LaunchOnTriggerComponent (#39871)

* LaunchOnTriggerComponent

Launches an object when a trigger on that
object is caused. The launch will be a certain
amount of force. The direction of the force
will be in the direction the object is facing.

* Fire stationary objects in the direction they're facing

* EVIL

* LESS EVIL

* Resolve @ScarKy0 comments

Content.Shared/Trigger/Systems/LaunchOnTriggerSystem.cs

@@ -0,0 +1,48 @@
+using System.Numerics;
+using Content.Shared.Trigger.Components.Effects;
+using Robust.Shared.Physics.Components;
+using Robust.Shared.Physics.Systems;
+
+namespace Content.Shared.Trigger.Systems;
+
+public sealed class LaunchOnTriggerSystem : EntitySystem
+{
+    [Dependency] private readonly SharedTransformSystem _transform = default!;
+    [Dependency] private readonly SharedPhysicsSystem _physics = default!;
+
+    public override void Initialize()
+    {
+        base.Initialize();
+
+        SubscribeLocalEvent<LaunchOnTriggerComponent, TriggerEvent>(OnTrigger);
+    }
+
+    private void OnTrigger(Entity<LaunchOnTriggerComponent> ent, ref TriggerEvent args)
+    {
+        if (args.Key != null && !ent.Comp.KeysIn.Contains(args.Key))
+            return;
+
+        var target = ent.Comp.TargetUser ? args.User : ent.Owner;
+
+        if (target is null)
+            return;
+
+        if (!TryComp(target, out PhysicsComponent? phys))
+            return;
+
+        var linearVelocity = _physics.GetMapLinearVelocity(target.Value);
+        // If the linear velocity is length 0, this means it's not moving. Given we want to move it in some direction...
+        if (linearVelocity.IsLengthZero())
+            // An object that isn't moving is launched in the direction its facing, not the direction it's rotated (objects face away from their rotation).
+            linearVelocity = _transform.GetWorldRotation(target.Value).RotateVec(Vector2.UnitY) * -1;
+
+        // When triggered, take the direction the target is moving in (the normalized vector) and multiply it by the speed.
+        // Then apply an impulse to the target on the new vector.
+        // (If the target is moving NE at 10 m/s, this impulses it NE at speed m/s)
+        _physics.ApplyLinearImpulse(target.Value,
+            linearVelocity.Normalized() * ent.Comp.Speed,
+            body: phys);
+
+        args.Handled = true;
+    }
+}