Replace a C++ Native Component with a Derived class

Development Programming & Scripting C++
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1

Hey, I’m trying to figure out how on earth to solve this gracefully -

I need to replace a Native Component with a Blueprint derived class.

Lets say we have:

UCLASS()
class Foo  : public AActor {
    ...

    UPROPERTY(EditAnywhere)
    UBarComponent* ReplaceableComponent;
    ...
    public Foo() {
        ReplaceableComponent= CreateDefaultSubobject< UBarComponent>( TEXT( "ReplaceableComponent" ) );
    }
}

Now, if I blueprint my class Foo, in the left hand pane, I can see my Native Component registered.
But I want to replace this instance, for this blueprint only.

Say I have created another Blueprint based off of UBarComponent called BP_MyBarComponent.
The blueprint editor will not let me replace “ReplaceableComponent” with an instance of BP_MyBarComponent.

Has anyone figured out how to deal with this?

2

You can’t replace them with Blueprint classes. The best you can do for that is expose a variable which you set in your Blueprint construction script or similar.

3

This seems to me like an oversight, the C++ class is blueprinted into a blueprint class, just like how we can override the CDO’s CreateDefaultSubobject in C++, we should be able to override the Default Subobjects in Blueprint… After all, that blueprint class has to be read from disk anyways…

I guess this could be done via the ConstructionScript in blueprint if the UPROPERTY() has BlueprintReadWrite on it.

4

I agree; it seems like it should be possible to override which class is used in a CreateDefaultSubobject<> instantiation. All the data necessary should be there for the engine to do the right thing.

5

It’s been asked for for a long time, but still isn’t supported. The only way to do this IMO, would be to inject into the “Simple Construction Script” that Blueprint uses to create components, and have a pop-up that lets you choose a derived component from the creation dialog box.

There is no CDO for Blueprints, and the C++ Constructor runs a long time before any Blueprint serialization, so no doubt it becomes a bit of a minefield, which is probably why they haven’t done it. For now, I’ve found the simplest method is to just set a Read/Write variable or call a Setter function that throws an error if called outside of the construction script.

6

this caught my eye. is there some method internally of doing this, or are you just setting a bool to flag it?

7

You can use the existing bActorIsBeingConstructed to guard against it.

1 Like
8

From what I understand the construction script is called when the blueprint is moved/updated as well though no? Wouldn’t this cause all the manually set data overrides to be cleared constantly?

9

Only when you move or update it in the editor, and if you’re only setting a value it shouldn’t be a problem.

10

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The VehicleMovementComponent is a native C++ class. It is possible to modifiy the component class here to use the bp version of the component (which I am not, but selecting it would instantiate the BP version of the component). Is it what you are trying to do?