🎚️ Lesson 6.2: Exposing Properties to Blueprint

The Two Axes, Applied

Recall the two independent axes from Lesson 2.3: editor access (EditAnywhere / VisibleAnywhere / Edit*Only) and Blueprint access (BlueprintReadWrite / BlueprintReadOnly). Here's how to combine them purposefully for a real class:

// Designer tunes per-instance; BP logic can also change it at runtime:
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Combat")
float Damage = 25.0f;

// A type-wide constant: set on the Blueprint default only, BP can read it:
UPROPERTY(EditDefaultsOnly, BlueprintReadOnly, Category = "Combat")
int32 MagazineSize = 30;

// Runtime state C++ owns; BP (e.g. HUD) reads but cannot set:
UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "State")
int32 CurrentAmmo = 30;

// A component reference — shown for inspection, not reassigned by hand:
UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "Components")
TObjectPtr<UStaticMeshComponent> Mesh;

ExposeOnSpawn

When a Blueprint spawns your actor via "Spawn Actor from Class," it can set exposed properties at spawn time — pins appear right on the spawn node. Mark a property with meta = (ExposeOnSpawn = true).

// A projectile whose damage and speed are set when a weapon spawns it:
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Projectile",
    meta = (ExposeOnSpawn = true))
float Damage = 10.0f;

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Projectile",
    meta = (ExposeOnSpawn = true))
float Speed = 2000.0f;

Inspector Polish

Small touches make a C++ class feel professional in the editor. Building on the meta options from Lesson 2.3:

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Combat|Ranged",
    meta = (ClampMin = "0.0", ClampMax = "1000.0", UIMin = "0.0", UIMax = "500.0",
            ToolTip = "Base damage per hit before modifiers."))
float Damage = 25.0f;

// A dependent property that only appears when relevant:
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Combat")
bool bUsesSpread = false;

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Combat",
    meta = (EditCondition = "bUsesSpread", ClampMin = "0.0"))
float SpreadAngle = 2.0f;

Guarding State

Some values are dangerous to let Blueprint set directly — health, ammo, an internal index — because a bad write breaks an invariant. Expose them read-only and provide a BlueprintCallable setter that enforces the rules.

// Read-only to Blueprint — no direct writes.
UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "State")
float CurrentHealth = 100.0f;

// The ONLY sanctioned way for Blueprint to change health — it clamps & notifies.
UFUNCTION(BlueprintCallable, Category = "State")
void SetHealth(float NewHealth)
{
    CurrentHealth = FMath::Clamp(NewHealth, 0.0f, MaxHealth);
    OnHealthChanged.Broadcast(CurrentHealth, MaxHealth);   // event from Lesson 5.4
}

Reacting to Edits

Sometimes you want C++ to respond the moment a designer changes a property in the editor — e.g. rebuild something when a size value changes. Override PostEditChangeProperty (editor-only).

#if WITH_EDITOR
void AMyActor::PostEditChangeProperty(FPropertyChangedEvent& Event)
{
    Super::PostEditChangeProperty(Event);

    // Which property changed?
    const FName Name = (Event.Property != nullptr) ? Event.Property->GetFName() : NAME_None;
    if (Name == GET_MEMBER_NAME_CHECKED(AMyActor, GridSize))
    {
        RebuildGrid();   // react to the designer's edit immediately
    }
}
#endif

Hands-on Exercise & Quiz

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Pickup",
    meta = (ClampMin = "1", ClampMax = "999", ExposeOnSpawn = true,
            ToolTip = "How much this pickup grants."))
int32 Amount = 25;

UPROPERTY(EditDefaultsOnly, BlueprintReadOnly, Category = "Pickup")
EPickupKind PickupType = EPickupKind::Health;

UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "State")
bool bConsumed = false;

UFUNCTION(BlueprintCallable, Category = "Pickup")
void Consume() { bConsumed = true; /* ...effects, Destroy... */ }

Summary

📚 Additional Resources

• UProperties (Epic)
• BenUI — UPROPERTY specifier reference

🚀 What's Next?

Functions and properties cross the boundary. But what about your own types — the USTRUCTs and UENUMs from Module 2? Next we make them first-class Blueprint citizens so you can pass rich data across the boundary, not just primitives.