109 lines
3.0 KiB
Markdown
109 lines
3.0 KiB
Markdown
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# Flags
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- **功能描述：** 把该枚举的值作为一个标志来拼接字符串输出。
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- **元数据类型：** bool
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- **引擎模块：** Trait
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- **作用机制：** 在EnumFlags中添加[Flags](../../../Flags/EEnumFlags/Flags.md)
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- **常用程度：★★★★★**
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把该枚举的值作为一个标志来拼接字符串输出。
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指定作用的地方是当把一个值输出为字符串输出的时候。转换成字符串的方式：一种是直接查找精确相等的值，然后查找特定的枚举值。第二种就是把它当做标志，输出符合相应标记的“A | B | C”这种格式的值。Flags就是指定采用第二种方式。
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但是要注意其枚举值是完全没有影响变化的。跟直接把枚举值定义为标记是不同的。
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注意和meta(bitflags)的区分，后者是标记该枚举可以作为一个标记，可以作为Bitmask被筛选中。
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## 示例代码：
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```cpp
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UENUM(BlueprintType)
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enum class EMyEnum_Normal:uint8
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{
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First,
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Second,
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Third,
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};
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/*
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[EMyEnum_Flags Enum->Field->Object /Script/Insider.EMyEnum_Flags]
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(BlueprintType = true, First.Name = EMyEnum_Flags::First, ModuleRelativePath = Enum/MyEnum_Flags.h, Second.Name = EMyEnum_Flags::Second, Third.Name = EMyEnum_Flags::Third)
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ObjectFlags: RF_Public | RF_Transient
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Outer: Package /Script/Insider
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EnumFlags: EEnumFlags::Flags
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EnumDisplayNameFn: 0
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CppType: EMyEnum_Flags
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CppForm: EnumClass
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{
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First = 0,
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Second = 1,
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Third = 2,
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EMyEnum_MAX = 3
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};
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*/
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UENUM(BlueprintType,Flags)
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enum class EMyEnum_Flags:uint8
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{
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First,
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Second,
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Third,
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};
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void UMyActor_EnumBitFlags_Test::TestFlags()
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{
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int value = 3;
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FString outStr_Normal = StaticEnum<EMyEnum_Normal>()->GetValueOrBitfieldAsString(value);
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FString outStr_Flags = StaticEnum<EMyEnum_Flags>()->GetValueOrBitfieldAsString(value);
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FString outStr_BitFlags = StaticEnum<EMyEnum_BitFlags>()->GetValueOrBitfieldAsString(value);
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}
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```
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## 示例效果：
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蓝图中的表示，依然只能选择单项。
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而测试代码里打印出来的字符串：
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可见outStr_Flags 的打印是字符串拼接的。
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## 原理：
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只在GetValueOrBitfieldAsString这个函数中生效，所以要用这个方法测试才生效。
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```cpp
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FString UEnum::GetValueOrBitfieldAsString(int64 InValue) const
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{
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if (!HasAnyEnumFlags(EEnumFlags::Flags) || InValue == 0)
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{
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return GetNameStringByValue(InValue);
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}
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else
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{
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FString BitfieldString;
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bool WroteFirstFlag = false;
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while (InValue != 0)
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{
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int64 NextValue = 1ll << FMath::CountTrailingZeros64(InValue);
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InValue = InValue & ~NextValue;
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if (WroteFirstFlag)
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{
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// We don't just want to use the NameValuePair.Key because we want to strip enum class prefixes
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BitfieldString.Appendf(TEXT(" | %s"), *GetNameStringByValue(NextValue));
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}
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else
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{
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// We don't just want to use the NameValuePair.Key because we want to strip enum class prefixes
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BitfieldString.Appendf(TEXT("%s"), *GetNameStringByValue(NextValue));
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WroteFirstFlag = true;
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}
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}
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return BitfieldString;
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}
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}
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```
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