Undo redo.md

2020-09-30_13:19:47

Undo redo

Undo-redo regions are created from C++ by creating an FScopedTransaction on the stack. The name of the transaction is displayed in the undo history and in a popup when being undone. All marked modifications done while the FScopedTransaction is active become a single undo step. An operation is considered marked if it modifies a UPROPERTY and Modify has been called on the modified object before the modification is done. There may also be something about whether or not the UPROPERTY is replicated. Not sure. Modify is a member of UObject. Example:

UCLASS()
class UActorMover : public UObject
{
public:
    GENERATED_BODY();

    UPROPERTY()
    AActory* MyActor;

    UPROPERTY()
    int32 NumMoves;

    void MoveActor();
}

void UMyClass::MoveActor()
{
    // Start a new undo transaction. Any marked changes made until
    // this object goes out of scope is considered a single atomic
    // operation in the undo history.
    FScopedTransaction Transaction(
        LOCTEXT("MoveActor", "Move actor."));

    // This member function will make two changes, one to itself and
    // one on the Actor it has a pointer to.

    // Mark this object as about to be modified.
    Modify();

    // Make the modification. NumMoves is a `UPROPERTY` on an objects on
    // which Modify has been called so it will be included in the undo
    // history.
    ++NumMoves;

    // Next we do the move. First mark the Actor for modification and then
    // do the actual modification. AActor is responsible for doing any
    // propagation of the Modify to sub-objects that may be necessary in
    // case the modifying member function, SetActorLocation in this case,
    // modifies a sub-object.
    MyActor->Modify();
    MyActor->SetActorLocation();

}

I believe undo only applies to objects with the RF_Transactional flag set. A pattern I've seen is that the undo management is handled by GUI-level code such as Component Visualizers and Details Customizations, not by the modified object itself.

FScopedTransaction transaction(); // will open up a slot in for data in GEditor
Modify(); // On what should this be called? The FScopedTransaction? Or is `Modify` a stand-in name for any modification on any object?
// Do you modifications.

//~FScopedTransaction() will finish the transaction, called when `transaction` goes out of scope.