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‫Hello and welcome.

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‫In this lecture, we're going to be getting our A.I. to be patrolling back and forth between our locations.

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‫So will go to my original start location, then I'll go back to his start location, wait a little bit,

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‫and then go again.

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‫We're going to be achieving this via behavior trees.

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‫So let's dive in and see how it's done.

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‫So now we haven't made much use of our behavior tree.

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‫Let's fix that.

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‫We're going to create a behavior that allows us to go from the place where we're starting to the player's

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‫location and then go back to our original starting location.

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‫Now, the way we're going to do this is using the following.

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‫So there is a distinction in behavior trees between things called tasks and things called composite

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‫nodes, or in this case, the example of the composite node is a sequence.

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‫So let's understand sequences and tasks as they kind of sound.

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‫A task is something you do.

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‫A sequence is a sequence of such tasks and it will run all of the tasks in order.

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‫So if we right click and expand out the tasks here, you can see an example of what could be used as

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‫a task.

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‫For example, we can have move two, which moves to a location that's moved directly to as well.

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‫I'm not entirely sure of the distinction we're going to be using.

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‫Move two.

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‫You can get it to play an animation such as the scratch your nose animation.

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‫You can get them to play a sound like, Hey you, that sort of stuff.

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‫You can get it to run another animal behavior if you want to.

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‫There's also a weight here which is very useful for adding in some pauses into your behavior.

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‫Now what's interesting about these tasks is that they can run for multiple frames, for multiple ticks.

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‫So Move two will be running the entire time that you are moving to a location and it'll only finish

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‫when you get there.

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‫So if we did a move too and we had, we can stick it in a sequence here by just dragging down from the

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‫sequence node.

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‫Then what you're going to get is, first of all, it's going to try and move to the destination.

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‫And then we could add in a second task node which tells it to wait, and then we can add in a third

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‫task task node.

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‫Notice I'm pulling down from the sequence here and releasing, which allows me to add in a task quite

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‫quickly underneath that sequence node.

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‫And then I can tell it.

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‫I want to move to another location and then I can tell it.

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‫I want to wait again for a certain amount of time.

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‫Now this is the basics of that moving to the player and then moving back to where we started from.

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‫And you can see how the sequence node is working here.

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‫Now there's a route node as well.

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‫The route node can only have one child node, so don't try and add more things under that.

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‫So we've got one sequence node is the route here and that's basically saying that I'm going to run all

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‫of these tasks in order until one of them fails and then once we're finished, we can start the process

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‫again.

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‫So the other thing to note here is that if you click on any one of these nodes, the details over here

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‫give us some impact over how that node works.

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‫For example, our move too currently takes a blackboard key in the blackboard section of its details

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‫pane of self active, but we can change that over to the player location.

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‫So that's basically saying this move to Node will now move to our player location.

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‫Then we're going to wait 5 seconds by default, but we can change that again to say we might want to

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‫wait 2 seconds and you might want to have a random deviation of 1/2 so that basically it will be waiting

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‫somewhere between 1/2 and 3 seconds.

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‫Then we want to move to a different location, and that's a location that we don't currently have,

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‫which is our start location.

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‫So we can go ahead and add that.

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‫Let's go over to our blackboard, add a new key, add a vector and have that as our start location.

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‫And then we can go back over to the behavior tree and see that our move to a node, if you go to the

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‫details pane now has the start location as an option and then we might say we want to wait 5 seconds

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‫and then the process will start again because we'll have successfully completed our sequence node.

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‫So this is how it works.

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‫You step down into the sequence node, the sequence node then says, okay, well where am I at?

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‫I want to start at the beginning.

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‫And you can see this number.

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‫Things here are telling us the execution order, we start at zero, which is the sequence node one is

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‫the move to then two is a weight three it's moved to four, etc. So we step through doing each task

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‫until completion or failure.

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‫If we fail, then the sequence node will fail as a whole.

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‫None of these are likely to fail, however, so we'll go through the whole thing and then the sequence

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‫node will complete and the route will start it again.

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‫So we're going to go through in a loop, going to the player location weighting, going back to the

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‫start location, waiting for a little bit longer.

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‫So that's the power of behavior trees and you can start to nest these deeper.

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‫You can have choices in here, you can have decorators, all the stuff that we're going to see in upcoming

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‫lectures.

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‫But this is the basis of it.

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‫A series of instructions that your AI can run and they can take more than one frame to complete.

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‫Now what we want to do is actually set this start location to be something meaningful.

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‫Now that should be reasonably straightforward.

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‫The only thing I probably want to tell you how to do is how do we get hold of the -- that we're in

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‫control of?

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‫So just.

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‫A reminder that if we go over to the controller h here, then we can have a look for the get -- and

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‫the get -- method for both player controllers and air controllers allows us to get hold of the --

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‫that is being controlled.

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‫So you can use this to get the -- being controlled and get its location at begin play in order to

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‫set our start location.

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‫So here's the challenge to you.

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‫Set a start location, get the --'s location as we've just talked a little bit about, set the correct

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‫blackboard key, use the player position that we've already used as a template for that.

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‫Set up the behavior tree the way I have in the lecture and test it out.

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‫See if that behavior gets the desired behavior of the player or the enemy moving to the player's --

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‫and then back to its original starting location.

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‫Pause the video and have a go.

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‫Okay.

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‫Welcome back.

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‫So hopefully that worked out for you.

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‫Let's give this a shot.

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‫So I'm going to copy the line for our player location.

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‫And instead of setting the player location, we go over to our blackboard, have a look at our start

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‫location.

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‫We're going to copy that key value in here.

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‫And then instead of getting the location of the player --, we're going to do a get -- and then

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‫get actor location, which is going to get the -- that we're currently controlling and get its location

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‫in the world so we can go ahead and go back to the editor and compile this.

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‫And once that's done, hit play and have a look at our --.

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‫He comes over, moves towards us.

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‫It waits 3 seconds or 2 seconds or 1/2, then goes back to his original start location.

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‫Then he waits 5 seconds, and then he comes back over to that original location of the player --.

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‫So he's not updating that every frame.

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‫That's okay.

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‫That's how we designed it at the moment.

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‫But you can see the behavior going on.

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‫He's patrolling back and forth.

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‫So let's have a look at the all of this in action.

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‫Have a look at our behavior tree.

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‫If we go over to our behavior tree and select the I in question, you can see where he's at.

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‫He's moving to he's waiting so many seconds.

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‫Then he's moving to his start location.

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‫He's waiting again so we can undock this and see both of these things happening at the same time in

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‫our game.

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‫You can see he's moving to that location.

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‫He's waiting now he is back to moving it to his start location.

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‫So this is really handy for us to be able to debug, especially when these behaviors get a little bit

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‫more complicated.

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‫And it also gives you a sense of how unreal is calculating the behavior tree and how that execution

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‫flow is working within our behavior tree.

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‫Great stuff.

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‫In the next lecture, we're going to make these behaviors even more complicated and add some element

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‫of decision to whether we choose to follow a player or whether we choose to stay put where we are.

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‫I'll see you in that lecture.