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

#pragma once

#include "CoreMinimal.h"
#include "UObject/StrongObjectPtr.h"
#include "Engine/AssetManager.h"
#include "Async/Future.h"
#include "Containers/Ticker.h"

class FAsyncCondition;
class UPrimaryDataAsset;

DECLARE_DELEGATE_OneParam(FStreamableHandleDelegate, TSharedPtr<FStreamableHandle>)

//TODO I think we need to introduce a retention policy, preloads automatically stay in memory until canceled
//     but what if you want to preload individual items just using the AsyncLoad functions?  I don't want to
//     introduce individual policies per call, or introduce a whole set of preload vs asyncloads, so would
//     would rather have a retention policy.  Should it be a member and actually create real memory when
//     you inherit from AsyncMixin, or should it be a template argument?
//enum class EAsyncMixinRetentionPolicy : uint8
//{
//	Default,
//	KeepResidentUntilComplete,
//	KeepResidentUntilCancel
//};

/**
 * The FAsyncMixin allows easier management of async loading requests, to ensure linear request handling, to make 
 * writing code much easier.  The usage pattern is as follows,
 *
 * First - inherit from FAsyncMixin, even if you're a UObject, you can also inherit from FAsyncMixin.
 *
 * Then - you can make your async loads as follows.
 * 
 * CancelAsyncLoading();			// Some objects get reused like in lists, so it's important to cancel anything you had pending doesn't complete.
 * AsyncLoad(ItemOne, CallbackOne);
 * AsyncLoad(ItemTwo, CallbackTwo);
 * StartAsyncLoading();
 * 
 * You can also include the 'this' scope safely, one of the benefits of the mix-in, is that none of the callbacks
 * are ever out of scope of the host AsyncMixin derived object.
 * e.g.
 * AsyncLoad(SomeSoftObjectPtr, [this, ...]() {
 *    
 * });
 * 
 *
 * What will happen is first we cancel any existing one(s), e.g. perhaps we are a widget that just got told to represent
 * some new thing.  What will happen is we'll Load ItemOne and ItemTwo, *THEN* we'll call the callbacks in the order you
 * requested the async loads - even if ItemOne or ItemTwo was already loaded when you request it.
 *
 * When all the async loading requests complete, OnFinishedLoading will be called.
 * 
 * If you forget to call StartAsyncLoading(), we'll call it next frame, but you should remember to call it
 * when you're done with your setup, as maybe everything is already loaded, and it will avoid a single frame
 * of a loading indicator flash, which is annoying.
 * 
 * NOTE: The FAsyncMixin also makes it safe to pass [this] as a captured input into your lambda, because it handles 
 * unhooking everything if either your owner class is destroyed, or you cancel everything.
 *
 * NOTE: FAsyncMixin doesn't add any additional memory to your class.  Several classes currently handling async loading 
 * internally allocate TSharedPtr<FStreamableHandle> members and tend to hold onto SoftObjectPaths temporary state.  The 
 * FAsyncMixin does all of this internally with a static TMap so that all of the async request memory is stored temporarily
 * and sparsely.
 * 
 * NOTE: For debugging and understanding what's going on, you should add -LogCmds="LogAsyncMixin Verbose" to the command line.
 */
class ASYNCMIXIN_API FAsyncMixin : public FNoncopyable
{
protected:
	FAsyncMixin();

public:
	virtual ~FAsyncMixin();

protected:
	/** Called when loading starts. */
	virtual void OnStartedLoading() { }
	/** Called when all loading has finished. */
	virtual void OnFinishedLoading() { }

protected:
	/** Async load a TSoftClassPtr<T>, call the Callback when complete. */
	template<typename T = UObject>
	void AsyncLoad(TSoftClassPtr<T> SoftClass, TFunction<void()>&& Callback)
	{
		AsyncLoad(SoftClass.ToSoftObjectPath(), FSimpleDelegate::CreateLambda(MoveTemp(Callback)));
	}

	/** Async load a TSoftClassPtr<T>, call the Callback when complete. */
	template<typename T = UObject>
	void AsyncLoad(TSoftClassPtr<T> SoftClass, TFunction<void(TSubclassOf<T>)>&& Callback)
	{
		AsyncLoad(SoftClass.ToSoftObjectPath(),
			FSimpleDelegate::CreateLambda([SoftClass, UserCallback = MoveTemp(Callback)]() mutable {
				UserCallback(SoftClass.Get());
			})
		);
	}

	/** Async load a TSoftClassPtr<T>, call the Callback when complete. */
	template<typename T = UObject>
	void AsyncLoad(TSoftClassPtr<T> SoftClass, const FSimpleDelegate& Callback = FSimpleDelegate())
	{
		AsyncLoad(SoftClass.ToSoftObjectPath(), Callback);
	}

	/** Async load a TSoftObjectPtr<T>, call the Callback when complete. */
	template<typename T = UObject>
	void AsyncLoad(TSoftObjectPtr<T> SoftObject, TFunction<void()>&& Callback)
	{
		AsyncLoad(SoftObject.ToSoftObjectPath(), FSimpleDelegate::CreateLambda(MoveTemp(Callback)));
	}

	/** Async load a TSoftObjectPtr<T>, call the Callback when complete. */
	template<typename T = UObject>
	void AsyncLoad(TSoftObjectPtr<T> SoftObject, TFunction<void(T*)>&& Callback)
	{
		AsyncLoad(SoftObject.ToSoftObjectPath(),
			FSimpleDelegate::CreateLambda([SoftObject, UserCallback = MoveTemp(Callback)]() mutable {
				UserCallback(SoftObject.Get());
			})
		);
	}

	/** Async load a TSoftObjectPtr<T>, call the Callback when complete. */
	template<typename T = UObject>
	void AsyncLoad(TSoftObjectPtr<T> SoftObject, const FSimpleDelegate& Callback = FSimpleDelegate())
	{
		AsyncLoad(SoftObject.ToSoftObjectPath(), Callback);
	}

	/** Async load a FSoftObjectPath, call the Callback when complete. */
	void AsyncLoad(FSoftObjectPath SoftObjectPath, const FSimpleDelegate& Callback = FSimpleDelegate());

	/** Async load an array of FSoftObjectPath, call the Callback when complete. */
	void AsyncLoad(const TArray<FSoftObjectPath>& SoftObjectPaths, TFunction<void()>&& Callback)
	{
		AsyncLoad(SoftObjectPaths, FSimpleDelegate::CreateLambda(MoveTemp(Callback)));
	}

	/** Async load an array of FSoftObjectPath, call the Callback when complete. */
	void AsyncLoad(const TArray<FSoftObjectPath>& SoftObjectPaths, const FSimpleDelegate& Callback = FSimpleDelegate());

	/** Given an array of primary assets, it loads all of the bundles referenced by properties of these assets specified in the LoadBundles array. */
	template<typename T = UPrimaryDataAsset>
	void AsyncPreloadPrimaryAssetsAndBundles(const TArray<T*>& Assets, const TArray<FName>& LoadBundles, const FSimpleDelegate& Callback = FSimpleDelegate())
	{
		TArray<FPrimaryAssetId> PrimaryAssetIds;
		for (const T* Item : Assets)
		{
			PrimaryAssetIds.Add(Item);
		}

		AsyncPreloadPrimaryAssetsAndBundles(PrimaryAssetIds, LoadBundles, Callback);
	}

	/** Given an array of primary asset ids, it loads all of the bundles referenced by properties of these assets specified in the LoadBundles array. */
	void AsyncPreloadPrimaryAssetsAndBundles(const TArray<FPrimaryAssetId>& AssetIds, const TArray<FName>& LoadBundles, TFunction<void()>&& Callback)
	{
		AsyncPreloadPrimaryAssetsAndBundles(AssetIds, LoadBundles, FSimpleDelegate::CreateLambda(MoveTemp(Callback)));
	}

	/** Given an array of primary asset ids, it loads all of the bundles referenced by properties of these assets specified in the LoadBundles array. */
	void AsyncPreloadPrimaryAssetsAndBundles(const TArray<FPrimaryAssetId>& AssetIds, const TArray<FName>& LoadBundles, const FSimpleDelegate& Callback = FSimpleDelegate());

	/** Add a future condition that must be true before we move forward. */
	void AsyncCondition(TSharedRef<FAsyncCondition> Condition, const FSimpleDelegate& Callback = FSimpleDelegate());

	/**
	 * Rather than load anything, this callback is just inserted into the callback sequence so that when async loading 
	 * completes this event will be called at the same point in the sequence.  Super useful if you don't want a step to be
	 * tied to a particular asset in case some of the assets are optional.
	 */
	void AsyncEvent(TFunction<void()>&& Callback)
	{
		AsyncEvent(FSimpleDelegate::CreateLambda(MoveTemp(Callback)));
	}

	/**
	 * Rather than load anything, this callback is just inserted into the callback sequence so that when async loading
	 * completes this event will be called at the same point in the sequence.  Super useful if you don't want a step to be
	 * tied to a particular asset in case some of the assets are optional.
	 */
	void AsyncEvent(const FSimpleDelegate& Callback);

	/** Flushes any async loading requests. */
	void StartAsyncLoading();

	/** Cancels any pending async loads. */
	void CancelAsyncLoading();

	/** Is async loading current in progress? */
	bool IsAsyncLoadingInProgress() const;

private:
	/**
	 * The FLoadingState is what actually is allocated for the FAsyncMixin in a big map so that the FAsyncMixin itself holds no
	 * no memory, and we dynamically create the FLoadingState only if needed, and destroy it when it's unneeded.
	 */
	class FLoadingState : public TSharedFromThis<FLoadingState>
	{
	public:
		FLoadingState(FAsyncMixin& InOwner);
		virtual ~FLoadingState();

		/** Starts the async sequence. */
		void Start();

		/** Cancels the async sequence. */
		void CancelAndDestroy();

		void AsyncLoad(FSoftObjectPath SoftObject, const FSimpleDelegate& DelegateToCall);
		void AsyncLoad(const TArray<FSoftObjectPath>& SoftObjectPaths, const FSimpleDelegate& DelegateToCall);
		void AsyncPreloadPrimaryAssetsAndBundles(const TArray<FPrimaryAssetId>& PrimaryAssetIds, const TArray<FName>& LoadBundles, const FSimpleDelegate& DelegateToCall);
		void AsyncCondition(TSharedRef<FAsyncCondition> Condition, const FSimpleDelegate& Callback);
		void AsyncEvent(const FSimpleDelegate& Callback);

		bool IsLoadingComplete() const { return !IsLoadingInProgress(); }
		bool IsLoadingInProgress() const;
		bool IsLoadingInProgressOrPending() const;
		bool IsPendingDestroy() const;

	private:
		void CancelOnly(bool bDestroying);
		void CancelStartTimer();
		void TryScheduleStart();
		void TryCompleteAsyncLoading();
		void CompleteAsyncLoading();

	private:
		void RequestDestroyThisMemory();
		void CancelDestroyThisMemory(bool bDestroying);

		/** Who owns the loading state?  We need this to call back into the owning mix-in object. */
		FAsyncMixin& OwnerRef;

		/**
		 * Did we need to pre-load bundles?  If we didn't pre-load bundles (which require you keep the streaming handle 
		 * around or they will be destroyed), then we can safely destroy the FLoadingState when everything is done loading.
		 */
		bool bPreloadedBundles = false;

		class FAsyncStep
		{
		public:
			FAsyncStep(const FSimpleDelegate& InUserCallback);
			FAsyncStep(const FSimpleDelegate& InUserCallback, const TSharedPtr<FStreamableHandle>& InStreamingHandle);
			FAsyncStep(const FSimpleDelegate& InUserCallback, const TSharedPtr<FAsyncCondition>& InCondition);

			~FAsyncStep();

			void ExecuteUserCallback();

			bool IsLoadingInProgress() const
			{
				return !IsComplete();
			}

			bool IsComplete() const;
			void Cancel();

			bool BindCompleteDelegate(const FSimpleDelegate& NewDelegate);
			bool IsCompleteDelegateBound() const;

		private:
			FSimpleDelegate UserCallback;
			bool bIsCompletionDelegateBound = false;

			// Possible Async 'thing'
			TSharedPtr<FStreamableHandle> StreamingHandle;
			TSharedPtr<FAsyncCondition> Condition;
		};

		bool bHasStarted = false;

		int32 CurrentAsyncStep = 0;
		TArray<TUniquePtr<FAsyncStep>> AsyncSteps;
		TArray<TUniquePtr<FAsyncStep>> AsyncStepsPendingDestruction;

		FTSTicker::FDelegateHandle StartTimerDelegate;
		FTSTicker::FDelegateHandle DestroyMemoryDelegate;
	};

	const FLoadingState& GetLoadingStateConst() const;
	
	FLoadingState& GetLoadingState();

	bool HasLoadingState() const;

	bool IsLoadingInProgressOrPending() const;

private:
	static TMap<FAsyncMixin*, TSharedRef<FLoadingState>> Loading;
};

/**
 * Sometimes a mix-in just doesn't make sense.  Perhaps the object has to manage many different jobs
 * that each have their own async dependency chain/scope.  For those situations you can use the FAsyncScope.
 * 
 * This class is a standalone Async dependency handler so that you can fire off several load jobs and always handle them
 * in the proper order, just like with combining FAsyncMixin with your class.
 */
class ASYNCMIXIN_API FAsyncScope : public FAsyncMixin
{
public:
	using FAsyncMixin::AsyncLoad;

	using FAsyncMixin::AsyncPreloadPrimaryAssetsAndBundles;

	using FAsyncMixin::AsyncCondition;

	using FAsyncMixin::AsyncEvent;

	using FAsyncMixin::CancelAsyncLoading;

	using FAsyncMixin::StartAsyncLoading;

	using FAsyncMixin::IsAsyncLoadingInProgress;
};

//------------------------------------------------------------------------------
//------------------------------------------------------------------------------

enum class EAsyncConditionResult : uint8
{
	TryAgain,
	Complete
};

DECLARE_DELEGATE_RetVal(EAsyncConditionResult, FAsyncConditionDelegate);

/**
 * The async condition allows you to have custom reasons to hault the async loading until some condition is met.
 */
class FAsyncCondition : public TSharedFromThis<FAsyncCondition>
{
public:
	FAsyncCondition(const FAsyncConditionDelegate& Condition);
	FAsyncCondition(TFunction<EAsyncConditionResult()>&& Condition);
	virtual ~FAsyncCondition();

protected:
	bool IsComplete() const;
	bool BindCompleteDelegate(const FSimpleDelegate& NewDelegate);

private:
	bool TryToContinue(float DeltaTime);

	FTSTicker::FDelegateHandle RepeatHandle;
	FAsyncConditionDelegate UserCondition;
	FSimpleDelegate CompletionDelegate;

	friend FAsyncMixin;
};