FrameValue.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "CoreGlobals.h"
#include "Misc/Optional.h"
 
/**
 * This struct allows you to cache a value for a frame, and automatically invalidates
 * when the frame advances.
 * If the value was set this frame, IsSet() returns true and GetValue() is valid.
 */
template<typename ValueType>
struct TFrameValue
{
private:
    uint64 FrameSet;
    TOptional<ValueType> Value;
 
public:
    /** Construct an OptionaType with a valid value. */
    TFrameValue(const ValueType& InValue)
        : FrameSet(GFrameCounter)
    {
        Value = InValue;
    }
 
    TFrameValue(ValueType&& InValue)
        : FrameSet(GFrameCounter)
    {
        Value = InValue;
    }
 
    /** Construct an OptionalType with no value; i.e. unset */
    TFrameValue()
        : FrameSet(GFrameCounter)
    {
    }
 
    /** Copy/Move construction */
    TFrameValue(const TFrameValue& InValue)
        : FrameSet(GFrameCounter)
    {
        Value = InValue;
    }
 
    TFrameValue(TFrameValue&& InValue)
        : FrameSet(GFrameCounter)
    {
        Value = InValue;
    }
 
    TFrameValue& operator=(const TFrameValue& InValue)
    {
        Value = InValue;
        FrameSet = GFrameCounter;
        return *this;
    }
 
    TFrameValue& operator=(TFrameValue&& InValue)
    {
        Value = InValue;
        FrameSet = GFrameCounter;
        return *this;
    }
 
    TFrameValue& operator=(const ValueType& InValue)
    {
        Value = InValue;
        FrameSet = GFrameCounter;
        return *this;
    }
 
    TFrameValue& operator=(ValueType&& InValue)
    {
        Value = InValue;
        FrameSet = GFrameCounter;
        return *this;
    }
 
public:
 
    bool IsSet() const
    {
        return Value.IsSet() && FrameSet == GFrameCounter;
    }
 
    const ValueType& GetValue() const
    {
        checkf(FrameSet == GFrameCounter, TEXT("Cannot get value on a different frame"));
        return Value.GetValue();
    }
 
    ValueType TryGetValue(ValueType UnsetValue) const
    {
        return IsSet() ? Value.GetValue() : UnsetValue;
    }
};