Step in the direction of RA2 paratroopers.

OpenRA.Mods.Common/Activities/Parachute.cs

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+#region Copyright & License Information
+/*
+ * Copyright 2007-2015 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation. For more information,
+ * see COPYING.
+ */
+#endregion
+
+using OpenRA.Activities;
+using OpenRA.Mods.Common.Traits;
+using OpenRA.Traits;
+
+namespace OpenRA.Mods.Common.Activities
+{
+	public class Parachute : Activity
+	{
+		readonly UpgradeManager um;
+		readonly IPositionable pos;
+		readonly ParachutableInfo para;
+		readonly WVec fallVector;
+
+		WPos dropPosition;
+		WPos currentPosition;
+		bool triggered = false;
+
+		public Parachute(Actor self, WPos dropPosition)
+		{
+			um = self.TraitOrDefault<UpgradeManager>();
+			pos = self.TraitOrDefault<IPositionable>();
+
+			// Parachutable trait is a prerequisite for running this activity
+			para = self.Info.Traits.Get<ParachutableInfo>();
+			fallVector = new WVec(0, 0, para.FallRate);
+			this.dropPosition = dropPosition;
+		}
+
+		Activity FirstTick(Actor self)
+		{
+			triggered = true;
+
+			if (um != null)
+				foreach (var u in para.ParachuteUpgrade)
+					um.GrantUpgrade(self, u, this);
+
+			// Place the actor and retrieve its visual position (CenterPosition)
+			pos.SetPosition(self, dropPosition);
+			currentPosition = self.CenterPosition;
+
+			return this;
+		}
+
+		Activity LastTick(Actor self)
+		{
+			pos.SetPosition(self, currentPosition - new WVec(0, 0, currentPosition.Z));
+
+			if (um != null)
+				foreach (var u in para.ParachuteUpgrade)
+					um.RevokeUpgrade(self, u, this);
+
+			foreach (var npl in self.TraitsImplementing<INotifyParachuteLanded>())
+				npl.OnLanded();
+
+			return NextActivity;
+		}
+
+		public override Activity Tick(Actor self)
+		{
+			// If this is the first tick
+			if (!triggered)
+				return FirstTick(self);
+
+			currentPosition -= fallVector;
+
+			// If the unit has landed, this will be the last tick
+			if (currentPosition.Z <= 0)
+				return LastTick(self);
+
+			pos.SetVisualPosition(self, currentPosition);
+
+			return this;
+		}
+
+		// Only the last queued activity (given order) is kept
+		public override void Queue(Activity activity)
+		{
+			NextActivity = activity;
+		}
+
+		// Cannot be cancelled
+		public override void Cancel(Actor self) { }
+	}
+}