Follow verb for ghosts (#6016)

* Follow verb for ghosts

* oop

* Use parenting instead

* Update Content.Shared/Follower/FollowerSystem.cs

Co-authored-by: ShadowCommander <10494922+ShadowCommander@users.noreply.github.com>

* mfw

* parent fixes

* figs

* oop

* porbis

Co-authored-by: ShadowCommander <10494922+ShadowCommander@users.noreply.github.com>

Content.Shared/Follower/Components/FollowedComponent.cs

@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+using Robust.Shared.Analyzers;
+using Robust.Shared.GameObjects;
+
+namespace Content.Shared.Follower.Components;
+
+/// <summary>
+///     Attached to entities that are currently being followed by a ghost.
+/// </summary>
+[RegisterComponent, ComponentProtoName("Followed"), Friend(typeof(FollowerSystem))]
+public class FollowedComponent : Component
+{
+    public HashSet<EntityUid> Following = new();
+}

Content.Shared/Follower/Components/FollowerComponent.cs

@@ -0,0 +1,11 @@
+using Robust.Shared.Analyzers;
+using Robust.Shared.GameObjects;
+
+namespace Content.Shared.Follower.Components;
+
+[RegisterComponent, ComponentProtoName("Follower")]
+[Friend(typeof(FollowerSystem))]
+public class FollowerComponent : Component
+{
+    public EntityUid Following;
+}

Content.Shared/Follower/FollowerSystem.cs

@@ -0,0 +1,106 @@
+using Content.Shared.Database;
+using Content.Shared.Ghost;
+using Content.Shared.Movement.EntitySystems;
+using Content.Shared.Verbs;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Localization;
+using Content.Shared.Follower.Components;
+using Robust.Shared.Maths;
+
+namespace Content.Shared.Follower;
+
+public class FollowerSystem : EntitySystem
+{
+    public override void Initialize()
+    {
+        base.Initialize();
+
+        SubscribeLocalEvent<GetAlternativeVerbsEvent>(OnGetAlternativeVerbs);
+        SubscribeLocalEvent<FollowerComponent, RelayMoveInputEvent>(OnFollowerMove);
+        SubscribeLocalEvent<FollowedComponent, EntityTerminatingEvent>(OnFollowedTerminating);
+    }
+
+    private void OnGetAlternativeVerbs(GetAlternativeVerbsEvent ev)
+    {
+        if (!HasComp<SharedGhostComponent>(ev.User))
+            return;
+
+        var verb = new Verb
+        {
+            Priority = 10,
+            Act = (() =>
+            {
+                StartFollowingEntity(ev.User, ev.Target);
+            }),
+            Impact = LogImpact.Low,
+            Text = Loc.GetString("verb-follow-text"),
+            IconTexture = "/Textures/Interface/VerbIcons/open.svg.192dpi.png",
+        };
+
+        ev.Verbs.Add(verb);
+    }
+
+    private void OnFollowerMove(EntityUid uid, FollowerComponent component, RelayMoveInputEvent args)
+    {
+        StopFollowingEntity(uid, component.Following);
+    }
+
+    // Since we parent our observer to the followed entity, we need to detach
+    // before they get deleted so that we don't get recursively deleted too.
+    private void OnFollowedTerminating(EntityUid uid, FollowedComponent component, EntityTerminatingEvent args)
+    {
+        StopAllFollowers(uid, component);
+    }
+
+    /// <summary>
+    ///     Makes an entity follow another entity, by parenting to it.
+    /// </summary>
+    /// <param name="follower">The entity that should follow</param>
+    /// <param name="entity">The entity to be followed</param>
+    public void StartFollowingEntity(EntityUid follower, EntityUid entity)
+    {
+        var followerComp = EnsureComp<FollowerComponent>(follower);
+        followerComp.Following = entity;
+
+        var followedComp = EnsureComp<FollowedComponent>(entity);
+        followedComp.Following.Add(follower);
+
+        var xform = Transform(follower);
+        xform.AttachParent(entity);
+        xform.LocalPosition = Vector2.Zero;
+    }
+
+    /// <summary>
+    ///     Forces an entity to stop following another entity, if it is doing so.
+    /// </summary>
+    public void StopFollowingEntity(EntityUid uid, EntityUid target,
+        FollowedComponent? followed=null)
+    {
+        if (!Resolve(target, ref followed))
+            return;
+
+        if (!HasComp<FollowerComponent>(uid))
+            return;
+
+        followed.Following.Remove(uid);
+        if (followed.Following.Count == 0)
+            RemComp<FollowedComponent>(target);
+        RemComp<FollowerComponent>(uid);
+        Transform(uid).AttachToGridOrMap();
+    }
+
+    /// <summary>
+    ///     Forces all of an entity's followers to stop following it.
+    /// </summary>
+    public void StopAllFollowers(EntityUid uid,
+        FollowedComponent? followed=null)
+    {
+        if (!Resolve(uid, ref followed))
+            return;
+
+        foreach (var player in followed.Following)
+        {
+            StopFollowingEntity(player, uid, followed);
+        }
+    }
+}

Resources/Locale/en-US/follower/follow-verb.ftl

@@ -0,0 +1 @@
+verb-follow-text = Follow