Ports mod link system and scryers (#28479)

* commiting

* and this

* the rest of the fucking owl

* name moment

* in my defense it printed

* Apply suggestions from code review

Co-authored-by: PollardTheDragon <144391971+PollardTheDragon@users.noreply.github.com>
Signed-off-by: Qwertytoforty <52090703+Qwertytoforty@users.noreply.github.com>

* woe old code removey

* and tgui / comment

* Apply suggestions from code review

Co-authored-by: Burzah <116982774+Burzah@users.noreply.github.com>
Signed-off-by: Qwertytoforty <52090703+Qwertytoforty@users.noreply.github.com>

---------

Signed-off-by: Qwertytoforty <52090703+Qwertytoforty@users.noreply.github.com>
Co-authored-by: PollardTheDragon <144391971+PollardTheDragon@users.noreply.github.com>
Co-authored-by: Burzah <116982774+Burzah@users.noreply.github.com>

code/modules/mod/mod_activation.dm

@@ -154,6 +154,7 @@
 		if(!module.active || (module.allow_flags & MODULE_ALLOW_INACTIVE))
 			continue
 		module.on_deactivation(display_message = FALSE)
+	mod_link.end_call()
 	activating = TRUE
 	to_chat(wearer, "<span class='notice'>MODsuit [active ? "shutting down" : "starting up"].</span>")
 	if(do_after(wearer, activation_step_time, FALSE, target = src, allow_moving = TRUE, use_default_checks = FALSE))
@@ -210,17 +211,15 @@
 		var/mob/living/carbon/human/H = wearer
 		H.regenerate_icons()
 
-/// Finishes the suit's activation, starts processing
+/// Finishes the suit's activation
 /obj/item/mod/control/proc/finish_activation(on)
 	active = on
 	if(active)
 		for(var/obj/item/mod/module/module as anything in modules)
 			module.on_suit_activation()
-		START_PROCESSING(SSobj, src)
 	else
 		for(var/obj/item/mod/module/module as anything in modules)
 			module.on_suit_deactivation()
-		STOP_PROCESSING(SSobj, src)
 	update_speed()
 	update_icon(UPDATE_ICON_STATE)
 	wearer.regenerate_icons()

code/modules/mod/mod_control.dm

@@ -91,6 +91,10 @@
 	var/datum/action/item_action/chameleon_change/modsuit/chameleon_action
 	/// Is the control unit disquised?
 	var/current_disguise = FALSE
+	/// The MODlink datum, letting us call people from the suit.
+	var/datum/mod_link/mod_link
+	/// The starting MODlink frequency, overridden on subtypes that want it to be something.
+	var/starting_frequency = null
 
 /obj/item/mod/control/serialize()
 	var/list/data = ..()
@@ -157,10 +161,19 @@
 		module = new module(src)
 		install(module)
 	ADD_TRAIT(src, TRAIT_ADJACENCY_TRANSPARENT, ROUNDSTART_TRAIT)
+	START_PROCESSING(SSobj, src)
+	mod_link = new(
+		src,
+		starting_frequency,
+		CALLBACK(src, PROC_REF(get_wearer)),
+		CALLBACK(src, PROC_REF(can_call)),
+		CALLBACK(src, PROC_REF(make_link_visual)),
+		CALLBACK(src, PROC_REF(get_link_visual)),
+		CALLBACK(src, PROC_REF(delete_link_visual))
+	)
 
 /obj/item/mod/control/Destroy()
-	if(active)
-		STOP_PROCESSING(SSobj, src)
+	STOP_PROCESSING(SSobj, src)
 	for(var/obj/item/mod/module/module as anything in modules)
 		uninstall(module, deleting = TRUE)
 	for(var/obj/item/part as anything in mod_parts)
@@ -179,6 +192,7 @@
 		QDEL_NULL(boots)
 	if(core)
 		QDEL_NULL(core)
+	QDEL_NULL(mod_link)
 	QDEL_NULL(wires)
 	wearer = null
 	selected_module = null
@@ -206,6 +220,7 @@
 			. += "You could remove [core] with a <b>wrench</b>."
 		else
 			. += "You could use a <b>MOD core</b> on it to install one."
+	. += "You could copy/set link frequency with a <b>multitool</b>."
 
 /obj/item/mod/control/examine_more(mob/user)
 	. = ..()
@@ -214,9 +229,13 @@
 /obj/item/mod/control/process()
 	if(seconds_electrified > 0)
 		seconds_electrified--
+	if(mod_link.link_call)
+		subtract_charge((DEFAULT_CHARGE_DRAIN * 0.25))
+	if(!active)
+		return
 	if(get_charge() <= 10 && active && !activating) //Sometimes we get power being funky, this should fix it.
 		power_off()
-		return PROCESS_KILL
+		return
 	var/malfunctioning_charge_drain = 0
 	if(malfunctioning)
 		malfunctioning_charge_drain = rand(1, 20)
@@ -294,6 +313,49 @@
 		return TRUE
 	return ..()
 
+/obj/item/mod/control/multitool_act(mob/living/user, obj/item/I)
+	. = TRUE
+	if(!I.use_tool(src, user, 0, volume = I.tool_volume))
+		return
+	if(!I.multitool_check_buffer(user))
+		return
+	var/obj/item/multitool/M = I
+	var/obj/item/linked_thing = locateUID(M.buffer_uid)
+
+	if(!linked_thing)
+		to_chat(user, "<span class='notice'>You save the frequency of [src] to the buffer.</span>")
+		M.buffer_uid = UID()
+		return TRUE
+	if(ismodcontrol(linked_thing))
+		var/obj/item/mod/control/chosen_control = linked_thing
+		var/response = tgui_alert(user, "Would you like to copy the frequency to the multitool or imprint the frequency to [src]?", "MODlink Frequency", list("Copy", "Imprint"))
+		if(!user.is_holding(I))
+			return FALSE
+		switch(response)
+			if("Copy")
+				to_chat(user, "<span class='notice'>You save the frequency of [src] to the buffer.</span>")
+				M.buffer_uid = UID()
+				return TRUE
+			if("Imprint")
+				mod_link.frequency = chosen_control.mod_link.frequency
+				to_chat(user, "<span class='notice'>You imprint the frequency to [src].</span>")
+				return TRUE
+	else
+		var/obj/item/clothing/neck/link_scryer/chosen_scryer = linked_thing
+		var/response = tgui_alert(user, "Would you like to copy or imprint the frequency?", "MODlink Frequency", list("Copy", "Imprint"))
+		if(!user.is_holding(I))
+			return FALSE
+		switch(response)
+			if("Copy")
+				mod_link.frequency = chosen_scryer.mod_link.frequency
+				to_chat(user, "<span class='notice'>You imprint the frequency to [src].</span>")
+				return TRUE
+			if("Imprint")
+				to_chat(user, "<span class='notice'>You save the frequency of [src] to the buffer.</span>")
+				M.buffer_uid = UID()
+				return TRUE
+
+
 /obj/item/mod/control/screwdriver_act(mob/living/user, obj/item/screwdriver)
 	if(..())
 		return TRUE
@@ -522,6 +584,7 @@
 		retract(null, part)
 	if(active)
 		finish_activation(on = FALSE)
+		mod_link?.end_call()
 	var/mob/old_wearer = wearer
 	unset_wearer()
 	old_wearer.drop_item()
@@ -605,6 +668,10 @@
 	QDEL_LIST_ASSOC_VAL(old_module.pinned_to)
 	old_module.mod = null
 
+/// Intended for callbacks, don't use normally, just get wearer by itself.
+/obj/item/mod/control/proc/get_wearer()
+	return wearer
+
 /obj/item/mod/control/proc/update_access(mob/user, obj/item/card/id/card)
 	if(!allowed(user))
 		to_chat(user, "<span class='warning'>Insufficient access!")

code/modules/mod/mod_link.dm

@@ -0,0 +1,557 @@
+/proc/make_link_visual_generic(datum/mod_link/mod_link, proc_path)
+	var/mob/living/user = mod_link.get_user_callback.Invoke()
+	var/obj/effect/overlay/link_visual = new()
+	link_visual.name = "holocall ([mod_link.id])"
+	link_visual.mouse_opacity = MOUSE_OPACITY_TRANSPARENT
+	link_visual.appearance_flags |= KEEP_TOGETHER
+	link_visual.makeHologram(0.5)
+	mod_link.visual_overlays = user.overlays
+	link_visual.add_overlay(mod_link.visual_overlays)
+	mod_link.visual = link_visual
+	mod_link.holder.RegisterSignals(user, list(COMSIG_CARBON_APPLY_OVERLAY, COMSIG_CARBON_REMOVE_OVERLAY), proc_path)
+	return link_visual
+
+/// Makes this atom look like a "hologram"
+/// So transparent, blue, with a scanline and an emissive glow
+/// This is acomplished using a combination of filters and render steps/overlays
+/// The degree of the opacity is optional, based off the opacity arg (0 -> 1)
+/atom/proc/makeHologram(opacity = 0.5)
+	// First, we'll make things blue (roughly) and sorta transparent
+	add_filter("HOLO: Color and Transparent", 1, color_matrix_filter(rgb(125, 180, 225, opacity * 255)))
+	// Now we're gonna do a scanline effect
+	// Gonna take this atom and give it a render target, then use it as a source for a filter
+	// (We use an atom because it seems as if setting render_target on an MA is just invalid. I hate this engine)
+	var/atom/movable/scanline = new(null)
+	scanline.icon = 'icons/effects/effects.dmi'
+	scanline.icon_state = "scanline"
+	scanline.appearance_flags |= RESET_TRANSFORM
+	// * so it doesn't render
+	var/static/uid_scan = 0
+	scanline.render_target = "*HoloScanline [uid_scan]"
+	uid_scan++
+	// Now we add it as a filter, and overlay the appearance so the render source is always around
+	add_filter("HOLO: Scanline", 2, alpha_mask_filter(render_source = scanline.render_target))
+	add_overlay(scanline)
+	qdel(scanline)
+	// And let's make the sucker emissive, so it glows in the dark
+	if(!render_target)
+		var/static/uid = 0
+		render_target = "HOLOGRAM [uid]"
+		uid++
+
+/proc/get_link_visual_generic(datum/mod_link/mod_link, atom/movable/visuals, proc_path)
+	var/mob/living/user = mod_link.get_user_callback.Invoke()
+	playsound(mod_link.holder, 'sound/machines/terminal_processing.ogg', 50, TRUE)
+	visuals.add_overlay(mutable_appearance('icons/effects/effects.dmi', "static_base", ABOVE_NORMAL_TURF_LAYER))
+	visuals.add_overlay(mutable_appearance('icons/effects/effects.dmi', "modlink", ABOVE_ALL_MOB_LAYER))
+	visuals.add_filter("crop_square", 1, alpha_mask_filter(icon = icon('icons/effects/effects.dmi', "modlink_filter")))
+	visuals.maptext_height = 6
+	visuals.alpha = 0
+	user.vis_contents += visuals
+	visuals.forceMove(user)
+	animate(visuals, 0.5 SECONDS, alpha = 255)
+	var/datum/callback/setdir_callback = CALLBACK(mod_link.holder, proc_path)
+	setdir_callback.Invoke(user, user.dir, user.dir)
+	mod_link.holder.RegisterSignal(mod_link.holder.loc, COMSIG_ATOM_DIR_CHANGE, proc_path)
+
+/proc/delete_link_visual_generic(datum/mod_link/mod_link)
+	var/mob/living/user = mod_link.get_user_callback.Invoke()
+	playsound(mod_link.get_other().holder, 'sound/machines/terminal_processing.ogg', 50, TRUE, -1)
+	mod_link.holder.UnregisterSignal(user, list(COMSIG_CARBON_APPLY_OVERLAY, COMSIG_CARBON_REMOVE_OVERLAY, COMSIG_ATOM_DIR_CHANGE))
+	QDEL_NULL(mod_link.visual)
+
+/proc/on_user_set_dir_generic(datum/mod_link/mod_link, newdir)
+	var/atom/other_visual = mod_link.get_other().visual
+	if(!newdir) // can sometimes be null or 0
+		return
+	other_visual.setDir(SOUTH)
+	other_visual.pixel_x = 0
+	other_visual.pixel_y = 0
+	var/matrix/new_transform = matrix()
+	if(newdir & NORTH)
+		other_visual.pixel_y = 13
+		other_visual.layer = BELOW_MOB_LAYER
+	if(newdir & SOUTH)
+		other_visual.pixel_y = -24
+		other_visual.layer = ABOVE_ALL_MOB_LAYER
+		new_transform.Scale(-1, 1)
+		new_transform.Translate(-1, 0)
+	if(newdir & EAST)
+		other_visual.pixel_x = 14
+		other_visual.layer = BELOW_MOB_LAYER
+		new_transform.Shear(0.5, 0)
+		new_transform.Scale(0.65, 1)
+	if(newdir & WEST)
+		other_visual.pixel_x = -14
+		other_visual.layer = BELOW_MOB_LAYER
+		new_transform.Shear(-0.5, 0)
+		new_transform.Scale(0.65, 1)
+	other_visual.transform = new_transform
+
+/obj/item/mod/control/proc/can_call()
+	return get_charge() && wearer && wearer.stat < DEAD
+
+/obj/item/mod/control/proc/make_link_visual()
+	return make_link_visual_generic(mod_link, PROC_REF(on_overlay_change))
+
+/obj/item/mod/control/proc/get_link_visual(atom/movable/visuals)
+	return get_link_visual_generic(mod_link, visuals, PROC_REF(on_wearer_set_dir))
+
+/obj/item/mod/control/proc/delete_link_visual()
+	return delete_link_visual_generic(mod_link)
+
+/obj/item/mod/control/hear_talk(mob/living/M, list/message_pieces)
+	. = ..()
+	if(M != wearer || !mod_link.visual)
+		return
+	if(ishuman(M))
+		var/mob/living/carbon/human/H = M
+		if(H.voice != M.real_name)
+			mod_link.visual.name = H.voice
+		else
+			mod_link.visual.name = M.name
+	else
+		mod_link.visual.name = M.name
+	mod_link.visual.atom_say(capitalize(multilingual_to_message(message_pieces)))
+
+/obj/item/mod/control/proc/on_overlay_change(atom/source, cache_index, overlay)
+	SIGNAL_HANDLER // COMSIG_CARBON_APPLY_OVERLAY, COMSIG_CARBON_REMOVE_OVERLAY
+	addtimer(CALLBACK(src, PROC_REF(update_link_visual)), 0.2 SECONDS, TIMER_UNIQUE)
+
+/obj/item/mod/control/proc/update_link_visual()
+	if(QDELETED(mod_link.link_call))
+		return
+	mod_link.visual.cut_overlay(mod_link.visual_overlays)
+	mod_link.visual_overlays = wearer.overlays
+	mod_link.visual.add_overlay(mod_link.visual_overlays)
+
+/obj/item/mod/control/proc/on_wearer_set_dir(atom/source, dir, newdir)
+	SIGNAL_HANDLER // COMSIG_ATOM_DIR_CHANGE
+	on_user_set_dir_generic(mod_link, newdir || SOUTH)
+
+/obj/item/clothing/neck/link_scryer
+	name = "\improper MODlink scryer"
+	desc = "An intricate piece of machinery that creates a holographic video call with another MODlink-compatible device. Essentially a video necklace."
+	icon_state = "modlink"
+	actions_types = list(/datum/action/item_action/call_link)
+	/// The installed power cell.
+	var/obj/item/stock_parts/cell/cell
+	/// The MODlink datum we operate.
+	var/datum/mod_link/mod_link
+	/// Initial frequency of the MODlink.
+	var/starting_frequency
+	/// An additional name tag for the scryer, seen as "MODlink scryer - [label]"
+	var/label
+
+/obj/item/clothing/neck/link_scryer/Initialize(mapload)
+	. = ..()
+	mod_link = new(
+		src,
+		starting_frequency,
+		CALLBACK(src, PROC_REF(get_user)),
+		CALLBACK(src, PROC_REF(can_call)),
+		CALLBACK(src, PROC_REF(make_link_visual)),
+		CALLBACK(src, PROC_REF(get_link_visual)),
+		CALLBACK(src, PROC_REF(delete_link_visual))
+	)
+	START_PROCESSING(SSobj, src)
+
+/obj/item/clothing/neck/link_scryer/Destroy()
+	QDEL_NULL(cell)
+	QDEL_NULL(mod_link)
+	STOP_PROCESSING(SSobj, src)
+	return ..()
+
+/obj/item/clothing/neck/link_scryer/examine(mob/user)
+	. = ..()
+	if(cell)
+		. += "<span class='notice'>The battery charge reads [cell.percent()]%. Use a <b>Screwdriver</b> to remove it.</span>"
+	else
+		. += "<span class='notice'>It is missing a battery, one can be installed by clicking with a power cell on it.</span>"
+	. += "<span class='notice'>The MODlink ID is [mod_link.id], frequency is [mod_link.frequency || "unset"]. <b>Use</b> a multitool to copy/imprint frequency.</span>"
+	. += "<span class='notice'>Use in hand to set name.</span>"
+
+/obj/item/clothing/neck/link_scryer/equipped(mob/living/user, slot)
+	. = ..()
+	if(slot != ITEM_SLOT_NECK)
+		mod_link?.end_call()
+
+/obj/item/clothing/neck/link_scryer/dropped(mob/living/user)
+	. = ..()
+	mod_link?.end_call()
+
+/obj/item/clothing/neck/link_scryer/attack_self__legacy__attackchain(mob/user)
+	var/new_label = reject_bad_text(tgui_input_text(user, "Change the visible name", "Set Name", label, MAX_NAME_LEN))
+	if(!user.is_holding(src))
+		return
+	if(!new_label)
+		to_chat(user, "<span class='warning'>That name is invalid!</span>")
+		return
+	label = new_label
+	to_chat(user, "<span class='notice'>You set the name as [label].</span>")
+	update_appearance(UPDATE_NAME)
+
+
+/obj/item/clothing/neck/link_scryer/multitool_act(mob/user, obj/item/I)
+	. = TRUE
+	if(!I.use_tool(src, user, 0, volume = I.tool_volume))
+		return
+	if(!I.multitool_check_buffer(user))
+		return
+	var/obj/item/multitool/M = I
+	var/obj/item/linked_thing = locateUID(M.buffer_uid)
+
+	if(!linked_thing)
+		to_chat(user, "<span class='notice'>You save the frequency of [src] to the buffer.</span>")
+		M.buffer_uid = UID()
+		return TRUE
+	if(ismodcontrol(linked_thing))
+		var/obj/item/mod/control/chosen_control = linked_thing
+		var/response = tgui_alert(user, "Would you like to copy the frequency to the multitool or imprint the frequency to [src]?", "MODlink Frequency", list("Copy", "Imprint"))
+		if(!user.is_holding(I))
+			return FALSE
+		switch(response)
+			if("Copy")
+				to_chat(user, "<span class='notice'>You save the frequency of [src] to the buffer.</span>")
+				M.buffer_uid = UID()
+				return TRUE
+			if("Imprint")
+				mod_link.frequency = chosen_control.mod_link.frequency
+				to_chat(user, "<span class='notice'>You imprint the frequency to [src].</span>")
+				return TRUE
+	else
+		var/obj/item/clothing/neck/link_scryer/chosen_scryer = linked_thing
+		var/response = tgui_alert(user, "Would you like to copy or imprint the frequency?", "MODlink Frequency", list("Copy", "Imprint"))
+		if(!user.is_holding(I))
+			return FALSE
+		switch(response)
+			if("Copy")
+				mod_link.frequency = chosen_scryer.mod_link.frequency
+				to_chat(user, "<span class='notice'>You imprint the frequency to [src].</span>")
+				return TRUE
+			if("Imprint")
+				to_chat(user, "<span class='notice'>You save the frequency of [src] to the buffer.</span>")
+				M.buffer_uid = UID()
+				return TRUE
+
+/obj/item/clothing/neck/link_scryer/get_cell()
+	return cell
+
+/obj/item/clothing/neck/link_scryer/process()
+	if(!mod_link.link_call)
+		return
+	cell.use(200)
+
+/obj/item/clothing/neck/link_scryer/attackby__legacy__attackchain(obj/item/O, mob/user, params)
+	. = ..()
+	if(cell || !istype(O, /obj/item/stock_parts/cell))
+		return
+	if(!user.drop_item())
+		to_chat(user, "<span class='warning'>[O] is stuck to your hand!</span>")
+		return
+	O.forceMove(src)
+	cell = O
+	to_chat(user, "<span class='notice'>You load [O] into [src].</span>")
+
+/obj/item/clothing/neck/link_scryer/update_name(updates)
+	. = ..()
+	name = "[initial(name)][label ? " - [label]" : ""]"
+
+/obj/item/clothing/neck/link_scryer/Exited(atom/movable/gone, direction)
+	. = ..()
+	if(gone == cell)
+		cell = null
+
+/obj/item/clothing/neck/link_scryer/screwdriver_act(mob/living/user, obj/item/I)
+	. = ..()
+	if(!cell)
+		return
+	to_chat(user, "<span class='notice'>You remove [cell] from [src].</span>")
+	user.put_in_hands(cell)
+	return
+
+/obj/item/clothing/neck/link_scryer/ui_action_click(mob/user)
+	if(mod_link.link_call)
+		mod_link.end_call()
+	else
+		call_link(user, mod_link)
+
+/obj/item/clothing/neck/link_scryer/proc/get_user()
+	var/mob/living/carbon/human/user = loc
+	return istype(user) && user.neck == src ? user : null
+
+/obj/item/clothing/neck/link_scryer/proc/can_call()
+	var/mob/living/user = loc
+	return istype(user) && cell?.charge && user.stat < DEAD
+
+/obj/item/clothing/neck/link_scryer/proc/make_link_visual()
+	var/mob/living/user = mod_link.get_user_callback.Invoke()
+	user.update_inv_neck()
+	return make_link_visual_generic(mod_link, PROC_REF(on_overlay_change))
+
+/obj/item/clothing/neck/link_scryer/proc/get_link_visual(atom/movable/visuals)
+	return get_link_visual_generic(mod_link, visuals, PROC_REF(on_user_set_dir))
+
+/obj/item/clothing/neck/link_scryer/proc/delete_link_visual()
+	var/mob/living/user = mod_link.get_user_callback.Invoke()
+	if(!QDELETED(user))
+		user.update_inv_neck()
+	return delete_link_visual_generic(mod_link)
+
+/obj/item/clothing/neck/link_scryer/hear_talk(mob/living/M, list/message_pieces)
+	. = ..()
+	if(M != loc || !mod_link.visual)
+		return
+	if(ishuman(M))
+		var/mob/living/carbon/human/H = M
+		if(H.voice != M.real_name)
+			mod_link.visual.name = H.voice
+	else
+		mod_link.visual.name = M.name
+	mod_link.visual.atom_say(capitalize(multilingual_to_message(message_pieces)))
+
+/obj/item/clothing/neck/link_scryer/proc/on_overlay_change(atom/source, cache_index, overlay)
+	SIGNAL_HANDLER // COMSIG_CARBON_APPLY_OVERLAY, COMSIG_CARBON_REMOVE_OVERLAY
+	addtimer(CALLBACK(src, PROC_REF(update_link_visual)), 0.2 SECONDS, TIMER_UNIQUE)
+
+/obj/item/clothing/neck/link_scryer/proc/update_link_visual()
+	if(QDELETED(mod_link.link_call))
+		return
+	var/mob/living/user = loc
+	mod_link.visual.cut_overlay(mod_link.visual_overlays)
+	mod_link.visual_overlays = user.overlays
+	mod_link.visual.add_overlay(mod_link.visual_overlays)
+
+/obj/item/clothing/neck/link_scryer/proc/on_user_set_dir(atom/source, dir, newdir)
+	SIGNAL_HANDLER // COMSIG_ATOM_DIR_CHANGE
+	on_user_set_dir_generic(mod_link, newdir || SOUTH)
+
+/obj/item/clothing/neck/link_scryer/loaded
+	starting_frequency = MODLINK_FREQ_NANOTRASEN
+
+/obj/item/clothing/neck/link_scryer/loaded/Initialize(mapload)
+	. = ..()
+	cell = new /obj/item/stock_parts/cell/high(src)
+
+/obj/item/clothing/neck/link_scryer/loaded/theta
+	starting_frequency = MODLINK_FREQ_THETA
+
+/// A MODlink datum, used to handle unique functions that will be used in the MODlink call.
+/datum/mod_link
+	/// Generic name for multitool buffers.
+	var/name = "MODlink"
+	/// The frequency of the MODlink. You can only call other MODlinks on the same frequency.
+	var/frequency
+	/// The unique ID of the MODlink.
+	var/id = ""
+	/// The atom that holds the MODlink.
+	var/atom/movable/holder
+	/// A reference to the visuals generated by the MODlink.
+	var/atom/movable/visual
+	/// A list of all overlays of the user, copied everytime they have an overlay change.
+	var/list/visual_overlays = list()
+	/// A reference to the call between two MODlinks.
+	var/datum/mod_link_call/link_call
+	/// A callback that returns the user of the MODlink.
+	var/datum/callback/get_user_callback
+	/// A callback that returns whether the MODlink can currently call.
+	var/datum/callback/can_call_callback
+	/// A callback that returns the visuals of the MODlink.
+	var/datum/callback/make_visual_callback
+	/// A callback that receives the visuals of the other MODlink.
+	var/datum/callback/get_visual_callback
+	/// A callback that deletes the visuals of the MODlink.
+	var/datum/callback/delete_visual_callback
+
+/datum/mod_link/New(
+	atom/holder,
+	frequency,
+	datum/callback/get_user_callback,
+	datum/callback/can_call_callback,
+	datum/callback/make_visual_callback,
+	datum/callback/get_visual_callback,
+	datum/callback/delete_visual_callback
+)
+	var/attempts = 0
+	var/digits_to_make = 3
+	do
+		if(attempts == 10)
+			attempts = 0
+			digits_to_make++
+		id = ""
+		for(var/i in 1 to digits_to_make)
+			id += num2text(rand(0, 9))
+		attempts++
+	while(GLOB.mod_link_ids[id])
+	GLOB.mod_link_ids[id] = src
+	src.frequency = frequency
+	src.holder = holder
+	src.get_user_callback = get_user_callback
+	src.can_call_callback = can_call_callback
+	src.make_visual_callback = make_visual_callback
+	src.get_visual_callback = get_visual_callback
+	src.delete_visual_callback = delete_visual_callback
+	RegisterSignal(holder, COMSIG_PARENT_QDELETING, PROC_REF(on_holder_delete))
+
+/datum/mod_link/Destroy()
+	GLOB.mod_link_ids -= id
+	if(link_call)
+		end_call()
+	get_user_callback = null
+	make_visual_callback = null
+	get_visual_callback = null
+	delete_visual_callback = null
+	return ..()
+
+/datum/mod_link/proc/get_other()
+	RETURN_TYPE(/datum/mod_link)
+	if(!link_call)
+		return
+	return link_call.link_caller == src ? link_call.link_receiver : link_call.link_caller
+
+/datum/mod_link/proc/call_link(datum/mod_link/called, mob/user)
+	if(!frequency)
+		return
+	if(!istype(called))
+		to_chat(user, "<span class='warning'>That target is invalid!</span>")
+		return
+	var/mob/living/link_user = get_user_callback.Invoke()
+	if(!link_user)
+		return
+	if(HAS_TRAIT(link_user, TRAIT_IN_CALL))
+		to_chat(user, "<span class='warning'>You are already calling someone!</span>")
+		return
+	var/mob/living/link_target = called.get_user_callback.Invoke()
+	if(!link_target)
+		to_chat(user, "<span class='warning'>That target is invalid!</span>")
+		return
+	if(HAS_TRAIT(link_target, TRAIT_IN_CALL))
+		to_chat(user, "<span class='warning'>The person you are calling is already in a call!</span>")
+		return
+	if(!can_call_callback.Invoke() || !called.can_call_callback.Invoke())
+		to_chat(user, "<span class='warning'>You are unable to call at this time!</span>")
+		return
+	link_target.playsound_local(get_turf(called.holder), 'sound/weapons/ring.ogg', 15, TRUE)
+	var/atom/movable/screen/alert/modlink_call/alert = link_target.throw_alert("[UID()]_modlink", /atom/movable/screen/alert/modlink_call)
+	alert.desc = "[holder] ([id]) is calling you! Left-click this to accept the call. Right-click to deny it."
+	alert.link_caller_uid = UID()
+	alert.link_receiver_uid = called.UID()
+	alert.user_uid = user.UID()
+
+/datum/mod_link/proc/end_call()
+	QDEL_NULL(link_call)
+
+/datum/mod_link/proc/on_holder_delete(atom/source)
+	SIGNAL_HANDLER
+	qdel(src)
+
+/// A MODlink call datum, used to handle the call between two MODlinks.
+/datum/mod_link_call
+	/// The MODlink that is calling.
+	var/datum/mod_link/link_caller
+	/// The MODlink that is being called.
+	var/datum/mod_link/link_receiver
+
+/datum/mod_link_call/New(datum/mod_link/link_caller, datum/mod_link/link_receiver)
+	link_caller.link_call = src
+	link_receiver.link_call = src
+	src.link_caller = link_caller
+	src.link_receiver = link_receiver
+	var/mob/living/caller_mob = link_caller.get_user_callback.Invoke()
+	ADD_TRAIT(caller_mob, TRAIT_IN_CALL, UID())
+	var/mob/living/receiver_mob = link_receiver.get_user_callback.Invoke()
+	ADD_TRAIT(receiver_mob, TRAIT_IN_CALL, UID())
+	make_visuals()
+	START_PROCESSING(SSprocessing, src)
+
+/datum/mod_link_call/Destroy()
+	var/mob/living/caller_mob = link_caller.get_user_callback.Invoke()
+	if(!QDELETED(caller_mob))
+		REMOVE_TRAIT(caller_mob, TRAIT_IN_CALL, UID())
+	var/mob/living/receiver_mob = link_receiver.get_user_callback.Invoke()
+	if(!QDELETED(receiver_mob))
+		REMOVE_TRAIT(receiver_mob, TRAIT_IN_CALL, UID())
+	STOP_PROCESSING(SSprocessing, src)
+	clear_visuals()
+	link_caller.link_call = null
+	link_receiver.link_call = null
+	return ..()
+
+/datum/mod_link_call/process(seconds_per_tick)
+	if(can_continue_call())
+		return
+	qdel(src)
+
+/datum/mod_link_call/proc/can_continue_call()
+	return link_caller.frequency == link_receiver.frequency && link_caller.can_call_callback.Invoke() && link_receiver.can_call_callback.Invoke()
+
+/datum/mod_link_call/proc/make_visuals()
+	var/caller_visual = link_caller.make_visual_callback.Invoke()
+	var/receiver_visual = link_receiver.make_visual_callback.Invoke()
+	link_caller.get_visual_callback.Invoke(receiver_visual)
+	link_receiver.get_visual_callback.Invoke(caller_visual)
+
+/datum/mod_link_call/proc/clear_visuals()
+	link_caller.delete_visual_callback.Invoke()
+	link_receiver.delete_visual_callback.Invoke()
+
+/proc/call_link(mob/user, datum/mod_link/calling_link)
+	if(!calling_link.frequency)
+		return
+	var/list/callers = list()
+	for(var/id in GLOB.mod_link_ids)
+		var/datum/mod_link/link = GLOB.mod_link_ids[id]
+		if(link.frequency != calling_link.frequency)
+			continue
+		if(link == calling_link)
+			continue
+		if(!link.can_call_callback.Invoke())
+			continue
+		callers["[link.holder] ([id])"] = id
+	if(!length(callers))
+		to_chat(user, "<span class='notice'>There are no targets on freq [calling_link.frequency].</span>")
+		return
+	var/chosen_link = tgui_input_list(user, "Choose ID to call from [calling_link.frequency] frequency", "MODlink", callers)
+	if(!chosen_link)
+		return
+	calling_link.call_link(GLOB.mod_link_ids[callers[chosen_link]], user)
+
+/atom/movable/screen/alert/modlink_call
+	name = "MODlink Call Incoming"
+	desc = "Someone is calling you! Left-click this to accept the call. Alt-click to deny it."
+	icon_state = "called"
+	timeout = 10 SECONDS
+	var/end_message = "call timed out!"
+	/// A UID to the MODlink that is calling.
+	var/link_caller_uid
+	/// A UID to the MODlink that is being called.
+	var/link_receiver_uid
+	/// A UID to the mob that is calling.
+	var/user_uid
+
+/atom/movable/screen/alert/modlink_call/Click(location, control, params)
+	. = ..()
+	if(usr != owner)
+		return
+	var/datum/mod_link/link_caller = locateUID(link_caller_uid)
+	var/datum/mod_link/link_receiver = locateUID(link_receiver_uid)
+	if(!link_caller || !link_receiver)
+		return
+	if(link_caller.link_call || link_receiver.link_call)
+		return
+	var/list/modifiers = params2list(params)
+	if(LAZYACCESS(modifiers, ALT_CLICK))
+		end_message = "The call was denied!"
+		owner.clear_alert("[link_caller.UID()]_modlink")
+		return
+	end_message = "The call was accepted!"
+	new /datum/mod_link_call(link_caller, link_receiver)
+	owner.clear_alert("[link_caller.UID()]_modlink")
+
+/atom/movable/screen/alert/modlink_call/Destroy()
+	var/mob/living/user = locateUID(user_uid)
+	var/datum/mod_link/link_caller = locateUID(link_caller_uid)
+	if(!user || !link_caller)
+		return ..()
+	to_chat(user, "<span class='notice'>[end_message]</span>")
+	return ..()

code/modules/mod/mod_types.dm

@@ -1,4 +1,5 @@
 /obj/item/mod/control/pre_equipped
+	starting_frequency = MODLINK_FREQ_NANOTRASEN
 	/// The skin we apply to the suit, defaults to the default_skin of the theme.
 	var/applied_skin
 	/// The MOD core we apply to the suit.
@@ -252,6 +253,7 @@
 	)
 
 /obj/item/mod/control/pre_equipped/traitor
+	starting_frequency = MODLINK_FREQ_SYNDICATE
 	theme = /datum/mod_theme/syndicate
 	applied_cell = /obj/item/stock_parts/cell/super
 	applied_modules = list(
@@ -272,6 +274,7 @@
 	new /obj/item/tank/internals/emergency_oxygen/engi/syndi(bag)
 
 /obj/item/mod/control/pre_equipped/traitor_elite
+	starting_frequency = MODLINK_FREQ_SYNDICATE
 	theme = /datum/mod_theme/elite
 	applied_cell = /obj/item/stock_parts/cell/bluespace
 	applied_modules = list(
@@ -293,6 +296,7 @@
 	new /obj/item/tank/internals/emergency_oxygen/engi/syndi(bag)
 
 /obj/item/mod/control/pre_equipped/nuclear
+	starting_frequency = MODLINK_FREQ_SYNDICATE
 	theme = /datum/mod_theme/syndicate
 	applied_cell = /obj/item/stock_parts/cell/hyper
 	req_access = list(ACCESS_SYNDICATE)
@@ -313,6 +317,7 @@
 	ADD_TRAIT(chestplate, TRAIT_HYPOSPRAY_IMMUNE, ROUNDSTART_TRAIT)
 
 /obj/item/mod/control/pre_equipped/elite
+	starting_frequency = MODLINK_FREQ_SYNDICATE
 	theme = /datum/mod_theme/elite
 	applied_cell = /obj/item/stock_parts/cell/bluespace
 	req_access = list(ACCESS_SYNDICATE)
@@ -334,6 +339,7 @@
 	ADD_TRAIT(chestplate, TRAIT_HYPOSPRAY_IMMUNE, ROUNDSTART_TRAIT)
 
 /obj/item/mod/control/pre_equipped/prototype
+	starting_frequency = MODLINK_FREQ_THETA
 	theme = /datum/mod_theme/prototype
 	req_access = list()
 	applied_modules = list(
@@ -349,6 +355,7 @@
 	)
 
 /obj/item/mod/control/pre_equipped/responsory
+	starting_frequency = MODLINK_FREQ_CENTCOM
 	theme = /datum/mod_theme/responsory
 	applied_cell = /obj/item/stock_parts/cell/bluespace
 	req_access = list(ACCESS_CENT_GENERAL)
@@ -437,6 +444,7 @@
 	additional_module = /obj/item/mod/module/power_kick //JUDGEMENT
 
 /obj/item/mod/control/pre_equipped/apocryphal
+	starting_frequency = MODLINK_FREQ_CENTCOM
 	theme = /datum/mod_theme/apocryphal
 	applied_cell = /obj/item/stock_parts/cell/bluespace
 	req_access = list(ACCESS_CENT_SPECOPS)
@@ -474,6 +482,7 @@
 	)
 
 /obj/item/mod/control/pre_equipped/corporate
+	starting_frequency = MODLINK_FREQ_CENTCOM
 	theme = /datum/mod_theme/corporate
 	applied_core = /obj/item/mod/core/infinite
 	req_access = list(ACCESS_CENT_SPECOPS)
@@ -490,6 +499,7 @@
 	)
 
 /obj/item/mod/control/pre_equipped/debug
+	starting_frequency = null
 	theme = /datum/mod_theme/debug
 	applied_core = /obj/item/mod/core/infinite
 	/// One of every type of module, for testing if they all work correctly // boy this isn't even 25% the modules
@@ -511,6 +521,7 @@
 	activation_step_time = 0.1 SECONDS // coders are cooler than admins
 
 /obj/item/mod/control/pre_equipped/administrative
+	starting_frequency = null
 	theme = /datum/mod_theme/administrative
 	applied_core = /obj/item/mod/core/infinite
 	applied_modules = list(
@@ -529,6 +540,7 @@
 
 //these exist for the prefs menu
 /obj/item/mod/control/pre_equipped/empty
+	starting_frequency = null
 
 /obj/item/mod/control/pre_equipped/empty/syndicate
 	theme = /datum/mod_theme/syndicate

code/modules/mod/mod_ui.dm

@@ -13,6 +13,9 @@
 	data["malfunctioning"] = malfunctioning
 	data["open"] = open
 	data["active"] = active
+	data["link_id"] = mod_link.id
+	data["link_freq"] = mod_link.frequency
+	data["link_call"] = mod_link.get_other()?.id
 	data["locked"] = locked
 	data["complexity"] = complexity
 	data["selected_module"] = selected_module?.name
@@ -65,6 +68,11 @@
 		to_chat(usr, "<span class='warning'>ERROR!</span>")
 		return
 	switch(action)
+		if("call")
+			if(!mod_link.link_call)
+				call_link(ui.user, mod_link)
+			else
+				mod_link.end_call()
 		if("lock")
 			locked = !locked
 			to_chat(usr, "<span class='notice'>ID [locked ? "locked" : "unlocked"].</span>")