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‫Welcome back.

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‫Now in this video, we're going to talk about hit events.

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‫We would like to detect when the projectile hits something and that way we can do something in response,

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‫such as destroy the projectile.

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‫And later we'll handle doing damage and causing explosion effects and things like that.

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‫So let's talk a little bit about hit events now.

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‫Our projectile has a mesh component.

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‫It's type is you static mesh component and you static mesh component is one class in a long hierarchy

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‫of classes and the unreal engine system one of its parents is you primitive component.

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‫Here is the documentation page for you primitive component.

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‫It says you primitive components are seen components that contain or generate some sort of geometry

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‫generally to be rendered or used as collision data.

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‫So they're basically scene components with some added features.

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‫And if you scroll down and look at the hierarchy, you'll see that we have you seen component and derived

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‫from that you primitive component and you can keep scrolling down and see all of the other classes that

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‫derive from you.

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‫Primitive component.

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‫Now the reason we're pointing this out is because you static mesh component inherits these features

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‫from you primitive component and you primitive component has the ability to generate hit events.

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‫Now, what do we mean by generate hit events?

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‫Well, let's say we have some geometry in our world represented by the rectangle to the right, and

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‫we have an object with a static mesh component represented by the circle to the left.

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‫Now, if this object that has a static mesh component is moving towards the block, then if this component

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‫is set up to handle hit events, then as soon as it hits that block, the hit event will be generated

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‫upon collision.

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‫Now a hit event will generate some data pertaining to that collision, including the component that

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‫was hit.

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‫So this block here may have a static mesh component.

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‫We can access that component upon colliding.

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‫We can also access the actor that was hit and the head event also generates an EF hit result.

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‫We've learned a little bit about the F hit result and we know that it contains lots of variables about

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‫the hit event, such as the hit location, which is an F vector representing a location in the world.

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‫So we have our projectile mesh.

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‫This is a variable of type you static mesh component pointer.

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‫Now whenever this static mesh component hits something in the world, we want to have some code that

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‫responds to that.

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‫Now being a static mesh component, it inherits things from the you primitive component class.

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‫One of the things it inherits is a variable called on component hit.

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‫Now here's the documentation for on component hit.

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‫It's described as an event called When a Component Hits or is hit by something solid.

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‫Now scrolling down, we see that on component hit has the type F component hit signature.

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‫And if we click on this, we see that F component hit signature is a struct.

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‫Now going back, we can see that the documentation says an event called when a component hits or is

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‫hit by something solid.

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‫This could happen due to things like character movement using set location with sweep enabled or physics

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‫simulation for events when objects overlap such as walking into a trigger, see the overlap event.

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‫So there are hit events and there are overlap events.

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‫We're concerned with hit events.

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‫So we know that on component hit is described in the documentation as a hit event and we know that on

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‫component hit is a struct of type F component hit signature.

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‫So far so good.

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‫Now this hit event on component hit is also something we refer to as a multicast delegate.

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‫A multicast delegate can have multiple functions bound to it.

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‫There can be functions that exist on multiple classes.

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‫And if those functions get bound to this multicast delegate, then they're added to something called

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‫an invocation list.

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‫This is a list of functions to be called when this hit event tells them all to be called.

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‫We say that this hit event or multicast delegate broadcasts when something happens.

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‫So in the event that our projectile mesh hits something, then the delegate will broadcast to all classes

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‫that have a function bound to this delegate.

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‫As soon as that broadcast is sent, then all of these bound functions are called in response to the

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‫broadcast.

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‫This is a very powerful feature of Unreal Engine.

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‫It allows us to have this a thing called a delegate, which waits for an event such as a collision.

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‫And then.

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‫And the delegate will be broadcast, resulting in all bound functions called in response to the broadcast.

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‫Since multiple functions can be bound to this delicate, we call it multicast.

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‫It's broadcasting to multiple classes that have their functions bound to it.

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‫Now, how do we bind a function to this delegate?

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‫Well, on component hit has a number of functions that allow us to do this, one of which is called

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‫add dynamic.

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‫So when we call the ADD dynamic function, all we need to do is pass in two parameters the user object

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‫which we can use this for, and the callback function, which is the address of some function on our

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‫class.

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‫So once we access this on component hit delegate and call the add dynamic function, we can pass in

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‫the user object and the callback.

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‫And this will result in adding that callback to the hit events invocation list.

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‫Now on component hit, we'll broadcast this delegate resulting in any functions bound to it to be called.

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‫So in order to bind to this delegate, we need to create a callback function.

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‫So here's projectile and we'd like a callback function for the hit event.

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‫So let's create a private function here and it's going to be void.

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‫And I'm going to just simply call this on hit.

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‫We could call this anything we want, but on seems descriptive enough.

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‫Now this callback function has to have the correct signature for hit events, which means its input

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‫parameter list needs to be very specific.

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‫The first input parameter has to be of type you primitive component pointer and it's called hit comp.

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‫This is the component doing the hitting.

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‫In our case, we're going to have our projectile mesh hitting things.

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‫The next parameter is an a actor pointer called Other Actor.

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‫This is going to be the actor that got hit.

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‫The next input parameter is another you primitive component pointer called other comp.

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‫This is the other component that was hit.

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‫So if we hit the static mesh component of some other actor, then this would be that static mesh component.

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‫Next is an F vector called normal impulse.

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‫If we're simulating physics, the physics engine will apply an impulse in response to the collision.

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‫And this f vector represents the direction and the magnitude of that impulse.

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‫And finally, we have an input parameter of type const F hit result and this is a const reference and

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‫it's called hit.

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‫So when on hit gets called, we'll have access to a hit result that contains even more information about

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‫the hit.

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‫Now, since we're binding this to a delegate, it has to be a U function.

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‫So we're going to mark this with you function.

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‫Now, you should be familiar with you function.

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‫It's just like you property only it's for functions rather than variables.

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‫So very important.

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‫Our callback has to be a u function for this to work.

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‫Now we've declared the function.

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‫Let's go ahead and define the function.

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‫So back in projectiles.

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‫CP Just under our tick function, let's define this function.

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‫We'll give it a function body and put the class name just before the function name, and now we have

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‫a callback function.

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‫So we're ready to bind this on hit function to our delegate and we need to do this in begin play.

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‫Doing it in the constructor is to early and sometimes you can run into issues like your delegate not

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‫being bound.

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‫So we're going to do it here and begin play.

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‫So we have our projectile mesh and we'll use the arrow operator to access the variable inherited from

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‫you.

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‫Primitive component called on component hit and on component hit has the function add dynamic.

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‫Now strangely, autocomplete is not going to work for add dynamic, so you're going to have to type

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‫the entire function name.

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‫This is the function we'll use to bind our callback to on component hit and add that function to its

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‫invocation list.

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‫Now first, we need the user object that's going to be this because of this is the class where binding

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‫the function for and next we need the address to the function that's going to be the address of on hit.

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‫So we'll use the address of operator and the function a projectile on hit.

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‫And with this we have our function bound and we're ready to generate hit events.

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‫Now we can test this out with a UEE log and we can use log temp warning.

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‫And for the text we can simply say on hit.

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‫And if we see this in the output log, we know that our on hit function has been called let's compile

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‫this.

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‫So I have the output log open here.

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‫I'm going to press play and fire a projectile and there's my log message on hit is being called.

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‫So this is working.

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‫Now just a side note, if we go into BP projectile and select projectile mesh in the details panel,

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‫we can scroll down to the collision section.

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‫And here we see that the collision presets is set to block all dynamic and that collision enabled is

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‫set to this setting collision enabled query and physics.

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‫This is necessary for hit events.

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‫We need to be able to query for a hit event.

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‫And of course, physics allows the physics engine to act on the projectile mesh.

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‫So now that we've bound to the on component hit delegate with our on hit function and we've placed a

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‫huge log to make sure that it's working, I'd like you to explore this function a little bit more so

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‫that we can understand some of these input parameters in our on hit function.

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‫So in the on hit function, add some more YOUI logs.

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‫I'd like you to print the names of some of these input parameters using the git name function.

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‫I'd like you to use a YUI log to print the name of hit comp, other actor and other comp so that we

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‫could see the names of these objects we're hitting.

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‫You're going to use the percent se format, specify the in the E log text macro.

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‫And just as a reminder, whenever we call get name, we have to use the asterisk when passing that string

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‫into the YUI log macro, for example, to get the name of other actor we would call other actor, get

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‫name and place that asterisk at the beginning just to return that C style string, which is what you

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‫log expects when you format it with a string.

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‫So pause the video and use Yui log in our on hit function to print the names of hit comp, other actor

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‫and other comp now.

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‫Okay.

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‫So here in projectile CP, I'm going to use you log here.

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‫So I'm going to copy this line where I'm using Yui log and for the string I'm going to say hit comp

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‫percent s and just after the text I'm going to call hit comp get name and I need this asterisk to convert

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‫from an F string to a C style string.

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‫Now I'm going to use this same line, only I'm going to change the text to say other actor and we're

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‫calling it name this time on other actor.

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‫And finally, I'm going to print other comp calling, get name on other comp.

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‫So let's compile and back here in the editor, I'm going to go ahead and hit play and shoot a projectile

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‫and check this out.

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‫We see that hit comp is projectile mesh.

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‫Remember we said that this is the component doing the hitting, which is our projectile mash and you'll

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‫see that it's using the name that we assigned to this component in create default sub object.

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‫So this is one of those situations where that name is being used.

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‫Now you'll see that it says other actor flaw underscore zero.

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‫And that's because this static mesh actor that is the flaw has the name flaw underscore zero.

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‫Sometimes the engine will place a number at the end of its internal name for actors and components,

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‫and you'll see that we have that same thing happening when we print out other comp.

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‫The flaw static mesh actor has a static mesh component and its name is static mesh component zero.

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‫So this gives us a little bit more information about those input parameters in our on hit callback function.

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‫And this is useful because we have access to those objects in C++ if we'd like to do something in response

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‫to the hit event.

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‫So in summary, we have learned about hit events and how these are implemented as multicast delegates

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‫and we learned how those work.

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‫We created a function called On Hit and Bound, that function to the on component hit delegate that

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‫our projectile mesh has inherited from the you primitive component class.

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‫Now we haven't destroyed the projectile on hit, but we now know that we can easily do so because we

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‫have this callback function and we can place logic in this function such as destroying the projectile.

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‫So we'll handle that in a future video.

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‫For now, pat yourself on the back because this is one of the more complex areas of Unreal Engine C++

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‫that you've learned so far.

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‫I'll see you in the next video.