[UE4] BitMask 使用介紹

作者：dorgon│2017-05-24 10:50:50│巴幣：0│人氣：1040

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BitMask是使用bit來表達True或false的概念，它可以使用一個int來表達多組的功能是否啟用。

先讓我們來看下圖：

若我們想要依照PlayerLevel跟Speed來進行排序的話，一般不知道這個技巧的人可能會使用二個int來處理這件事。在使用了BitMask之後我們就可以藉由將第一跟第三個bit設為1來表達這件事。在一般的C++中我們常常在enum的設定中就直接將該bit位移，如下面的code:

	enum class EHorizonPlayerSortType
	{
	Invalidated = ( 1 << 0),
	Leve= ( 1 << 1),
	Power = ( 1 << 2),
	Speed= ( 1 << 3),
	PlaceHolder1 = ( 1 << 4),
	PlaceHolder2= ( 1 << 5),
	PlaceHolder3= ( 1 << 6),
	PlaceHolder4 = ( 1 << 7),
	};

	void Usage(){
	int flag = EHorizonPlayerSortType::Level \| EHorizonPlayerSortType::Speed;
	}

但在UE4中的Bitmask沒辦法使用跟上面一樣的做法，由於enum class被限制成只能使用uint8，因此在UE4中是直接將enum中的值當成位移數，如下面的code:

	UENUM(BlueprintType, meta = (Bitflags))
	enum class EHorizonPlayerSortType : uint8
	{
	Invalidated = 0,
	LevelUMETA(DisplayName = "PlayerLevel"),
	PowerUMETA(DisplayName = "Power"),
	SpeedUMETA(DisplayName = "Speed"),
	PlaceHolder1 UMETA(DisplayName = "PlaceHolder1"),
	PlaceHolder2UMETA(DisplayName = "PlaceHolder2"),
	PlaceHolder3UMETA(DisplayName = "PlaceHolder3"),
	PlaceHolder4UMETA(DisplayName = "PlaceHolder4"),
	};

	ENUM_CLASS_FLAGS(EHorizonPlayerSortType)


	USTRUCT(BlueprintType)
	struct HORIZONTWEEN_API FHorizonPlayerParameters
	{
	GENERATED_BODY()
	FHorizonPlayerParameters()
	: PlayerSortTypeFlags((int)EHorizonPlayerSortType::Level)
	{
	}
	public:

	UPROPERTY(EditAnywhere, BlueprintReadWrite, meta = (Bitmask, BitmaskEnum = "EHorizonPlayerSortType"))
	int PlayerSortTypeFlags;
	};



	bool HasFlag(const INT32& Flags, EHorizonPlayerSortType TestFlag)
	{
	INT32 bitFlag = static_cast<int32>(1 << (INT32)TestFlag); //shift to the bit corresponding to the enum value
	return (Flags & bitFlag ) != 0;
	}

	void Usage(const FHorizonPlayerParameters& param){
	INT32 bitFlag = static_cast<int32>((1 << (INT32)EHorizonPlayerSortType::Level) \|
	(1 << (INT32)EHorizonPlayerSortType::Speed)); // 10
	bool b1 = HasFlag(param.PlayerSortTypeFlags, EHorizonPlayerSortType::Level); //true
	bool b2 = HasFlag(param.PlayerSortTypeFlags, EHorizonPlayerSortType::Power); //false
	bool b3 = HasFlag(param.PlayerSortTypeFlags, EHorizonPlayerSortType::Speed); //true
	}