Normally in UnityUnreal, if you wish to spawn a Prefab an Actor at runtime, you would simply call GameObject.Instantiate “Spawn Actor From Class” in Blueprint, or UWorld::SpawnActor from C++, to make a new instance of it.
There are a couple of problems you run into when doing this in Cavrnus however.
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The spawn isn’t synchronized for everyone, so only you will see the new object.
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The CavrnusFunctionLibrary provides an API call that solves two of the main challenges doing this.
The object spawned must be synchronized for everyone.
Every instance should share the same property paths as the base prefab, so they couldn’t can behave individually.
Both of these problems are solved using the CavrnusFunctionLibrary API call SpawnObject.
What this does is it posts the object spawn to the journal and, in doing that, assigns it a new Property Path. Then when the object is Instantiated() instantiated it goes through all the PropertiesContainers and updates their property paths to reflect the unique journal identifier for the spawned instance.
Setting up a Prefab to be Spawnable
To make your prefab spawnable, you need only right-click on it, and then select Cavrnus → Make Selection Spawnable.
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What this does under-the-hood is it adds that prefab to the Cavrnus Spatial Connector’s Spawnable Objects list. It also gives it a uniqueId (generally the name of the prefab, but we modify it slightly for duplicate names).
This UniqueId to add it to the list of actor classes in the CavrnusSpatialConnector in your level, with a unique name as the key.
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This unique string key is what you will use to actually spawn the object. (Note that this means you can modify or change the prefab and, as long as the UniqueId key name doesn’t change, old journal operations will now spawn the new prefab.)
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Spawning your Prefab
Now, let’s write up a simple UI button script to spawn the object:
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create an object in the local map that will be able to create one or more Cavrnus objects:
As you can see, we’re simply passing in that UniqueId unique string ID from above. The value we get back, meanwhile, is the Container Name of the new Instance. So, when the new prefab is spawned, the Cavrnus Properties Container at it’s its root will use this Container Name.
Properties on Spawned Objects
Spawned object property names are handled in a very similar way to Avatar Instances.
Essentially what happens is the initial root Cavrnus Properties Container’s Container Name is replaced with the new spawned instance ID. Then, every sub-object that starts with the root Container Name swaps out that part of the string for new new Id.
To give an example, let’s look at the above prefab.
The Root has a Unique no Container Name defined:
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While the sub-parts have Container Names starting with that same root name:
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Now let’s look at the runtime instance that we got from the SpawnObject call. As you can see, it has a unique name do distinguish it from other instances
Notice that the Unique Contain Container Name on the root has been completely replaced with the new ID.
Note: There is a Cavrnus Spawned Object Flag component also attached here. This will be needed later when we want to delete the object.
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Meanwhile the Left Arm has replaced the first part of its Container Name only:
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The new root Container Name is returned synchronously from the SpawnObject call, and can immediately be used to post properties. Alternatively, any ValueSync scripts components on the object will function normally too.
| Code Block |
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public void SpawnPrefab()
{
// SpaceConnection here would be a class member of type FCavrnusSpaceConnection, that would be set on Space joined
if (spaceConnSpaceConnection == null)
return;
// SpawnedObjectArrived here would be a delegate class member of type FCavrnusSpawnedObjectArrived, that would be bound to a callback during setup
stringFString instanceContainerNameInstanceContainerName = spaceConn.UCavrnusFunctionLibrary::SpawnObject("Cuby McCubeface"SpaceConnection, "PerfTestSphere", SpawnedObjectArrived);
spaceConn.UCavrnusFunctionLibrary::PostBoolPropertyUpdate(instanceContainerNameSpaceConnection, InstanceContainerName, "Visibility", true);
} |
Deleting Spawned Objects
When deleting a spawned object, you will need to access the Cavrnus Spawned Object Flag attached to ita FCavrnusSpawnedObject is provided. This can be done with a simple GetComponent callsaved off when FCavrnusSpawnedObjectArrived is called, or it can be retrieved from the SpawnedObjectsManager by calling UCavrnusFunctionLibrary::GetIfIsSpawnedObject. Once you have it, simply call DestroyObject on it.
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The above system is a good way to spawn well-known prefabs. However, if you want to spawn more advanced or imported objects, you can read this guide here (TODO: LINK).