109 lines
3.9 KiB
Markdown
109 lines
3.9 KiB
Markdown
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# Getter
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- **功能描述：** 为属性增加一个C++的Get函数，只在C++层面应用。
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- **元数据类型：** string="abc"
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- **引擎模块：** Blueprint
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- **常用程度：** ★★★
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为属性增加一个C++的Get函数，只在C++层面应用。
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- Getter上如不提供函数名，那就用默认的GetXXX的名字。也可以提供另外一个函数名。
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- 这些Getter函数是不加UFUNCTION的，这点要和BlueprintGetter区分。
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- 感觉更好的名字是NativeGetter。
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- GetXXX的函数必须自己手写，否则UHT会报错。
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- 我们当然也可以自己写GetSet函数，不需要写Getter和Setter的元数据。但写上Getter和Settter的好处是，万一在项目里别的地方，用到了反射来获取和设置值，这个时候如果没有标上Getter和Setter，就会直接从属性上获取值，从而跳过我们想要的自定义Get/Set流程。
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## 测试代码：
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```cpp
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public:
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//GetterFunc: Has Native Getter
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UPROPERTY(BlueprintReadWrite, Getter)
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float MyFloat = 1.0f;
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//GetterFunc: Has Native Getter
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UPROPERTY(BlueprintReadWrite, Getter = GetMyCustomFloat)
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float MyFloat2 = 1.0f;
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public:
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float GetMyFloat()const { return MyFloat + 100.f; }
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float GetMyCustomFloat()const { return MyFloat2 + 100.f; }
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void UMyProperty_Get::RunTest()
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{
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float Value1=MyFloat;
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FProperty* prop=GetClass()->FindPropertyByName(TEXT("MyFloat"));
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float Value2=0.f;
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prop->GetValue_InContainer(this,&Value2);
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}
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```
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## 蓝图表现：
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在测试的时候，可见如果是用GetValue_InContainer这种反射的方式来获取值，就会自动的调用到GetMyFloat，从而返回不同的值。
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在蓝图里直接Get MyFloat 是依然是1.
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## 原理：
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UHT在分析Getter标记后，会在gen.cpp里生成相应的函数包装。在构建FProperty的时候，就会创建TPropertyWithSetterAndGetter，之后在GetSingleValue_InContainer的时候就会调用到CallGetter。
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```cpp
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void UMyProperty_Get::GetMyFloat_WrapperImpl(const void* Object, void* OutValue)
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{
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const UMyProperty_Get* Obj = (const UMyProperty_Get*)Object;
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float& Result = *(float*)OutValue;
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Result = (float)Obj->GetMyFloat();
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}
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const UECodeGen_Private::FFloatPropertyParams Z_Construct_UClass_UMyProperty_Get_Statics::NewProp_MyFloat = { "MyFloat", nullptr, (EPropertyFlags)0x0010000000000004, UECodeGen_Private::EPropertyGenFlags::Float, RF_Public|RF_Transient|RF_MarkAsNative, nullptr, &UMyProperty_Get::GetMyFloat_WrapperImpl, 1, STRUCT_OFFSET(UMyProperty_Get, MyFloat), METADATA_PARAMS(UE_ARRAY_COUNT(NewProp_MyFloat_MetaData), NewProp_MyFloat_MetaData) };
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template <typename PropertyType, typename PropertyParamsType>
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PropertyType* NewFProperty(FFieldVariant Outer, const FPropertyParamsBase& PropBase)
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{
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const PropertyParamsType& Prop = (const PropertyParamsType&)PropBase;
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PropertyType* NewProp = nullptr;
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if (Prop.SetterFunc || Prop.GetterFunc)
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{
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NewProp = new TPropertyWithSetterAndGetter<PropertyType>(Outer, Prop);
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}
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else
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{
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NewProp = new PropertyType(Outer, Prop);
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}
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}
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void FProperty::GetSingleValue_InContainer(const void* InContainer, void* OutValue, int32 ArrayIndex) const
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{
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checkf(ArrayIndex <= ArrayDim, TEXT("ArrayIndex (%d) must be less than the property %s array size (%d)"), ArrayIndex, *GetFullName(), ArrayDim);
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if (!HasGetter())
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{
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// Fast path - direct memory access
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CopySingleValue(OutValue, ContainerVoidPtrToValuePtrInternal((void*)InContainer, ArrayIndex));
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}
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else
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{
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if (ArrayDim == 1)
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{
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// Slower but no mallocs. We can copy the value directly to the resulting param
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CallGetter(InContainer, OutValue);
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}
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else
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{
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// Malloc a temp value that is the size of the array. Getter will then copy the entire array to the temp value
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uint8* ValueArray = (uint8*)AllocateAndInitializeValue();
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GetValue_InContainer(InContainer, ValueArray);
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// Copy the item we care about and free the temp array
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CopySingleValue(OutValue, ValueArray + ArrayIndex * ElementSize);
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DestroyAndFreeValue(ValueArray);
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}
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}
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}
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```
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