UnrealEngine/Engine/Source/Editor/DataHierarchyEditor/Public/DataHierarchyViewModelBase.h

// Copyright Epic Games, Inc. All Rights Reserved.

#pragma once

#include "CoreMinimal.h"
#include "PropertyEditorDelegates.h"
#include "ToolMenuSection.h"
#include "IPropertyRowGenerator.h"
#include "Misc/TransactionObjectEvent.h"
#include "EditorUndoClient.h"
#include "Widgets/Views/STableRow.h"
#include "TickableEditorObject.h"
#include "Engine/TimerHandle.h"
#include "Templates/SubclassOf.h"
#include "UObject/Package.h"
#include "UObject/UObjectGlobals.h"
#include "DataHierarchyViewModelBase.generated.h"

/** HIERARCHY EDITOR
 *	The hierarchy editor is a generic tool to organize and structure all kinds of data.
 *	It inherently supports sections, categories, and items. You can add your own items and customize how they are visualized.
 *	Each hierarchy element is a UObject, and some hierarchy elements will represent externally owned data.
 *	For example, categories and sections defined within the hierarchy are also owned by the hierarchy, but an item might represent a parameter defined elsewhere.
 *
 *  Each hierarchy element is pure data and should not reference externally owned data that could become invalid.
 *  To define per-element rules, each hierarchy element gets assigned one view model.
 *  
 *	To use the Hierarchy Editor, you need multiple things:
 *	1) A UHierarchyRoot object that the Hierarchy Editor uses to store the created hierarchy
 *	2) A UDataHierarchyViewModelBase-derived object that defines core hierarchy rules. This is the main object responsible for configuring your hierarchy.
 *	-- The UDataHierarchyViewModelBase-derived class has multiple virtual functions you need to override. The key functions are:
 *	--- a) GetHierarchyRoot(), pointing to the UHierarchyRoot object you created in 1)
 *	--- b) PrepareSourceItems(UHierarchyRoot* SourceRoot, ...), which you need to use to populate the list of elements to be organized.
 *	------ These are transient items and a transient root. You can also add sections and categories here if you wish to structure the source element view.
 *	------ To add a child, call NewObject with ParentElement as the outer (in a flat list the SourceRoot).
 *	------ Then use ParentElement->GetChildrenMutable().Add(NewElement).
 *	------ This way, you can write your own initialization functions
 *	------ You can also add categories and sections to the SourceRoot
 *	--- c) Optionally but likely: CreateCustomViewModelForData(UHierarchyElement* Element, ...), which is used to create and assign non-default view models for each hierarchy element
 *	------ You don't need to implement this, but any slightly advanced use case that requires modification to hierarchy logic will need to implement this
 *	3) An SHierarchyEditor widget, which takes in the UDataHierarchyViewModelBase-derived object you created in 2). This way, the hierarchy editor knows where to store data, and how to initialize.
 *	--- Additionally, it has arguments such as 'OnGenerateRowContentWidget', which gives you an ElementViewModel.
 *	--- After determining the type, you can use this to define widgets for each hierarchy element.
 *
 *	Tips and tricks:
 *	1) Each hierarchy element can have a FHierarchyElementIdentity consisting of guid(s) and/or name(s).
 *	--- This can be used to uniquely identify an element, navigate to it and so on.
 *	2) To deal with automated cleanup of stale hierarchy elements that represent external data, you can set a UHierarchyDataRefreshContext-derived object on the UDataHierarchyViewModelBase object.
 *	--- The idea is to create a new derived class with objects and properties of the system that uses the Hierarchy Editor.
 *	--- For example, in a graph based system, the DataRefreshContext object can point to the graph.
 *	--- In an FHierarchyElementViewModel's 'DoesExternalDataStillExist' function,
 *	--- you can access the DataRefreshContext to determine whether your externally represented hierarchy elements should still exist.
 *	--- If not, it is automatically deleted.
 *	3) In the details panel, You can edit the hierarchy elements themselves, or external objects 
 *
 *  To make use of the created hierarchy, you access the UHierarchyRoot object you created in 1), and query it for its children, sections etc.
 *  The HierarchyEditor does not define how to use the created hierarchy data in your own UI. It only lets you structure and edit data.
 *  How you use it from a 'data consumption' point of view is up to you.
 */

struct FHierarchyRootViewModel;
/** This struct is used to identify a given hierarchy element and can be based on guids and/or names.
 *  This is particularly useful when a hierarchy element represents an object or a property that is not owned by the hierarchy itself.
 *  
 *  
 */
USTRUCT()
struct FHierarchyElementIdentity
{
	GENERATED_BODY()

	FHierarchyElementIdentity() {}
	FHierarchyElementIdentity(TArray<FGuid> InGuids, TArray<FName> InNames) : Guids(InGuids), Names(InNames) {}
	
	/** An array of guids that have to be satisfied in order to match. */
	UPROPERTY()
	TArray<FGuid> Guids;

	/** Optionally, an array of names can be specified in place of guids. If guids & names are present, guids have to be satisfied first, then names. */
	UPROPERTY()
	TArray<FName> Names;

	bool IsValid() const
	{
		return Guids.Num() > 0 || Names.Num() > 0;
	}
	
	bool operator==(const FHierarchyElementIdentity& OtherIdentity) const
	{
		if(Guids.Num() != OtherIdentity.Guids.Num() || Names.Num() != OtherIdentity.Names.Num())
		{
			return false;
		}

		for(int32 GuidIndex = 0; GuidIndex < Guids.Num(); GuidIndex++)
		{
			if(Guids[GuidIndex] != OtherIdentity.Guids[GuidIndex])
			{
				return false;
			}
		}

		for(int32 NameIndex = 0; NameIndex < Names.Num(); NameIndex++)
		{
			if(!Names[NameIndex].IsEqual(OtherIdentity.Names[NameIndex]))
			{
				return false;
			}
		}

		return true;
	}

	bool operator!=(const FHierarchyElementIdentity& OtherIdentity) const
	{
		return !(*this == OtherIdentity);
	}
};

FORCEINLINE uint32 GetTypeHash(const FHierarchyElementIdentity& Identity)
{
	uint32 Hash = 0;
	
	for(const FGuid& Guid : Identity.Guids)
	{
		Hash = HashCombine(Hash, GetTypeHash(Guid));
	}
	
	for(const FName& Name : Identity.Names)
	{
		Hash = HashCombine(Hash, GetTypeHash(Name));
	}
	
	return Hash;
}

/** A base class that is used to refresh data that represents external data. Inherit from this class if you need more context data. */
UCLASS(Transient)
class DATAHIERARCHYEDITOR_API UHierarchyDataRefreshContext : public UObject
{
	GENERATED_BODY()
};

UCLASS()
class DATAHIERARCHYEDITOR_API UHierarchyElement : public UObject
{
	GENERATED_BODY()

public:
	UHierarchyElement() { Identity.Guids.Add(FGuid::NewGuid()); }
	virtual ~UHierarchyElement() override {}

	TArray<TObjectPtr<UHierarchyElement>>& GetChildrenMutable() { return Children; }
	const TArray<UHierarchyElement*>& GetChildren() const;

	template<class ChildClass>
	ChildClass* AddChild();

	UHierarchyElement* FindChildWithIdentity(FHierarchyElementIdentity ChildIdentity, bool bSearchRecursively = false);

	UHierarchyElement* CopyAndAddItemAsChild(const UHierarchyElement& ItemToCopy);
	UHierarchyElement* CopyAndAddItemUnderParentIdentity(const UHierarchyElement& ItemToCopy, FHierarchyElementIdentity ParentIdentity);
	
	/** Remove a child with a given identity. Can be searched recursively. This function operates under the assumption there will be only one item with a given identity. */
	bool RemoveChildWithIdentity(FHierarchyElementIdentity ChildIdentity, bool bSearchRecursively = false);
	
	template<class ChildClass>
	bool DoesOneChildExist(bool bRecursive = false) const;

	template<class ChildClass>
	TArray<ChildClass*> GetChildrenOfType(TArray<ChildClass*>& Out, bool bRecursive = false) const;

	template<class PREDICATE_CLASS>
	void SortChildren(const PREDICATE_CLASS& Predicate, bool bRecursive = false);
	
	virtual FString ToString() const { return GetName(); }
	FText ToText() const { return FText::FromString(ToString()); }
	
	/** An identity can be optionally set to create a mapping from previously existing guids or names to hierarchy items that represent them. */
	void SetIdentity(FHierarchyElementIdentity InIdentity) { Identity = InIdentity; }
	FHierarchyElementIdentity GetPersistentIdentity() const { return Identity; }
	TArray<FHierarchyElementIdentity> GetParentIdentities() const;
	
	/** Overridden modify method to also mark all children as modified */
	virtual bool Modify(bool bAlwaysMarkDirty = true) override;

protected:
	virtual void PostLoad() override;

	UPROPERTY()
	TArray<TObjectPtr<UHierarchyElement>> Children;

	UPROPERTY()
	FHierarchyElementIdentity Identity;
	
	/** An optional guid; can be used if hierarchy items represent outside items */
	UPROPERTY()
	FGuid Guid_DEPRECATED;
};

template <class ChildClass>
ChildClass* UHierarchyElement::AddChild()
{
	ChildClass* NewChild = NewObject<ChildClass>(this);
	GetChildrenMutable().Add(NewChild);

	return NewChild;
}

template <class ChildClass>
bool UHierarchyElement::DoesOneChildExist(bool bRecursive) const
{
	for(UHierarchyElement* ChildElement : Children)
	{
		if(ChildElement->IsA<ChildClass>())
		{
			return true;
		}
	}

	if(bRecursive)
	{
		for(UHierarchyElement* ChildElement : Children)
		{
			if(ChildElement->DoesOneChildExist<ChildClass>(bRecursive))
			{
				return true;
			}
		}
	}

	return false;
}

template <class ChildClass>
TArray<ChildClass*> UHierarchyElement::GetChildrenOfType(TArray<ChildClass*>& Out, bool bRecursive) const
{
	for(UHierarchyElement* ChildElement : Children)
	{
		if(ChildElement->IsA<ChildClass>())
		{
			Out.Add(Cast<ChildClass>(ChildElement));
		}
	}

	if(bRecursive)
	{
		for(UHierarchyElement* ChildElement : Children)
		{
			ChildElement->GetChildrenOfType<ChildClass>(Out, bRecursive);
		}
	}

	return Out;
}

template <class PREDICATE_CLASS>
void UHierarchyElement::SortChildren(const PREDICATE_CLASS& Predicate, bool bRecursive)
{
	Children.Sort(Predicate);

	if(bRecursive)
	{
		for(TObjectPtr<UHierarchyElement> ChildElement : Children)
		{
			ChildElement->SortChildren(Predicate, bRecursive);
		}
	}
}

/** A minimal implementation of a section. */
UCLASS()
class DATAHIERARCHYEDITOR_API UHierarchySection : public UHierarchyElement
{
	GENERATED_BODY()

public:
	UHierarchySection() {}

	void SetSectionName(FName InSectionName) { Section = InSectionName; }
	FName GetSectionName() const { return Section; }
	
	void SetSectionNameAsText(const FText& Text);
	FText GetSectionNameAsText() const { return FText::FromName(Section); }

	void SetTooltip(const FText& InTooltip) { Tooltip = InTooltip; }
	FText GetTooltip() const { return Tooltip; }

	virtual FString ToString() const override { return Section.ToString(); }
private:
	UPROPERTY()
	FName Section;

	/** The tooltip used when the user is hovering this section */
	UPROPERTY(EditAnywhere, Category = "Section", meta = (MultiLine = "true"))
	FText Tooltip;
};

/** UHierarchyRoot is used as the main object for serialization purposes, and a transient root is created automatically by the widget to populate the source list of items. */
UCLASS()
class DATAHIERARCHYEDITOR_API UHierarchyRoot : public UHierarchyElement
{
	GENERATED_BODY()
public:
	UHierarchyRoot() {}
	virtual ~UHierarchyRoot() override {}

	const TArray<UHierarchySection*>& GetSectionData() const { return Sections; }
	TArray<TObjectPtr<UHierarchySection>>& GetSectionDataMutable() { return Sections; }

	TSet<FName> GetSections() const;
	int32 GetSectionIndex(UHierarchySection* Section) const;

	UHierarchySection* AddSection(FText InNewSectionName, int32 InsertIndex = INDEX_NONE, TSubclassOf<UHierarchySection> SectionClass = UHierarchySection::StaticClass());
	UHierarchySection* FindSectionByIdentity(FHierarchyElementIdentity SectionIdentity);
	/** This will copy the section element itself */
	void DuplicateSectionFromOtherRoot(const UHierarchySection& SectionToCopy);
	void RemoveSection(FText SectionName);
	void RemoveSectionByIdentity(FHierarchyElementIdentity SectionIdentity);
	
	virtual bool Modify(bool bAlwaysMarkDirty = true) override;
	virtual void Serialize(FStructuredArchive::FRecord Record) override;

	virtual void EmptyAllData();
protected:

	UPROPERTY()
	TArray<TObjectPtr<UHierarchySection>> Sections;
};

/** A minimal implementation of an item. Inherit from this and add your own properties. */
UCLASS(MinimalAPI)
class UHierarchyItem : public UHierarchyElement
{
	GENERATED_BODY()
public:
	UHierarchyItem() {}
	virtual ~UHierarchyItem() override {}
};

/** A category, potentially pointing at the section it belongs to. Only top-level categories can belong to sections by default.
 *  Inherit from this to add your own properties.  */
UCLASS()
class DATAHIERARCHYEDITOR_API UHierarchyCategory : public UHierarchyElement
{
	GENERATED_BODY()
public:
	UHierarchyCategory() {}
	UHierarchyCategory(FName InCategory) : Category(InCategory) {}
	
	void SetCategoryName(FName NewCategory) { Category = NewCategory; }
	FName GetCategoryName() const { return Category; }

	FText GetCategoryAsText() const { return FText::FromName(Category); }
	FText GetTooltip() const { return Tooltip; }

	void SetSection(UHierarchySection* InSection) { Section = InSection; }
	const UHierarchySection* GetSection() const { return Section; }

	virtual FString ToString() const override { return Category.ToString(); }

	/** Since the category points to a section object, during merge or copy paste etc. it is possible the section pointer will point at a section from another root.
	 *  We fix this up by looking through our available sections and match up via persistent identity.
	 *  This function expects the correct section with the same identity to exist already at the root level
	 */
	void FixupSectionLinkage();

	static FHierarchyElementIdentity ConstructIdentity();

protected:
	virtual void PostLoad() override;
private:
	UPROPERTY()
	FName Category;

	/** The tooltip used when the user is hovering this category */
	UPROPERTY(EditAnywhere, Category = "Category", meta = (MultiLine = "true"))
	FText Tooltip;

	UPROPERTY()
	TObjectPtr<UHierarchySection> Section = nullptr;
};

struct FHierarchyElementViewModel;
struct FHierarchySectionViewModel;

/** Inherit from this to allow UI customization for your drag & drop operation by overriding CreateCustomDecorator. */
class DATAHIERARCHYEDITOR_API FHierarchyDragDropOp : public FDragDropOperation
{
public:
	DRAG_DROP_OPERATOR_TYPE(FHierarchyDragDropOp, FDragDropOperation)

	FHierarchyDragDropOp(TSharedPtr<FHierarchyElementViewModel> InDraggedElementViewModel);

	virtual void Construct() override { FDragDropOperation::Construct(); }
	virtual TSharedPtr<SWidget> GetDefaultDecorator() const override final;
	
	/** Override this custom decorator function to provide custom widget visuals. If not specified, you can still use Label and Description. */
	virtual TSharedRef<SWidget> CreateCustomDecorator() const { return SNullWidget::NullWidget; }
	
	TWeakPtr<FHierarchyElementViewModel> GetDraggedElement() { return DraggedElement; }
	
	void SetLabel(FText InText) { Label = InText; }
	FText GetLabel() const { return Label; }

	void SetDescription(FText InText) { Description = InText; }
	FText GetDescription() const { return Description; }

	void SetFromSourceList(bool bInFromSourceList) { bFromSourceList = bInFromSourceList; }
	bool GetIsFromSourceList() const { return bFromSourceList; }
protected:
	TWeakPtr<FHierarchyElementViewModel> DraggedElement;
	
	/** Label will be displayed if no custom decorator has been specified. */
	FText Label;
	/** Useful for runtime tweaking of the tooltip based on what we are hovering. Always displayed if not-empty */
	FText Description;
	/** If the drag drop op is from the source list, we can further customize the actions */
	bool bFromSourceList = false;
};

UCLASS(BlueprintType)
class DATAHIERARCHYEDITOR_API UHierarchyMenuContext : public UObject
{
	GENERATED_BODY()

public:
	TWeakObjectPtr<class UDataHierarchyViewModelBase> HierarchyViewModel;
	TArray<TSharedPtr<FHierarchyElementViewModel>> MenuHierarchyElements;
};

/** The main controller class for the SHierarchyEditor widget. Defines core hierarchy rules.
 *  Inherit from this and override the required virtual functions, instantiate an object, Initialize it and pass it to the SHierarchyEditor widget. */
UCLASS(Abstract, Transient)
class DATAHIERARCHYEDITOR_API UDataHierarchyViewModelBase : public UObject, public FSelfRegisteringEditorUndoClient
{
public:
	DECLARE_MULTICAST_DELEGATE(FOnHierarchyChanged)
	DECLARE_MULTICAST_DELEGATE(FOnHierarchyPropertiesChanged)
	DECLARE_DELEGATE_OneParam(FOnSectionActivated, TSharedPtr<FHierarchySectionViewModel> Section)
	DECLARE_DELEGATE_OneParam(FOnElementAdded, TSharedPtr<FHierarchyElementViewModel> AddedElement)
	DECLARE_DELEGATE_OneParam(FOnRefreshViewRequested, bool bForceFullRefresh)
	DECLARE_DELEGATE_OneParam(FOnNavigateToElementIdentityInHierarchyRequested, FHierarchyElementIdentity Identity)
	DECLARE_DELEGATE_OneParam(FOnNavigateToElementInHierarchyRequested, TSharedPtr<FHierarchyElementViewModel> ElementViewModel)

	GENERATED_BODY()

	UDataHierarchyViewModelBase();
	virtual ~UDataHierarchyViewModelBase() override;

	/** Initialize is called automatically for you, but it is recommended to call it manually after creating the HierarchyViewModel in your own Initialize function. This lets you access external data. */
	void Initialize();
	/** Call Finalize manually when you no longer need the HierarchyViewModel. */
	void Finalize();
	
	bool IsInitialized() const { return bIsInitialized; }
	bool IsFinalized() const { return bIsFinalized; }
	bool IsValid() const { return IsInitialized() && !IsFinalized(); }

	FName GetContextMenuName () const;

	/** Creates view model hierarchy elements. To create custom view models, override CreateCustomViewModelForElement. */
	TSharedPtr<FHierarchyElementViewModel> CreateViewModelForElement(UHierarchyElement* Element, TSharedPtr<FHierarchyElementViewModel> Parent);
	/** Get the root view model associated with the hierarchy. */
	TSharedPtr<struct FHierarchyRootViewModel> GetHierarchyRootViewModel() const { return HierarchyRootViewModel; }
	/** Hierarchy items reflect the already edited hierarchy. This should generally be constructed from persistent serialized data. */
	const TArray<TSharedPtr<FHierarchyElementViewModel>>& GetHierarchyItems() const;

	TSharedPtr<struct FHierarchySectionViewModel> GetDefaultHierarchySectionViewModel() const { return DefaultHierarchySectionViewModel; }
public:
	/** The hierarchy root the widget is editing. This should point to persistent data stored somewhere else as the serialized root of the hierarchy. */
	virtual UHierarchyRoot* GetHierarchyRoot() const PURE_VIRTUAL(UDataHierarchyViewModelBase::GetHierarchyRoot, return nullptr;);
	/** The outer for the transient source root creation can be overridden. */
	virtual UObject* GetOuterForSourceRoot() const { return GetTransientPackage(); }
	/** Prepares the items we want to create a hierarchy for. Primary purpose is to add children to the source root to gather the items to display in the source panel.
	 * The root view model is also given as a way to forcefully sync view models to access additional functionality, if needed */
	virtual void PrepareSourceItems(UHierarchyRoot* SourceRoot, TSharedPtr<FHierarchyRootViewModel> SourceRootViewModel) PURE_VIRTUAL(UDataHierarchyViewModelBase::PrepareSourceItems,);
	/** The class used for creating categories. You can subclass UHierarchyCategory to add new properties. */
	virtual TSubclassOf<UHierarchyCategory> GetCategoryDataClass() const;
	/** The class used for creating sections. You can subclass UHierarchySection to add new properties. */
	virtual TSubclassOf<UHierarchySection> GetSectionDataClass() const;
	/** Function to implement drag drop ops. FHierarchyDragDropOp is a default implementation for a single hierarchy element. Inherit from it and override CreateCustomDecorator for custom UI. */
	virtual TSharedRef<FHierarchyDragDropOp> CreateDragDropOp(TSharedRef<FHierarchyElementViewModel> Item);
	/** This needs to return true if you want the details panel to show up. */
	virtual bool SupportsDetailsPanel() { return true; }
	/** Overriding this allows to define details panel instance customizations for specific UClasses */
	virtual TArray<TTuple<UClass*, FOnGetDetailCustomizationInstance>> GetInstanceCustomizations() { return {}; }

protected:
	/** Additional commands can be specified overriding the SetupCommands function. */
	virtual void SetupCommands() {}
private:
	/** Lets you add some additional logic to the Initialize function. */
	virtual void InitializeInternal() {}
	virtual void FinalizeInternal() {}

	/** This function is used to determine custom view models for Hierarchy Elements. Called by CreateViewModelForElement. */
	virtual TSharedPtr<FHierarchyElementViewModel> CreateCustomViewModelForElement(UHierarchyElement* Element, TSharedPtr<FHierarchyElementViewModel> Parent) { return nullptr; }
	/** Override this to add custom context menu options. Defaults to common actions such as finding, renaming and deleting.
	 * The hierarchy elements to generate this menu for are contained in a UHierarchyMenuContext object accessible through the ToolMenu. */
	virtual void GenerateDynamicContextMenuInternal(UToolMenu* DynamicToolMenu) const;
public:
	const UHierarchyDataRefreshContext* GetRefreshContext() const { return RefreshContext; }
	/** Set the refresh context to easily allow Hierarchy Elements representing external data to access whether the external data still exists. */
	void SetRefreshContext(UHierarchyDataRefreshContext* InContext) { RefreshContext = InContext; }
	
	UHierarchyElement* AddElementUnderRoot(TSubclassOf<UHierarchyElement> NewChildClass, FHierarchyElementIdentity ChildIdentity);
	void AddCategory(TSharedPtr<FHierarchyElementViewModel> CategoryParent = nullptr) const;
	void AddSection() const;

	/** Delete all specified elements. */
	void DeleteElements(TArray<TSharedPtr<FHierarchyElementViewModel>> ViewModels) const;
	/** Special case for deleting a specific element based on its identity.
	 *  Useful for externally removing an element from the hierarchy when you don't have access to the view model.*/
	void DeleteElementWithIdentity(FHierarchyElementIdentity Identity);

	void NavigateToElementInHierarchy(const FHierarchyElementIdentity& HierarchyIdentity) const;
	void NavigateToElementInHierarchy(const TSharedRef<FHierarchyElementViewModel> HierarchyElement) const;
	
	/** Refreshes all data and widgets */
	void ForceFullRefresh();
	void ForceFullRefreshOnTimer();
	void RequestFullRefreshNextFrame();
	
	TSharedRef<FUICommandList> GetCommands() const { return Commands.ToSharedRef(); }
	
	void OnGetChildren(TSharedPtr<FHierarchyElementViewModel> Element, TArray<TSharedPtr<FHierarchyElementViewModel>>& OutChildren) const;
	
	void RefreshAllViews(bool bFullRefresh = false) const;
	void RefreshSourceView(bool bFullRefresh = false) const;
	void RefreshHierarchyView(bool bFullRefresh = false) const;
	void RefreshSectionsView() const;
	
	// Delegate that call functions from SHierarchyEditor
	FSimpleDelegate& OnRefreshSourceItemsRequested() { return RefreshSourceItemsRequestedDelegate; }
	FOnRefreshViewRequested& OnRefreshSourceView() { return RefreshSourceViewDelegate; }
	FOnRefreshViewRequested& OnRefreshHierarchyView() { return RefreshHierarchyWidgetDelegate; }
	FSimpleDelegate& OnRefreshSectionsView() { return RefreshSectionsViewDelegate; }

	// Delegates for external systems
	FOnHierarchyChanged& OnHierarchyChanged() { return OnHierarchyChangedDelegate; } 
	FOnHierarchyChanged& OnHierarchyPropertiesChanged() { return OnHierarchyPropertiesChangedDelegate; } 
	FOnElementAdded& OnElementAdded() { return OnElementAddedDelegate; }
	FOnRefreshViewRequested& OnRefreshViewRequested() { return RefreshAllViewsRequestedDelegate; }
	FOnNavigateToElementIdentityInHierarchyRequested& OnNavigateToElementIdentityInHierarchyRequested() { return OnNavigateToElementIdentityInHierarchyRequestedDelegate; }
	FOnNavigateToElementInHierarchyRequested& OnNavigateToElementInHierarchyRequested() { return OnNavigateToElementInHierarchyRequestedDelegate; }
	FSimpleDelegate& OnInitialized() { return OnInitializedDelegate; }
	
	// Sections
	void SetActiveHierarchySection(TSharedPtr<struct FHierarchySectionViewModel>);
	TSharedPtr<FHierarchySectionViewModel> GetActiveHierarchySection() const;
	UHierarchySection* GetActiveHierarchySectionData() const;
	bool IsHierarchySectionActive(const UHierarchySection* Section) const;
	FOnSectionActivated& OnHierarchySectionActivated() { return OnHierarchySectionActivatedDelegate; }

	FString OnElementToStringDebug(TSharedPtr<FHierarchyElementViewModel> ElementViewModel) const;

protected:	
	virtual void PostUndo(bool bSuccess) override;
	virtual void PostRedo(bool bSuccess) override;
	virtual bool MatchesContext(const FTransactionContext& InContext, const TArray<TPair<UObject*, FTransactionObjectEvent>>& TransactionObjectContexts) const override;
private:
	bool FilterForHierarchySection(TSharedPtr<const FHierarchyElementViewModel> ViewModel) const;
	bool FilterForUncategorizedRootItemsInAllSection(TSharedPtr<const FHierarchyElementViewModel> ViewModel) const;

	static void GenerateDynamicContextMenu(UToolMenu* ToolMenu);

	void ToolMenuRequestRename(const FToolMenuContext& Context) const;
	bool ToolMenuCanRequestRename(const FToolMenuContext& Context) const;

	void ToolMenuDelete(const FToolMenuContext& Context) const;
	bool ToolMenuCanDelete(const FToolMenuContext& Context) const;

	void ToolMenuNavigateTo(const FToolMenuContext& Context) const;
	bool ToolMenuCanNavigateTo(const FToolMenuContext& Context) const;
protected:
	UPROPERTY()
	TObjectPtr<UHierarchyRoot> HierarchyRoot;
	
	TSharedPtr<struct FHierarchyRootViewModel> HierarchyRootViewModel;

	TSharedPtr<FHierarchySectionViewModel> DefaultHierarchySectionViewModel;
	TWeakPtr<struct FHierarchySectionViewModel> ActiveHierarchySection;

	TSharedPtr<FUICommandList> Commands;

	UPROPERTY(Transient)
	TObjectPtr<UHierarchyDataRefreshContext> RefreshContext = nullptr;
	
protected:
	// delegate collection to call UI functions
	FSimpleDelegate RefreshSourceItemsRequestedDelegate;
	FOnRefreshViewRequested RefreshAllViewsRequestedDelegate;
	FOnRefreshViewRequested RefreshSourceViewDelegate;
	FOnRefreshViewRequested RefreshHierarchyWidgetDelegate;
	FSimpleDelegate RefreshSectionsViewDelegate;
	FOnNavigateToElementIdentityInHierarchyRequested OnNavigateToElementIdentityInHierarchyRequestedDelegate;
	FOnNavigateToElementInHierarchyRequested OnNavigateToElementInHierarchyRequestedDelegate;
	
	FOnElementAdded OnElementAddedDelegate;
	FOnSectionActivated OnHierarchySectionActivatedDelegate;
	FOnSectionActivated OnSourceSectionActivatedDelegate;
	FOnHierarchyChanged OnHierarchyChangedDelegate;
	FOnHierarchyPropertiesChanged OnHierarchyPropertiesChangedDelegate;
	
	FSimpleDelegate OnInitializedDelegate;

	FTimerHandle FullRefreshNextFrameHandle;

private:
	UPROPERTY(Transient)
	bool bIsInitialized = false;
	
	UPROPERTY(Transient)
	bool bIsFinalized = false;
};

/** The base view model for all elements in the hierarchy. There are four base view models inheriting from this; for roots, items, categories, and sections.
 *  When creating a new view model, you should inherit from one of those four base view models.
 */
struct DATAHIERARCHYEDITOR_API FHierarchyElementViewModel : TSharedFromThis<FHierarchyElementViewModel>, public FTickableEditorObject
{
	DECLARE_DELEGATE(FOnSynced)
	DECLARE_DELEGATE_RetVal_OneParam(bool, FOnFilterChild, const TSharedPtr<const FHierarchyElementViewModel> Child);
	DECLARE_DELEGATE_OneParam(FOnChildRequestedDeletion, TSharedPtr<FHierarchyElementViewModel> Child)
	
	FHierarchyElementViewModel(UHierarchyElement* InElement, TSharedPtr<FHierarchyElementViewModel> InParent, TWeakObjectPtr<UDataHierarchyViewModelBase> InHierarchyViewModel, bool bInIsForHierarchy)
		: Element(InElement)
		, Parent(InParent)
		, HierarchyViewModel(InHierarchyViewModel)
		, bIsForHierarchy(bInIsForHierarchy)
	{		
		
	}

	/** Can be implemented for additional logic that the constructor isn't valid for. */
	virtual void Initialize() {}
	
	virtual ~FHierarchyElementViewModel() override;
	
	UHierarchyElement* GetDataMutable() const { return Element; }
	const UHierarchyElement* GetData() const { return Element; }
	
	template<class T>
	T* GetDataMutable() const { return Cast<T>(Element); }
	
	template<class T>
	const T* GetData() const { return Cast<T>(Element); }
	
	virtual void Tick(float DeltaTime) override;
	virtual TStatId GetStatId() const override;
	
	virtual FString ToString() const { return Element->ToString(); }
	FText ToStringAsText() const { return FText::FromString(ToString()); }
	virtual TArray<FString> GetSearchTerms() const { return {ToString()} ;}

	void RefreshChildrenData();
	void SyncViewModelsToData();
	
	/** Every item view model can define its own sort order for its children. By default we put categories above items. */
	virtual void SortChildrenData() const;
	
	const TArray<TSharedPtr<FHierarchyElementViewModel>>& GetChildren() const { return Children; }
	TArray<TSharedPtr<FHierarchyElementViewModel>>& GetChildrenMutable() { return Children; }
	const TArray<TSharedPtr<FHierarchyElementViewModel>>& GetFilteredChildren() const;
	
	void AddChildFilter(FOnFilterChild InFilterChild);
	
	template<class DataClass, class ViewModelChildClass>
	void GetChildrenViewModelsForType(TArray<TSharedPtr<ViewModelChildClass>>& OutChildren, bool bRecursive = false);

	/** Returns the hierarchy depth via number of parents above. */
	int32 GetHierarchyDepth() const;
	bool HasParent(TSharedPtr<FHierarchyElementViewModel> ParentCandidate, bool bRecursive = false) const;
	
	TSharedRef<FHierarchyElementViewModel> DuplicateToThis(TSharedPtr<FHierarchyElementViewModel> ItemToDuplicate, int32 InsertIndex = INDEX_NONE);
	TSharedRef<FHierarchyElementViewModel> ReparentToThis(TSharedPtr<FHierarchyElementViewModel> ItemToMove, int32 InsertIndex = INDEX_NONE);

	TSharedPtr<FHierarchyElementViewModel> FindViewModelForChild(UHierarchyElement* Child, bool bSearchRecursively = false) const;
	TSharedPtr<FHierarchyElementViewModel> FindViewModelForChild(FHierarchyElementIdentity ChildIdentity, bool bSearchRecursively = false) const;
	int32 FindIndexOfChild(UHierarchyElement* Child) const;
	int32 FindIndexOfDataChild(TSharedPtr<FHierarchyElementViewModel> Child) const;
	int32 FindIndexOfDataChild(UHierarchyElement* Child) const;

	UHierarchyElement* AddChild(TSubclassOf<UHierarchyElement> NewChildClass, FHierarchyElementIdentity ChildIdentity);
	
	/** Deleting will ask the parent to delete its child */
	void Delete();
	void DeleteChild(TSharedPtr<FHierarchyElementViewModel> Child);

	TWeakObjectPtr<UDataHierarchyViewModelBase> GetHierarchyViewModel() const { return HierarchyViewModel; }

	/** Returns a set result if the item can accept a drop either above/onto/below the item.  */
	TOptional<EItemDropZone> OnCanRowAcceptDrop(const FDragDropEvent& DragDropEvent, EItemDropZone ItemDropZone, TSharedPtr<FHierarchyElementViewModel> Item);
	virtual FReply OnDroppedOnRow(const FDragDropEvent& DragDropEvent, EItemDropZone ItemDropZone, TSharedPtr<FHierarchyElementViewModel> Item);
	void OnRowDragLeave(const FDragDropEvent& DragDropEvent);

	struct FCanPerformActionResults
	{
		FCanPerformActionResults(bool bInCanPerform) : bCanPerform(bInCanPerform) {}
		
		bool bCanPerform = false;
		/** A message that is used when bCanPerform is false. Will either be used in tooltips in the hierarchy editor or as popup message. */
		FText CanPerformMessage;

		bool operator==(const bool& bOther) const
		{
			return bCanPerform == bOther;
		}

		bool operator!=(const bool& bOther) const
		{
			return !(*this==bOther);
		}
	};

	/** Should return true if properties are supposed to be editable & needs to be true if typical operations should work on it (renaming, dragging, deleting etc.) */
	virtual FCanPerformActionResults IsEditableByUser() { return FCanPerformActionResults(false); }
	
	/** Needs to be true in order to allow drag & drop operations to parent items to this item */
	virtual bool CanHaveChildren() const { return false; }
	
	/** Should return true if an item should be draggable. An uneditable item can not be dragged even if CanDragInternal returns true. */
	FCanPerformActionResults CanDrag();
	
	/** Returns true if renamable */
	bool CanRename() { return IsEditableByUser().bCanPerform && CanRenameInternal(); }

	void Rename(FName NewName) { RenameInternal(NewName); HierarchyViewModel->OnHierarchyPropertiesChanged().Broadcast(); }

	void RequestRename()
	{
		if(CanRename() && OnRequestRenameDelegate.IsBound())
		{
			bRenamePending = false;
			OnRequestRenameDelegate.Execute();
		}
	}

	void RequestRenamePending()
	{
		if(CanRename())
		{
			bRenamePending = true;
		}
	}
	
	/** Returns true if deletable */
	bool CanDelete() { return IsEditableByUser().bCanPerform && CanDeleteInternal(); }

	/** Returns true if the given item can be dropped on the given target area. */
	FCanPerformActionResults CanDropOn(TSharedPtr<FHierarchyElementViewModel> DraggedItem, EItemDropZone ItemDropZone) { return CanDropOnInternal(DraggedItem, ItemDropZone); }

	/** Gets executed when an item was dropped on this. */
	void OnDroppedOn(TSharedPtr<FHierarchyElementViewModel> DroppedItem, EItemDropZone ItemDropZone) { OnDroppedOnInternal(DroppedItem, ItemDropZone); }

	/** Determines the section this item belongs to. */
	const UHierarchySection* GetSection() const { return GetSectionInternal(); }
	
	/** For data cleanup that represents external data, this needs to return true in order for live cleanup to work. */
	virtual bool RepresentsExternalData() const { return false; }
	/** This function determines whether a hierarchy item that represents that external data should be maintained during data refresh
	 * Needs to be implemented if RepresentsExternalData return true.
	 * The context object can be used to add arbitrary data. */
	virtual bool DoesExternalDataStillExist(const UHierarchyDataRefreshContext* Context) const { return false; }

	/** The UObject we display in the details panel when this item is selected. By default it's the hierarchy element the view model represents. */
	virtual UObject* GetDataForEditing() { return Element; }
	/** Source items are transient, which is why we don't allow editing by default.
	 * This is useful to override if source data points at actual data to edit. */
	virtual bool AllowEditingInDetailsPanel() const { return bIsForHierarchy; }

	bool IsForHierarchy() const { return bIsForHierarchy; }

	/** Override this to register dynamic context menu entries when right clicking a single hierarchy item */
	virtual void AppendDynamicContextMenuForSingleElement(UToolMenu* ToolMenu) {}
	
	FReply OnDragDetected(const FGeometry& Geometry, const FPointerEvent& PointerEvent, bool bIsSource);

	FSimpleDelegate& GetOnRequestRename() { return OnRequestRenameDelegate; }
	FOnSynced& GetOnSynced() { return OnSyncedDelegate; }
	FOnChildRequestedDeletion& OnChildRequestedDeletion() { return OnChildRequestedDeletionDelegate; }

	TWeakPtr<FHierarchyElementViewModel> GetParent() { return Parent; }

protected:
	/** Should return true if draggable. An optional message can be provided if false that will show as a slate notification. */
	virtual FCanPerformActionResults CanDragInternal() { return false; }

	/** Should return true if renamable */
	virtual bool CanRenameInternal() { return false; }

	virtual void RenameInternal(FName NewName) {}
	
	/** Should return true if deletable. By default, we can delete items in the hierarchy, not in the source. */
	virtual bool CanDeleteInternal() { return IsForHierarchy(); }

	/** Should return true if the given drag drop operation is allowed to succeed. */
	virtual FCanPerformActionResults CanDropOnInternal(TSharedPtr<FHierarchyElementViewModel>, EItemDropZone ItemDropZone);
	
	/** Override this to handle drop-on logic. This is called when an item has been dropped on the item that has implemented this function. */
	virtual void OnDroppedOnInternal(TSharedPtr<FHierarchyElementViewModel>, EItemDropZone ItemDropZone) { }

	/** Can be overridden to support sections in the source list.
	 * In the hierarchy only categories can be parented directly to the root, but using this it is possible to add items to custom sections in the source panel.
	 * This will only work for top-level objects, i.e. anything directly under the root. */
	virtual const UHierarchySection* GetSectionInternal() const { return nullptr; }
private:
	/** Optionally implement this to refresh dependent data. */
	virtual void RefreshChildrenDataInternal() {}
	/** Optionally implement this to further customize the view model sync process.
	 * An example for this is how the root view model handles sections, as sections exist outside the children hierarchy */
	virtual void SyncViewModelsToDataInternal() {}
	/** Optionally implement this to handle shutdown logic.
	 * An example for this is when a section gets deleted, it iterates over all categories to null out the associated section  */
	virtual void FinalizeInternal() {}

	void PropagateOnChildSynced();
protected:
	/** The hierarchy element this view model represents. Assumed valid while this view model exists. */
	UHierarchyElement* const Element;

	/** Parent should be valid for all instances of this struct except for root objects */
	TWeakPtr<FHierarchyElementViewModel> Parent;
	TArray<TSharedPtr<FHierarchyElementViewModel>> Children;
	
	TWeakObjectPtr<UDataHierarchyViewModelBase> HierarchyViewModel;
	
	TArray<FOnFilterChild> ChildFilters;
	mutable TArray<TSharedPtr<FHierarchyElementViewModel>> FilteredChildren;
	
	FSimpleDelegate OnRequestRenameDelegate;
	FOnSynced OnSyncedDelegate;
	FOnChildRequestedDeletion OnChildRequestedDeletionDelegate;
	
	bool bRenamePending = false;
	bool bIsForHierarchy = false;
};

template <class DataClass, class ViewModelClass>
void FHierarchyElementViewModel::GetChildrenViewModelsForType(TArray<TSharedPtr<ViewModelClass>>& OutChildren, bool bRecursive)
{
	for(auto& Child : Children)
	{
		if(Child->GetData()->IsA<DataClass>())
		{
			OutChildren.Add(StaticCastSharedPtr<ViewModelClass>(Child));
		}
	}

	if(bRecursive)
	{
		for(auto& Child : Children)
		{
			Child->GetChildrenViewModelsForType<DataClass, ViewModelClass>(OutChildren, bRecursive);
		}
	}
}

struct DATAHIERARCHYEDITOR_API FHierarchyRootViewModel : FHierarchyElementViewModel
{
	DECLARE_DELEGATE(FOnSyncPropagated)
	DECLARE_DELEGATE(FOnSectionsChanged)
	DECLARE_DELEGATE_OneParam(FOnSingleSectionChanged, TSharedPtr<FHierarchySectionViewModel> AddedSection)
	
	FHierarchyRootViewModel(UHierarchyElement* InItem, TWeakObjectPtr<UDataHierarchyViewModelBase> InHierarchyViewModel, bool bInIsForHierarchy) : FHierarchyElementViewModel(InItem, nullptr, InHierarchyViewModel, bInIsForHierarchy) {}
	virtual ~FHierarchyRootViewModel() override;

	virtual void Initialize() override;
	
	virtual FCanPerformActionResults CanDropOnInternal(TSharedPtr<FHierarchyElementViewModel> DraggedElement, EItemDropZone ItemDropZone) override;
	virtual void OnDroppedOnInternal(TSharedPtr<FHierarchyElementViewModel> DroppedItem, EItemDropZone ItemDropZone) override;

	virtual bool CanHaveChildren() const override { return true; }

	TSharedPtr<struct FHierarchySectionViewModel> AddSection();
	void DeleteSection(TSharedPtr<FHierarchyElementViewModel> SectionViewModel);
	TArray<TSharedPtr<struct FHierarchySectionViewModel>>& GetSectionViewModels() { return SectionViewModels; }

	FOnSyncPropagated& OnSyncPropagated() { return OnSyncPropagatedDelegate; }

	/** General purpose delegate for when sections change */
	FOnSectionsChanged& OnSectionsChanged() { return OnSectionsChangedDelegate; }
	/** Delegates for when a section is added or removed */
	FOnSingleSectionChanged& OnSectionAdded() { return OnSectionAddedDelegate; }
	FOnSingleSectionChanged& OnSectionDeleted() { return OnSectionDeletedDelegate; }

private:
	void PropagateOnSynced();
	virtual void SyncViewModelsToDataInternal() override;
	TArray<TSharedPtr<struct FHierarchySectionViewModel>> SectionViewModels;

	FOnSyncPropagated OnSyncPropagatedDelegate;
	FOnSingleSectionChanged OnSectionAddedDelegate;
	FOnSingleSectionChanged OnSectionDeletedDelegate;
	FOnSectionsChanged OnSectionsChangedDelegate;
};

struct DATAHIERARCHYEDITOR_API FHierarchySectionViewModel : FHierarchyElementViewModel
{
	FHierarchySectionViewModel(UHierarchySection* InItem, TSharedRef<FHierarchyRootViewModel> InParent, TWeakObjectPtr<UDataHierarchyViewModelBase> InHierarchyViewModel) : FHierarchyElementViewModel(InItem, InParent, InHierarchyViewModel, InParent->IsForHierarchy())
	{
		if(bIsForHierarchy == false)
		{
			SetDropDisallowed(true);
		}
	}
	
	virtual ~FHierarchySectionViewModel() override {}

	virtual FString ToString() const override;
	
	void SetSectionName(FName InSectionName);
	FName GetSectionName() const;

	void SetSectionNameAsText(const FText& Text);
	FText GetSectionNameAsText() const;
	FText GetSectionTooltip() const;

	void SetSectionImage(const FSlateBrush* InSectionImage) { SectionImage = InSectionImage; }
	const FSlateBrush* GetSectionImage() const { return SectionImage; }

	void SetDropDisallowed(bool bInDropDisallowed) { bDropDisallowed = bInDropDisallowed; }
protected:
	/** Only hierarchy sections are editable */
	virtual FCanPerformActionResults IsEditableByUser() override { return FCanPerformActionResults(IsForHierarchy()); }
	virtual bool CanHaveChildren() const override { return false; }
	virtual FCanPerformActionResults CanDragInternal() override;
	
	/** We can only rename hierarchy sections */
	virtual bool CanRenameInternal() override;
	virtual void RenameInternal(FName NewName) override { GetDataMutable<UHierarchySection>()->SetSectionName(NewName); }

	virtual bool CanDeleteInternal() override;
	
	virtual FCanPerformActionResults CanDropOnInternal(TSharedPtr<FHierarchyElementViewModel> DraggedItem, EItemDropZone ItemDropZone) override;
	virtual void OnDroppedOnInternal(TSharedPtr<FHierarchyElementViewModel> DroppedItem, EItemDropZone ItemDropZone) override;

	virtual void FinalizeInternal() override;

private:
	const FSlateBrush* SectionImage = nullptr;
	
	bool bDropDisallowed = false;
};

struct DATAHIERARCHYEDITOR_API FHierarchyItemViewModel : FHierarchyElementViewModel
{
	FHierarchyItemViewModel(UHierarchyItem* InElement, TSharedRef<FHierarchyElementViewModel> InParent, TWeakObjectPtr<UDataHierarchyViewModelBase> InHierarchyViewModel) : FHierarchyElementViewModel(InElement, InParent, InHierarchyViewModel, InParent->IsForHierarchy()) {}
	
	virtual ~FHierarchyItemViewModel() override {}

	virtual FCanPerformActionResults IsEditableByUser() override { return FCanPerformActionResults(true); }
	virtual bool CanHaveChildren() const override { return false; }
	virtual FCanPerformActionResults CanDragInternal() override { return true; }

	virtual FCanPerformActionResults CanDropOnInternal(TSharedPtr<FHierarchyElementViewModel> DraggedItem, EItemDropZone ItemDropZone) override;
	virtual void OnDroppedOnInternal(TSharedPtr<FHierarchyElementViewModel> DroppedItem, EItemDropZone ItemDropZone) override;
};

struct DATAHIERARCHYEDITOR_API FHierarchyCategoryViewModel : FHierarchyElementViewModel
{
	FHierarchyCategoryViewModel(UHierarchyCategory* InCategory, TSharedRef<FHierarchyElementViewModel> InParent, TWeakObjectPtr<UDataHierarchyViewModelBase> InHierarchyViewModel) : FHierarchyElementViewModel(InCategory, InParent, InHierarchyViewModel, InParent->IsForHierarchy()) {}
	virtual ~FHierarchyCategoryViewModel() override{}

	FText GetCategoryName() const { return GetData<UHierarchyCategory>()->GetCategoryAsText(); }
	
	virtual FCanPerformActionResults IsEditableByUser() override { return FCanPerformActionResults(true); }
	virtual bool CanHaveChildren() const override { return true; }
	virtual FCanPerformActionResults CanDragInternal() override { return true; }
	virtual bool CanRenameInternal() override { return true; }
	virtual void RenameInternal(FName NewName) override { GetDataMutable<UHierarchyCategory>()->SetCategoryName(NewName); }
	virtual const UHierarchySection* GetSectionInternal() const override { return Cast<UHierarchyCategory>(Element)->GetSection(); }

	bool IsTopCategoryActive() const;

	virtual FCanPerformActionResults CanDropOnInternal(TSharedPtr<FHierarchyElementViewModel>, EItemDropZone ItemDropZone) override;
	virtual void OnDroppedOnInternal(TSharedPtr<FHierarchyElementViewModel> DroppedItem, EItemDropZone ItemDropZone) override;
};

class DATAHIERARCHYEDITOR_API FSectionDragDropOp : public FHierarchyDragDropOp
{
public:
	DRAG_DROP_OPERATOR_TYPE(FSectionDragDropOp, FHierarchyDragDropOp)

	FSectionDragDropOp(TSharedPtr<FHierarchySectionViewModel> SectionViewModel) : FHierarchyDragDropOp(SectionViewModel) {}
	
	TWeakPtr<FHierarchySectionViewModel> GetDraggedSection() const { return StaticCastSharedPtr<FHierarchySectionViewModel>(DraggedElement.Pin()); }
private:
	virtual TSharedRef<SWidget> CreateCustomDecorator() const override;
};