595,37 @@ sequence of events. Finally, because we constantly check the parent proc, all
 signals that are expected to be sent, are, so any other components or listeners
 can take appropriate action and cancel the attack chain themselves, if requested.
 
+### Cancelling All Behavior
+
+Frequently, a subtype will want to completely prevent any of its parent type
+behavior from running. Examples may be a holofloor, which should prevent any
+attempts to deconstruct it, or a destroyed variant of an object, which cancels
+out the existing functionality of the parent type.
+
+Attack chain methods must always call their parent procs, so this presents a
+problem.
+
+In order to implement behavior such as this, the child type should register to
+listen for the signal that applies to the attack chain proc, and respond by
+calling one of the procs which return a signal preventing the rest of the attack
+chain from running.
+
+For `attack_by` prevention, this proc is [/datum/proc/signal_cancel_attack_by][]. For
+`activate_self` prevention, this proc is [/datum/proc/signal_cancel_activate_self][].
+
+[/datum/proc/signal/cancel_attack_by]: https://codedocs.paradisestation.org/datum.html#proc/signal_cancel_attack_by
+[/datum/proc/signal/cancel_activate_self]: https://codedocs.paradisestation.org/datum.html#proc/signal_cancel_activate_self
+
+For example, when we migrated the airlock electronics above, we neglected to
+handle the `/destroyed` subtype, which prevents any interaction via
+`activate_self`. To ensure this, we make the following change:
+
+```diff
++/obj/item/airlock_electronics/destroyed/Initialize(mapload)
++	. = ..()
++	RegisterSignal(src, COMSIG_ACTIVATE_SELF, TYPE_PROC_REF(/datum, signal_cancel_activate_self))
+```
+
 ## Migration Helpers
 
 There are two important tools which can help make the migration process easier: