How to make guard NPCs using a state machine — Transcript

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1. 0:00So, let's say you want to create
2. 0:01exciting NPCs to fill your game world,
3. 0:04but dumping all your logic into one big
4. 0:06file leads to chaotic code you'll never
5. 0:08be able to debug.
6. 0:09If you want NPCs that you can customize
7. 0:11quickly, reuse all over your world, and
8. 0:14debug easily, you're going to want a
9. 0:15state machine.
10. 0:17Hi, I'm Miller, a software engineer here
11. 0:19at Roblox. Today, we're going to learn
12. 0:21how to use a state machine library to
13. 0:23make a guard NPC that patrols and gets
14. 0:26alerted if you come too close. All the
15. 0:28code I'm going to show today, plus the
16. 0:30place file, you can download and follow
17. 0:32along if you want to.
18. 0:34So, let's have a look at the world we'll
19. 0:35be working in today.
20. 0:36>> [music]
21. 0:37>> As you can see, I've already gone ahead
22. 0:38and set up a castle, as well as a simple
23. 0:40NPC. This guy just has a humanoid,
24. 0:43nothing else, and no other code telling
25. 0:45him what to do. I've also set up these
26. 0:47four flags that we use as waypoints for
27. 0:49the guard's patrol and tag them. And
28. 0:52lastly, I've put a sword in the middle
29. 0:53on a golden pedestal that we can try to
30. 0:55grab before the guards attack us. So,
31. 0:57let's get started by having a look at
32. 0:59our code. Here, I've got our types file.
33. 1:01We're going to check out the different
34. 1:02kind of tables we're going to be using
35. 1:04in our state machine. We're going to
36. 1:05have a context table, which is going to
37. 1:07be a [music] global blackboard for all
38. 1:08the information our NPCs need. We're
39. 1:10going to have state tables, which are
40. 1:12the possible configurations of each NPC.
41. 1:14Test functions, which are run by
42. 1:16transitions to connect the states, and
43. 1:18finally, an actual state machine,
44. 1:19[music]
45. 1:20which is all the transitions we have,
46. 1:22our starting context, our starting
47. 1:23state, and we give it a name. Now, this
48. 1:25right here is the meat and potatoes of
49. 1:27the state machine. This is the tick
50. 1:28function. This takes in one of those
51. 1:30state machine tables we just saw, and
52. 1:32it's going to run the tick function,
53. 1:33checking each transition coming out of
54. 1:35the active state to see if we should
55. 1:37take it or not.
56. 1:38>> [music]
57. 1:38>> If we take a transition, we're going to
58. 1:39run an on exit and an on enter function
59. 1:42on the state we're leaving and entering,
60. 1:44respectively. And if we don't
61. 1:45transition, we're going to run an on
62. 1:47stay function. That's really all the
63. 1:49code that actually matters for using the
64. 1:50state machine. I've also provided a
65. 1:52composition test thing that lets you
66. 1:54combine all multiple test functions
67. 1:56[music] to create a combination, like a
68. 1:58boolean and, or, or not. And then also,
69. 2:02a single module that you can access all
70. 2:03of them from called simple state
71. 2:05machine. All right, now we're going to
72. 2:06create an NPC that I'll call the walker.
73. 2:08This NPC is going to walk between flags
74. 2:10one and two. So, here we've got the code
75. 2:12for the walker. Uh if you have a look
76. 2:14here at the top, we've got the types,
77. 2:16which we've just imported from simple
78. 2:17state machine. I've set up a transition
79. 2:19test, and then what I've also done is
80. 2:21set up the actual states. So, if we have
81. 2:22a look at walk to flag one, now when the
82. 2:25NPC is in this state, it's just going to
83. 2:27make them walk towards flag one. We've
84. 2:29got a second state called [music] walk
85. 2:30from to flag two. It's going to make
86. 2:32them walk towards flag two. Now, let's
87. 2:33talk about what this state does. What
88. 2:35happens is when the NPC enters the
89. 2:38state, we're going to set
90. 2:39context.targetflag
91. 2:41to flag one. Context, that's a global
92. 2:43blackboard. We can keep information
93. 2:45about the NPC in there. Then, in the on
94. 2:47stay function, which is run every tick,
95. 2:50we're going to have the humanoid set its
96. 2:52move to to the flag one's pivot
97. 2:55position. Walk to flag two is going to
98. 2:57be the exact same thing, just replacing
99. 2:59flag one with flag two. Coming back to
100. 3:01our original file, you can see I've set
101. 3:03up transitions. If we're in walk to flag
102. 3:05one, we check if we're near the target
103. 3:07flag. That's flag one. If we are, we go
104. 3:09to flag two, and I made a transition to
105. 3:12do the opposite. I just load those
106. 3:14things into the state machine, and I set
107. 3:16up some context information. The
108. 3:17script's going to need to know what our
109. 3:18character is, what our humanoid is, and
110. 3:21also which flag we should start going
111. 3:23to. We're going to start by going to
112. 3:24flag two. And then all I've got to do is
113. 3:26set heartbeat to run the tick function I
114. 3:28showed you earlier. Now, all I've got to
115. 3:30do is drag our walker script to the
116. 3:31guard, and we'll see what happens.
117. 3:38As you can see, the guard's walking to
118. 3:39flag two, and when he reaches there, he
119. 3:41switches back to the first state, walk
120. 3:43to flag one,
121. 3:45and he goes back and forth.
122. 3:48So, that's a start, but we can do
123. 3:49better. You might have noticed in the
124. 3:50last one that walk to flag Now, let's
125. 3:52try that again. You might have noticed
126. 3:54for the walker that the states walk to
127. 3:55flag one and walk to flag two [music]
128. 3:57are pretty similar. So, the first thing
129. 3:59I'm going to do is create a more generic
130. 4:01called walk to random flag.
131. 4:04What this [music] state is going to do
132. 4:05is it's going to pick the next flag to
133. 4:07go to randomly. So, instead of just
134. 4:08going between one and two,
135. 4:10>> [music]
136. 4:10>> we're going to go to one of the two
137. 4:12flags on an adjacent corner. So, the
138. 4:14walk to random flag, when we enter it,
139. 4:16is going to have us set the next flag.
140. 4:19So, we'll figure out which flag we
141. 4:21should go to randomly. And then what
142. 4:23we'll do is in the on stay, we'll do the
143. 4:24exact same thing we saw earlier with
144. 4:26humanoid move to to have the character
145. 4:28walk towards the next flag. [music] When
146. 4:30you take that logic and you add on to it
147. 4:32a simple idle state that has the
148. 4:34character hang around for a second after
149. 4:36reaching a flag, we end up with the
150. 4:38patroller. The patroller's going to have
151. 4:41a lot of the same code, but we're going
152. 4:42to switch around the transitions. We're
153. 4:44going to go [music] from idling, waiting
154. 4:46around for a second, to walking to the
155. 4:48next random flag. We're going to have
156. 4:50two tests for this one. We're going to
157. 4:52have is near target flag, which checks
158. 4:54if we're close enough to the flag, and
159. 4:56we're also going to have is idle timer
160. 4:58done, which tells us if we've been
161. 5:00idling for long enough that it's time to
162. 5:02go back to patrolling.
163. 5:03Again, we're going to take all that and
164. 5:05put it into a state machine. We're going
165. 5:06to add some new values to our context.
166. 5:09Now, we're going to have an idle start
167. 5:11time, and we're going to have a current
168. 5:13flag number, so we can track all four
169. 5:15flags. Connect it to heartbeat, and
170. 5:17we'll watch it go.
171. 5:30As you can see, our NPC now chooses
172. 5:32flags randomly, letting it explore the
173. 5:34entire castle and keep it protected from
174. 5:36all sides.
175. 5:38That said, although this guard does
176. 5:40patrol between all the flags, it doesn't
177. 5:42really protect it. So, the next thing
178. 5:44we're going to add is new states for the
179. 5:46guard to attack us if we get too close
180. 5:48to prevent us from stealing the sword.
181. 5:50So, let's soup up our guard with the
182. 5:51ability to attack. The first thing I'm
183. 5:53going to do is [music]
184. 5:56We're going to add in this attack state.
185. 5:58What this attack state does is while
186. 6:01we're in the state, we're going to
187. 6:03record the last player we've seen, and
188. 6:05we're going to chase them down. And when
189. 6:07we get close by, we're going to attack
190. 6:09them on a cool down. Having a look at
191. 6:12the final version of our NPC, which I'm
192. 6:14calling the defender, you can see that
193. 6:16we've added a new transition test, is
194. 6:18player nearby. Just like before, this
195. 6:20takes in the context, and we can use it
196. 6:22to determine if a transition should
197. 6:24occur. This test checks to see if a
198. 6:26player is within a range of the NPC. And
199. 6:29if it is, it sets our
200. 6:30context.targetplayer
201. 6:32to that value and returns true, letting
202. 6:34us know that it's time to attack.
203. 6:37If we come down and have a look at our
204. 6:38transitions, you'll see that I'm
205. 6:40leveraging a feature of the state
206. 6:41machine. If you see the first transition
207. 6:44there, I'm providing two different start
208. 6:46states, letting us essentially have two
209. 6:48arrows pointing at the attack state. If
210. 6:51you have a look at the transitions
211. 6:52table, I've added a new transition from
212. 6:55idle or walk to random flag to our
213. 6:57attack state, and that's using our is
214. 7:00player nearby test. Coming down to look
215. 7:02at our defender state machine, the only
216. 7:04thing that's changed [music] is that
217. 7:05I've added in a target player value,
218. 7:08which is set to nil until the transition
219. 7:11is player nearby finds a player to
220. 7:13attack. Now that the defender is ready,
221. 7:15let's see how the guard acts when we add
222. 7:17that script to it.
223. 7:20Let's try getting close to the guard and
224. 7:22see what happens.
225. 7:26Now, let's see how state machine NPCs
226. 7:28can make a scene really exciting by
227. 7:30adding tons of guards. All right, I've
228. 7:32got nine guards walking around the
229. 7:34palace. Let's see if I can get in, get
230. 7:36the sword, and get out without dying.
231. 7:38Okay, I've already aggroed a couple of
232. 7:40them. This is not good. Okay, let me try
233. 7:42that again.
234. 7:47All right, sneaking past while they're
235. 7:49idle.
236. 7:50I'm in.
237. 7:53Oh, shoot. Got the sword. Take one down.
238. 7:57Took two down. Oh, there's a lot more.
239. 7:59I got them all.
240. 8:02Victory! So, there you have it. A guard
241. 8:05that patrols, alerts, and battles you
242. 8:07when you get too close. Next time you're
243. 8:09making an NPC, feel free to try out the
244. 8:11state machine library, or design your
245. 8:12own. You'll be surprised how active and
246. 8:14engaging your NPCs can be with just a
247. 8:16few states and transitions. That's all
248. 8:18from me. I hope you learned something
249. 8:20new.