Refactor var/can_be_held to an element (#96917)

## About The Pull Request

We were doing a lot of mental gymnastics in a bunch of other places, so
let's change this wonky var to a streamlined element that will rely on
the same signals that a lot of stuff was already using/accounting for in
its own signal handling pathways, instead of being a weird coverage gap.
This patch should also make the whole "checking if someone is attempting
to pick a mob up" thing make a lot more sense and use a unified proc
instead of spot-checking whatever random things it wants to spot-check.
## Why It's Good For The Game

I didn't know this was a thing until I looked at #96873 and it made me
sad because literally everything else involving mob drag-and-drop is
already signal-based except this weird stinker that relied on proc
overrides. Never mind that now, let's use nice traits to avoid
typecasting and elements to avoid duplicating code. It should also be
much cleaner to add holdability to a mob isntead of having to do
`can_be_held = FALSE` as a weird behavior (at least one instance had
this non-necessarily). All this really is is just middleware on the
extant /mob/living code but still making it in proper lockstep with the
other signalling procs.

I did port over raptor code faithfully but I'm not 100% sure if it was
meant to be like this? Regardless, that's how it is.
## Changelog
🆑
refactor: Picking up mobs has been altered a bit, please report any bugs
or glitches.
/🆑

code/modules/mob/living/basic/bots/repairbot/repairbot.dm

@@ -9,7 +9,6 @@
 	layer = BELOW_MOB_LAYER
 	anchored = FALSE
 	health = 35
-	can_be_held = TRUE
 	maxHealth = 35
 	custom_materials = list(/datum/material/iron = SHEET_MATERIAL_AMOUNT * 7.8, /datum/material/glass = SMALL_MATERIAL_AMOUNT * 2)
 	path_image_color = "#80dae7"
@@ -93,6 +92,7 @@
 	our_screwdriver = new(src)
 	our_rods = new(src, our_rods::max_amount)
 	set_color(toolbox_color)
+	AddElement(/datum/element/can_be_held)
 	START_PROCESSING(SSobj, src)
 
 /mob/living/basic/bot/repairbot/proc/set_color(new_color)

code/modules/mob/living/basic/drone/_drone.dm

@@ -46,7 +46,6 @@
 	lighting_cutoff_red = 30
 	lighting_cutoff_green = 35
 	lighting_cutoff_blue = 25
-	can_be_held = TRUE
 	worn_slot_flags = ITEM_SLOT_HEAD
 	inhand_holder_type = /obj/item/mob_holder/drone
 	/// `TRUE` if we have picked our visual appearance, `FALSE` otherwise (default)
@@ -155,6 +154,8 @@
 	listener.RegisterSignal(src, COMSIG_LIVING_DEATH, TYPE_PROC_REF(/datum/alarm_listener, prevent_alarm_changes))
 	listener.RegisterSignal(src, COMSIG_LIVING_REVIVE, TYPE_PROC_REF(/datum/alarm_listener, allow_alarm_changes))
 
+	AddElement(/datum/element/can_be_held)
+
 /mob/living/basic/drone/med_hud_set_health()
 	set_hud_image_state(DIAG_HUD, "huddiag[RoundDiagBar(health/maxHealth)]")
 

code/modules/mob/living/basic/farm_animals/bee/_bee.dm

@@ -36,7 +36,6 @@
 	mob_biotypes = MOB_ORGANIC|MOB_BUG
 	density = FALSE
 	gold_core_spawnable = FRIENDLY_SPAWN
-	can_be_held = TRUE
 	held_w_class = WEIGHT_CLASS_TINY
 	environment_smash  = ENVIRONMENT_SMASH_NONE
 	habitable_atmos = null
@@ -71,6 +70,7 @@
 	AddComponent(/datum/component/obeys_commands, pet_commands)
 	AddElement(/datum/element/swabable, CELL_LINE_TABLE_QUEEN_BEE, CELL_VIRUS_TABLE_GENERIC_MOB, 1, 5)
 	AddElement(/datum/element/basic_allergenic_attack, allergen = BUGS, allergen_chance = 33, histamine_add = 5)
+	AddElement(/datum/element/can_be_held)
 
 /mob/living/basic/bee/mob_pickup(mob/living/picker)
 	if(flags_1 & HOLOGRAM_1)

code/modules/mob/living/basic/farm_animals/rabbit.dm

@@ -18,7 +18,6 @@
 	health = 15
 	maxHealth = 15
 	mob_size = MOB_SIZE_SMALL
-	can_be_held = TRUE
 	density = FALSE
 	gold_core_spawnable = FRIENDLY_SPAWN
 	speak_emote = list("sniffles", "twitches")
@@ -55,6 +54,7 @@
 	AddElement(/datum/element/ai_retaliate)
 	AddElement(/datum/element/pet_bonus, "hop")
 	AddElement(/datum/element/animal_variety, icon_prefix, pick("brown", "black", "white"), TRUE)
+	AddElement(/datum/element/can_be_held)
 	if(prob(20)) // bunny
 		name = "bunny"
 

code/modules/mob/living/basic/lavaland/raptor/_raptor.dm

@@ -78,6 +78,8 @@ GLOBAL_LIST_EMPTY(raptor_population)
 	var/datum/raptor_inheritance/inherited_stats = null
 	/// Current happiness value of the raptor
 	var/happiness_percentage = 0
+	/// The ability for this raptor to be picked up and held. Defaults to FALSE as it's meant to be in lockstep with the element being added/removed.
+	var/could_be_held = FALSE
 
 /mob/living/basic/raptor/Initialize(mapload, datum/raptor_color/color_type, datum/raptor_inheritance/passed_stats)
 	. = ..()
@@ -283,6 +285,16 @@ GLOBAL_LIST_EMPTY(raptor_population)
 
 	return pick_weight(prob_list)
 
+/// Updates the presence of the can_be_held element based on what we want from the raptor
+/mob/living/basic/raptor/proc/update_holdability(bool)
+	if(bool && !could_be_held)
+		AddElement(/datum/element/can_be_held)
+		could_be_held = TRUE
+
+	if(!bool && could_be_held)
+		RemoveElement(/datum/element/can_be_held)
+		could_be_held = FALSE
+
 /mob/living/basic/raptor/proc/on_picked_up(mob/living/basic/raptor/source, mob/living/user, obj/item/mob_holder/holder)
 	SIGNAL_HANDLER
 	// Our inventory code sucks so we have to do this
@@ -364,16 +376,16 @@ GLOBAL_LIST_EMPTY(raptor_population)
 			base_pixel_w = initial(base_pixel_w)
 			mob_size = initial(mob_size)
 
-	can_be_held = initial(can_be_held)
 	density = initial(density)
 	move_resist = initial(move_resist)
 	can_breed = initial(can_breed)
+	update_holdability(initial(could_be_held))
 
 	if (new_stage == RAPTOR_ADULT)
 		// Adults need to be tamed with skill rather than snacks
 		qdel(GetComponent(/datum/component/tameable))
 	else // Make us teeny-tiny
-		can_be_held = TRUE
+		update_holdability(TRUE)
 		density = FALSE
 		can_breed = FALSE
 		move_resist = MOVE_RESIST_DEFAULT
@@ -389,7 +401,7 @@ GLOBAL_LIST_EMPTY(raptor_population)
 	var/obj/item/mob_holder/holder = null
 	if (istype(loc, /obj/item/mob_holder))
 		holder = loc
-		if (!can_be_held)
+		if (!could_be_held)
 			holder.release()
 			holder = null
 

code/modules/mob/living/basic/lavaland/raptor/raptor_color.dm

@@ -128,7 +128,7 @@ GLOBAL_LIST_INIT(raptor_colors, init_raptor_colors())
 
 // Purple raptors never "fully" grow up, and remain usable as backpacks
 /datum/raptor_color/purple/setup_adult(mob/living/basic/raptor/raptor)
-	raptor.can_be_held = TRUE
+	raptor.update_holdability(TRUE)
 	raptor.density = FALSE
 	raptor.move_resist = MOVE_RESIST_DEFAULT
 	raptor.held_w_class = WEIGHT_CLASS_BULKY

code/modules/mob/living/basic/pets/cat/cat.dm

@@ -24,7 +24,6 @@
 	response_harm_simple = "kick"
 	mobility_flags = MOBILITY_FLAGS_REST_CAPABLE_DEFAULT
 	gold_core_spawnable = FRIENDLY_SPAWN
-	can_be_held = TRUE
 	ai_controller = /datum/ai_controller/basic_controller/cat
 	held_state = "cat2"
 	attack_verb_continuous = "claws"
@@ -91,6 +90,7 @@
 	AddElement(/datum/element/ai_retaliate)
 	AddElement(/datum/element/pet_bonus, "purr", /datum/mood_event/pet_animal)
 	AddElement(/datum/element/footstep, footstep_type = FOOTSTEP_MOB_CLAW)
+	AddElement(/datum/element/can_be_held)
 	add_cell_sample()
 	add_verb(src, /mob/living/proc/toggle_resting)
 	add_traits(list(TRAIT_CATLIKE_GRACE, TRAIT_VENTCRAWLER_ALWAYS, TRAIT_WOUND_LICKER, TRAIT_COLORBLIND), INNATE_TRAIT)

code/modules/mob/living/basic/pets/dog/_dog.dm

@@ -27,7 +27,6 @@
 	response_harm_simple = "kick"
 	speak_emote = list("barks", "woofs")
 	faction = list(FACTION_NEUTRAL)
-	can_be_held = TRUE
 	ai_controller = /datum/ai_controller/basic_controller/dog
 	// The dog attack pet command can raise melee attack above 0
 	attack_verb_continuous = "bites"
@@ -74,6 +73,7 @@
 	AddElement(/datum/element/pet_bonus, "woof")
 	AddElement(/datum/element/footstep, FOOTSTEP_MOB_CLAW)
 	AddElement(/datum/element/unfriend_attacker, untamed_reaction = "%SOURCE% fixes %TARGET% with a look of betrayal.")
+	AddElement(/datum/element/can_be_held)
 	var/static/list/food_types = list(
 		/obj/item/food/meat/slab/human/mutant/skeleton,
 		/obj/item/stack/sheet/bone,

code/modules/mob/living/basic/pets/fox.dm

@@ -19,7 +19,6 @@
 	response_harm_continuous = "kicks"
 	response_harm_simple = "kick"
 	gold_core_spawnable = FRIENDLY_SPAWN
-	can_be_held = TRUE
 	held_state = "fox"
 	melee_damage_lower = 5
 	melee_damage_upper = 5
@@ -58,6 +57,7 @@
 	AddElement(/datum/element/footstep, footstep_type = FOOTSTEP_MOB_CLAW)
 	AddElement(/datum/element/tiny_mob_hunter, MOB_SIZE_SMALL)
 	AddElement(/datum/element/ai_retaliate)
+	AddElement(/datum/element/can_be_held)
 
 /datum/ai_controller/basic_controller/fox
 	blackboard = list(

code/modules/mob/living/basic/pets/sloth.dm

@@ -10,7 +10,6 @@ GLOBAL_DATUM(cargo_sloth, /mob/living/basic/sloth)
 
 	speak_emote = list("yawns")
 
-	can_be_held = TRUE
 	held_state = "sloth"
 
 	response_help_continuous = "pets"
@@ -53,6 +52,7 @@ GLOBAL_DATUM(cargo_sloth, /mob/living/basic/sloth)
 	AddElement(/datum/element/pet_bonus, "ssmile")
 	AddElement(/datum/element/footstep, footstep_type = FOOTSTEP_MOB_CLAW)
 	AddElement(/datum/element/ai_retaliate)
+	AddElement(/datum/element/can_be_held)
 	AddComponent(/datum/component/tree_climber)
 
 	if(!mapload || !isnull(GLOB.cargo_sloth) || !is_station_level(z))

code/modules/mob/living/basic/snails/snail.dm

@@ -19,7 +19,6 @@
 	speed = 6
 	verb_say = "gurgles"
 	verb_ask = "gurgles curiously"
-	can_be_held = TRUE
 	verb_exclaim = "gurgles loudly"
 	verb_yell = "gurgles loudly"
 	worn_slot_flags = ITEM_SLOT_HEAD
@@ -28,6 +27,8 @@
 	ai_controller = /datum/ai_controller/basic_controller/snail
 	/// What do we turn into if effected by a regal rat?
 	var/minion_path = /mob/living/basic/snail/angry
+	/// Are we able to be held by a player?
+	var/should_be_holdable = TRUE
 
 /mob/living/basic/snail/Initialize(mapload)
 	. = ..()
@@ -53,6 +54,9 @@
 	if (minion_path)
 		AddElement(/datum/element/regal_rat_minion, converted_path = minion_path, success_balloon = "gurgle", pet_commands = GLOB.regal_rat_minion_commands)
 
+	if(should_be_holdable)
+		AddElement(/datum/element/can_be_held)
+
 /mob/living/basic/snail/proc/on_entered(datum/source, obj/effect/decal/cleanable/food/salt/potential_salt)
 	SIGNAL_HANDLER
 	if(istype(potential_salt))
@@ -105,7 +109,7 @@
 	melee_damage_lower = 5
 	melee_damage_upper = 8
 	obj_damage = 8
-	can_be_held = FALSE
+	should_be_holdable = FALSE
 	minion_path = null
 	ai_controller = /datum/ai_controller/basic_controller/snail/trash
 

code/modules/mob/living/basic/space_fauna/ant.dm

@@ -26,7 +26,6 @@
 	response_harm_simple = "kick"
 	gold_core_spawnable = FRIENDLY_SPAWN
 	faction = list(FACTION_NEUTRAL)
-	can_be_held = FALSE
 	health = 100
 	maxHealth = 100
 	light_range = 1.5 // Bioluminescence!

code/modules/mob/living/basic/space_fauna/spider/spider.dm

@@ -200,7 +200,6 @@
 	icon_state = "maint_spider"
 	icon_living = "maint_spider"
 	icon_dead = "maint_spider_dead"
-	can_be_held = TRUE
 	mob_size = MOB_SIZE_TINY
 	held_w_class = WEIGHT_CLASS_TINY
 	worn_slot_flags = ITEM_SLOT_HEAD
@@ -234,3 +233,4 @@
 	AddElement(/datum/element/ai_retaliate)
 	AddComponent(/datum/component/obeys_commands, pet_commands)
 	AddElement(/datum/element/tiny_mob_hunter)
+	AddElement(/datum/element/can_be_held)

code/modules/mob/living/basic/stoats/stoat.dm

@@ -19,7 +19,6 @@
 	response_help_simple = "pet"
 	verb_say = "chips"
 	verb_ask = "chips curiously"
-	can_be_held = TRUE
 	verb_exclaim = "chips loudly"
 	verb_yell = "chips loudly"
 	faction = list(FACTION_NEUTRAL)
@@ -47,6 +46,7 @@
 	AddComponent(/datum/component/tameable, food_types = eatable_food, tame_chance = 70, bonus_tame_chance = 0)
 	ai_controller.set_blackboard_key(BB_BASIC_FOODS, typecacheof(eatable_food))
 	AddElement(/datum/element/wears_collar)
+	AddElement(/datum/element/can_be_held)
 	AddComponent(/datum/component/obeys_commands, pet_commands)
 	if(can_breed)
 		add_breeding_component()

code/modules/mob/living/basic/vermin/axolotl.dm

@@ -22,7 +22,6 @@
 	response_harm_continuous = "splats"
 	response_harm_simple = "splat"
 
-	can_be_held = TRUE
 	held_w_class = WEIGHT_CLASS_TINY
 	held_lh = 'icons/mob/inhands/animal_item_lefthand.dmi'
 	held_rh = 'icons/mob/inhands/animal_item_righthand.dmi'
@@ -35,6 +34,7 @@
 	. = ..()
 	add_traits(list(TRAIT_NODROWN, TRAIT_SWIMMER, TRAIT_VENTCRAWLER_ALWAYS), INNATE_TRAIT)
 	AddElement(/datum/element/swabable, CELL_LINE_TABLE_AXOLOTL, CELL_VIRUS_TABLE_GENERIC_MOB, 1, 5)
+	AddElement(/datum/element/can_be_held)
 
 /datum/ai_controller/basic_controller/axolotl
 	ai_traits = PASSIVE_AI_FLAGS

code/modules/mob/living/basic/vermin/cockroach/cockroach.dm

@@ -10,7 +10,6 @@
 	health = 1
 	maxHealth = 1
 	speed = 1.25
-	can_be_held = TRUE
 	gold_core_spawnable = FRIENDLY_SPAWN
 	pass_flags = PASSTABLE | PASSGRILLE | PASSMOB
 
@@ -50,6 +49,7 @@
 	. = ..()
 	AddElement(/datum/element/death_drops, /obj/effect/decal/cleanable/insectguts)
 	AddElement(/datum/element/swabable, cockroach_cell_line, CELL_VIRUS_TABLE_GENERIC_MOB, 1, 7)
+	AddElement(/datum/element/can_be_held)
 	AddComponent( \
 		/datum/component/squashable, \
 		squash_chance = 50, \

code/modules/mob/living/basic/vermin/frog.dm

@@ -31,7 +31,6 @@
 	pass_flags = PASSTABLE | PASSGRILLE | PASSMOB
 	mob_size = MOB_SIZE_TINY
 	gold_core_spawnable = FRIENDLY_SPAWN
-	can_be_held = TRUE
 	held_w_class = WEIGHT_CLASS_TINY
 	worn_slot_flags = ITEM_SLOT_HEAD
 	head_icon = 'icons/mob/clothing/head/pets_head.dmi'
@@ -60,6 +59,7 @@
 	AddElement(/datum/element/venomous, poison_type, poison_per_bite)
 	AddElement(/datum/element/ai_retaliate)
 	AddElement(/datum/element/swabable, CELL_LINE_TABLE_FROG, CELL_VIRUS_TABLE_GENERIC_MOB, 1, 5)
+	AddElement(/datum/element/can_be_held)
 	if (minion_type)
 		AddElement(/datum/element/regal_rat_minion, converted_path = minion_type, success_balloon = "ribbit", pet_commands = GLOB.regal_rat_minion_commands)
 

code/modules/mob/living/basic/vermin/lizard.dm

@@ -32,7 +32,6 @@
 	gold_core_spawnable = FRIENDLY_SPAWN
 	obj_damage = 0
 	environment_smash = ENVIRONMENT_SMASH_NONE
-	can_be_held = TRUE
 	held_w_class = WEIGHT_CLASS_TINY
 	held_lh = 'icons/mob/inhands/animal_item_lefthand.dmi'
 	held_rh = 'icons/mob/inhands/animal_item_righthand.dmi'
@@ -60,6 +59,7 @@
 	. = ..()
 	ADD_TRAIT(src, TRAIT_VENTCRAWLER_ALWAYS, INNATE_TRAIT)
 	AddElement(/datum/element/pet_bonus, "tongue")
+	AddElement(/datum/element/can_be_held)
 	AddElement(/datum/element/basic_eating, heal_amt = 5, food_types = edibles)
 	ai_controller.set_blackboard_key(BB_BASIC_FOODS, typecacheof(edibles))
 

code/modules/mob/living/basic/vermin/mothroach/mothroach.dm

@@ -16,7 +16,6 @@
 	maxHealth = 25
 	speed = 1.25
 	gold_core_spawnable = FRIENDLY_SPAWN
-	can_be_held = TRUE
 	worn_slot_flags = ITEM_SLOT_HEAD
 
 	verb_say = "flutters"
@@ -60,6 +59,7 @@
 	ai_controller.set_blackboard_key(BB_BASIC_FOODS, typecacheof(food_types))
 	AddElement(/datum/element/ai_retaliate)
 	AddElement(/datum/element/pet_bonus, "squeak")
+	AddElement(/datum/element/can_be_held)
 	add_verb(src, /mob/living/proc/toggle_resting)
 	ADD_TRAIT(src, TRAIT_VENTCRAWLER_ALWAYS, INNATE_TRAIT)
 

code/modules/mob/living/basic/vermin/mouse.dm

@@ -11,7 +11,6 @@
 	density = FALSE
 	pass_flags = PASSTABLE|PASSGRILLE|PASSMOB
 	mob_size = MOB_SIZE_TINY
-	can_be_held = TRUE
 	held_w_class = WEIGHT_CLASS_TINY
 	mob_biotypes = MOB_ORGANIC|MOB_BEAST
 	gold_core_spawnable = FRIENDLY_SPAWN
@@ -78,6 +77,7 @@
 	AddElement(/datum/element/connect_loc, loc_connections)
 	make_tameable()
 	AddComponent(/datum/component/swarming, 16, 16) //max_x, max_y
+	AddElement(/datum/element/can_be_held)
 
 /mob/living/basic/mouse/proc/make_tameable()
 	if (HAS_TRAIT(src, TRAIT_TAMED))