Unreal Blueprint Codegen

Unreal Engine Blueprint Code Generation (UE 5.4–5.7)

Generate complete .uasset files (Blueprints, Widget Blueprints, domain assets) from C++ in an editor module. Validated against UE 5.7. Most patterns work back to 5.4.

When this skill applies

User wants to author asset content programmatically — variables, function graphs, event-graph wiring, widget hierarchies, UMG animations, custom asset graphs — instead of clicking through the editor. Typical use case: generating Marketplace sample content (Quick-Start, walkthrough, demo conversations) so it can be regenerated reproducibly instead of hand-pruning binary .uasset files.

If the user only wants runtime widget instantiation (CreateWidget / WidgetTree->ConstructWidget at game time), this skill is overkill — that's just normal UMG.

Hard prerequisites

• C++ editor module with Type=Editor (or UncookedOnly). Python cannot author event graphs — UEdGraph/UK2Node_* are not exposed. Set up a UBlueprintFunctionLibrary in C++ and call its UFUNCTIONs from Python if a Python entry point is needed.
• Module must depend on at least: UnrealEd, BlueprintGraph, Kismet, KismetCompiler, AssetTools, AssetRegistry. Add UMG, UMGEditor, MovieScene, MovieSceneTracks for Widget Blueprints. See assets/experiment-module-template/ for a working .Build.cs.
• IWYU: include each engine header explicitly. Forward-declare UPackage/UObject etc. in your own headers; pull full headers in .cpp only.

The 30-second mental model

Copy code
Asset = UPackage  +  UObject (e.g. UBlueprint / UWidgetBlueprint / UMyAsset)
                     |
                     +-- source UEdGraph (editor-only, what designers see)
                     |     +-- UK2Node_* / UEdGraphNode_* (the boxes)
                     |           +-- UEdGraphPin (the wires)
                     |
                     +-- generated UClass  <- produced by CompileBlueprint
                           +-- FProperty / UFunction (what code uses at runtime)

Spawn editor nodes onto a graph, wire their pins, then call CompileBlueprint to bake the generated class. Save the package.

Decision tree

Standard workflow

For every generator function:

1. Create / load package: CreatePackage(*FullPath) then Package->FullyLoad().
2. Create the asset with FKismetEditorUtilities::CreateBlueprint(...) (covers both UBlueprint and UWidgetBlueprint — pass the right Blueprint class + GeneratedClass).
3. Build content — variables, function graphs, widget tree, animations.
4. Wire event graph / call user functions — only AFTER an intermediate compile if the wiring references content created in step 3 (see two-pass-compile.md).
5. MarkBlueprintAsStructurallyModified + CompileBlueprint — once at the end (or twice, two-pass).
6. Save: FAssetRegistryModule::AssetCreated(Asset) + Package->MarkPackageDirty() + UPackage::SavePackage(...).

Skeleton:

cppCopy code
#include "Engine/Blueprint.h"
#include "Kismet2/BlueprintEditorUtils.h"
#include "Kismet2/KismetEditorUtilities.h"
#include "AssetRegistry/AssetRegistryModule.h"
#include "UObject/SavePackage.h"

UBlueprint* CreateBP(const FString& PackagePath, const FString& AssetName, UClass* ParentClass)
{
    const FString FullPath = PackagePath / AssetName;
    UPackage* Package = CreatePackage(*FullPath);
    Package->FullyLoad();

    UBlueprint* BP = FKismetEditorUtilities::CreateBlueprint(
        ParentClass,                  // e.g. AActor::StaticClass() or UUserWidget::StaticClass()
        Package,
        FName(*AssetName),
        BPTYPE_Normal,
        UBlueprint::StaticClass(),    // or UWidgetBlueprint::StaticClass()
        UBlueprintGeneratedClass::StaticClass(),  // or UWidgetBlueprintGeneratedClass::StaticClass()
        FName("MyGenerator"));        // calling-context tag; any FName

    // ... build content here ...

    FBlueprintEditorUtils::MarkBlueprintAsStructurallyModified(BP);
    FKismetEditorUtilities::CompileBlueprint(BP);

    FAssetRegistryModule::AssetCreated(BP);
    Package->MarkPackageDirty();
    const FString Filename = FPackageName::LongPackageNameToFilename(
        Package->GetName(), FPackageName::GetAssetPackageExtension());
    FSavePackageArgs Args;
    Args.TopLevelFlags = RF_Public | RF_Standalone;
    UPackage::SavePackage(Package, BP, *Filename, Args);

    return BP;
}

Critical rules (read these even if skipping cookbooks)

1. SetExternalMember BEFORE AllocateDefaultPins on UK2Node_CallFunction

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UK2Node_CallFunction* Node = NewObject<UK2Node_CallFunction>(Graph);
Node->FunctionReference.SetExternalMember(
    GET_FUNCTION_NAME_CHECKED(UKismetSystemLibrary, PrintString),
    UKismetSystemLibrary::StaticClass());
Node->AllocateDefaultPins();   // AFTER. Reverse and the node spawns pinless.

Same rule for UK2Node_VariableSet/Get: set the variable reference first, then AllocateDefaultPins.

2. MarkBlueprintAsStructurallyModified vs Modified

• StructurallyModified: any node added/removed, any variable added/removed, any pin topology change. Forces class layout regen.
• Modified: only property tweaks on existing nodes/vars (e.g. changed a default value).

For codegen, almost always use MarkBlueprintAsStructurallyModified.

3. Function calls must be in the exec chain or get pruned

Non-pure functions (with exec pins) must be wired into the exec chain. If you connect their data output without wiring exec, the BP compiler prunes the call and the data pin reads default. Symptom: warning "X was pruned because its Exec pin is not connected".

cppCopy code
// WRONG — Compute is non-pure, exec not wired, gets pruned:
EvtThen->MakeLinkTo(SetCounter->GetExecPin());
ComputeRet->MakeLinkTo(CounterValPin);  // reads default, not actual return

// RIGHT — Compute in the chain:
EvtThen->MakeLinkTo(CallCompute->GetExecPin());
CallCompute->GetThenPin()->MakeLinkTo(SetCounter->GetExecPin());
ComputeRet->MakeLinkTo(CounterValPin);

4. Avoid (0,0) — auto-spawned default events live there

Both AActor-based BPs and Widget Blueprints auto-spawn placeholder events (BeginPlay, Tick, ActorBeginOverlap, PreConstruct, Construct) at (0, 0). Position your nodes well off, e.g. Y=-300, X=-400. Otherwise the user opens the asset to a tangled mess.

5. Two-pass compile when nodes reference newly-created content

If you create a function Compute and then wire CallCompute referencing it in the same pass, AllocateDefaultPins on CallCompute runs before Compute is reflected on the generated class — the pins for InValue/ReturnValue won't exist, and FindPinChecked asserts. Fix: compile after building the function, then wire the call. Same for UMG animations referenced via UK2Node_VariableGet. See references/two-pass-compile.md.

Common UE 5.7 gotchas (full list in caveats.md)

• PlayAnimationByName does NOT exist on UUserWidget in 5.7. Only PlayAnimation(UWidgetAnimation*, ...). Wire via UK2Node_VariableGet for the animation, not a string.
• Animation FName must equal display label. UUserWidget::GetAnimationByName matches GetFName(). Set both: NewObject<UWidgetAnimation>(WBP, FName("SlideIn"), ...) plus Anim->SetDisplayLabel(TEXT("SlideIn")).
• Outer hierarchy is load-bearing. UWidgetAnimation's outer must be the WBP. UMovieScene's outer must be the animation. Wrong outer → animation does not appear in the editor's Animations panel.
• UE_DEFINE_GAMEPLAY_TAG_STATIC for any tags the generated assets reference. Runtime UGameplayTagsManager::AddNativeGameplayTag from inside a generator function is too late — the editor UI's tag tree is already built. Register at module load.
• Python cannot reach UEdGraph / UK2Node_*. The only way to author event graphs is C++. Expose your generator as BlueprintCallable UFUNCTIONs on a UBlueprintFunctionLibrary if Python invocation is desired.
• FBlueprintEditorUtils::AddFunctionGraph does NOT spawn a Result node by default. Only Entry. If your function returns a value, manually create one with FGraphNodeCreator<UK2Node_FunctionResult>.

Quick FEdGraphPinType cheat sheet

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FEdGraphPinType T;
T.PinCategory = UEdGraphSchema_K2::PC_Boolean;            // bool
T.PinCategory = UEdGraphSchema_K2::PC_Int;                // int32
T.PinCategory = UEdGraphSchema_K2::PC_Real;
T.PinSubCategory = UEdGraphSchema_K2::PC_Float;           // float
T.PinSubCategory = UEdGraphSchema_K2::PC_Double;          // double
T.PinCategory = UEdGraphSchema_K2::PC_String;             // FString
T.PinCategory = UEdGraphSchema_K2::PC_Name;               // FName
T.PinCategory = UEdGraphSchema_K2::PC_Text;               // FText
T.PinCategory = UEdGraphSchema_K2::PC_Object;             // object ref
T.PinSubCategoryObject = AActor::StaticClass();           //   (with target class)
T.ContainerType = EPinContainerType::Array;               // wrap as TArray<>
T.ContainerType = EPinContainerType::Set;                 // TSet<>
T.ContainerType = EPinContainerType::Map;                 // TMap<>  (PinValueType for value)

Use with: FBlueprintEditorUtils::AddMemberVariable(BP, FName("MyVar"), T, FString(TEXT("DefaultValueAsString")));

Workflow checklist

When implementing a generator from this skill:

1. Confirm the editor module exists with the right dependencies (template in assets/experiment-module-template/).
2. Read references/k2node-cookbook.md for the patterns relevant to the asset type.
3. For Widget Blueprints, also read references/widget-blueprint-cookbook.md.
4. If wiring crosses content created in the same function, plan the two-pass compile structure (references/two-pass-compile.md).
5. Check references/caveats.md for known traps before assuming new behavior is a bug.
6. Test by building the editor target, opening the editor, calling the UFUNCTION (often via Python in the Output Log), and visually inspecting the asset.
7. Iterate: when a node looks wrong (red error border, missing pin, "X pruned" warning, default values where data should flow), the answer is almost always one of: missing exec wire, wrong allocation order, missing two-pass compile, or wrong pin name.

What this skill does NOT cover

• Cooking / packaging the generated assets — they are regular .uasset files, ship like any other.
• Editor Utility Widgets / Blutilities — you can call your generator UFUNCTIONs from those, but designing the EUW is separate UMG work.
• Runtime UMG — CreateWidget, AddToViewport, animation PlayAnimation calls in game code. That is not codegen.
• Custom UEdGraph schemas — if generating into a domain-specific graph (dialogue, quest, ability tree), use the plugin's own compiler entry point. Static compiler classes are often non-exported, so call the graph's update method (e.g. UMyGraph::UpdateAsset()) instead of the static compiler if linkage fails.