182 lines
6.5 KiB
Markdown
182 lines
6.5 KiB
Markdown
# ScriptMethodMutable
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- **功能描述：** 把ScriptMethod的第一个const结构参数在调用上改成引用参数，函数内修改的值会保存下来。
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- **使用位置：** UFUNCTION
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- **引擎模块：** Script
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- **元数据类型：** bool
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- **限制类型：** 第一个参数是结构类型
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- **关联项：** [ScriptMethod](ScriptMethod.md)
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- **常用程度：** ★★
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把ScriptMethod的第一个const结构参数在调用上改成引用参数，函数内修改的值会保存下来。
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- 在const参数上如果想改变值，依然要标记c++里的mutable。
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- 虽然py生成的代码一模一样，但实际调用上ScriptMethodMutable会真正改变参数的值，而没有ScriptMethodMutable的函数并不会改变参数的原始值。
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- ScriptMethodMutable和UPARAM(ref) 在调用效果上，都可以改变参数的值。但区别是UPARAM(ref) 生成的py代码会返回第一个参数作为返回值。
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```cpp
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USTRUCT(BlueprintType)
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struct INSIDER_API FMyPythonStruct_ScriptMethod
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{
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GENERATED_BODY()
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public:
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UPROPERTY(BlueprintReadWrite, EditAnywhere)
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mutable FString MyString;
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};
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UCLASS(Blueprintable, BlueprintType)
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class INSIDER_API UMyPython_ScriptMethod_Test :public UObject
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{
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GENERATED_BODY()
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public:
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UFUNCTION(BlueprintCallable, meta = (ScriptMethod))
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static void SetStringOnStruct(const FMyPythonStruct_ScriptMethod& myStruct, FString val);
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UFUNCTION(BlueprintCallable, meta = (ScriptMethod, ScriptMethodMutable))
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static void SetStringOnStructMutable(const FMyPythonStruct_ScriptMethod& myStruct, FString val);
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UFUNCTION(BlueprintCallable, meta = (ScriptMethod, ScriptMethodMutable))
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static void SetStringOnStructViaRef(UPARAM(ref) FMyPythonStruct_ScriptMethod& myStruct, FString val);
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};
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```
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## 测试效果：
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看py里生成的代码是一致的，如果用UPARAM(ref)，则在MyPython_ScriptMethod_Test里面生成的my_func_on_struct_via_ref会返回结构MyPythonStruct_ScriptMethod来达成引用的效果。
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然而my_func_on_struct_mutable返回的是None，同不加ScriptMethodMutable的my_func_on_struct并没有区别。但是实际上在真正调用的时候会真正有区别。
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```cpp
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class MyPythonStruct_ScriptMethod(StructBase):
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r"""
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My Python Struct Script Method
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**C++ Source:**
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- **Module**: Insider
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- **File**: MyPython_ScriptMethod.h
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"""
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def __init__(self) -> None:
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...
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def my_func_on_struct_via_ref(self, val: str) -> None:
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r"""
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x.my_func_on_struct_via_ref(val) -> None
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My Func on Struct Via Ref
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Args:
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val (str):
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"""
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...
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def my_func_on_struct_mutable(self, val: str) -> None:
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r"""
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x.my_func_on_struct_mutable(val) -> None
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My Func on Struct Mutable
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Args:
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val (str):
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"""
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...
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def my_func_on_struct(self, val: str) -> None:
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r"""
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x.my_func_on_struct(val) -> None
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My Func on Struct
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Args:
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val (str):
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"""
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...
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class MyPython_ScriptMethod_Test(Object):
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r"""
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My Python Script Method Test
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**C++ Source:**
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- **Module**: Insider
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- **File**: MyPython_ScriptMethod.h
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"""
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@classmethod
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def my_func_on_struct_via_ref(cls, my_struct: MyPythonStruct_ScriptMethod, val: str) -> MyPythonStruct_ScriptMethod:
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r"""
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X.my_func_on_struct_via_ref(my_struct, val) -> MyPythonStruct_ScriptMethod
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My Func on Struct Via Ref
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Args:
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my_struct (MyPythonStruct_ScriptMethod):
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val (str):
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Returns:
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MyPythonStruct_ScriptMethod:
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my_struct (MyPythonStruct_ScriptMethod):
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"""
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...
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@classmethod
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def my_func_on_struct_mutable(cls, my_struct: MyPythonStruct_ScriptMethod, val: str) -> None:
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r"""
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X.my_func_on_struct_mutable(my_struct, val) -> None
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My Func on Struct Mutable
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Args:
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my_struct (MyPythonStruct_ScriptMethod):
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val (str):
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"""
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...
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@classmethod
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def my_func_on_struct(cls, my_struct: MyPythonStruct_ScriptMethod, val: str) -> None:
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r"""
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X.my_func_on_struct(my_struct, val) -> None
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My Func on Struct
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Args:
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my_struct (MyPythonStruct_ScriptMethod):
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val (str):
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"""
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...
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```
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在UE Python控制台里调用的记录，分析调用顺序：
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- 一开始调用set_string_on_struct_mutable，再print(a)，可以打印出Hello，说明值真正的设置到了a结构里。
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- 再尝试set_string_on_struct，再print(a)，无法打印出FFF，说明值并没有设置到a结构里。说明py在调用的时候很可能构造了一个临时值来当作调对象，调用完成的新值并没有设置到a对象上。
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- 再尝试set_string_on_struct_via_ref，再print(a)，可以打印出First，说明用UPARAM(ref)可以也达成改变参数的效果。
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```cpp
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LogPython: a=unreal.MyPythonStruct_ScriptMethod()
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LogPython: print(a)
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LogPython: <Struct 'MyPythonStruct_ScriptMethod' (0x0000092D08CD6ED0) {my_string: ""}>
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LogPython: a.set_string_on_struct_mutable("Hello")
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LogBlueprintUserMessages: [None] UMyPython_ScriptMethod_Test::SetStringOnStructMutable
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LogPython: print(a)
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LogPython: <Struct 'MyPythonStruct_ScriptMethod' (0x0000092D08CD6ED0) {my_string: "Hello"}>
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LogPython: a.set_string_on_struct("FFF")
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LogBlueprintUserMessages: [None] UMyPython_ScriptMethod_Test::SetStringOnStruct
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LogPython: print(a)
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LogPython: <Struct 'MyPythonStruct_ScriptMethod' (0x0000092D08CD6ED0) {my_string: "Hello"}>
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LogPython: a.set_string_on_struct_via_ref("First")
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LogBlueprintUserMessages: [None] UMyPython_ScriptMethod_Test::SetStringOnStructViaRef
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LogPython: print(a)
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LogPython: <Struct 'MyPythonStruct_ScriptMethod' (0x0000092D08CD6ED0) {my_string: "First"}>
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```
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## 原理：
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判断如果有ScriptMethodMutable，会设置SelfReturn，从而再最后把函数调用中的临时值复制给原本的参数值，达成可变引用调用的效果。
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```cpp
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// The function may have been flagged as mutable, in which case we always consider it to need a 'self' return
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if (!GeneratedWrappedDynamicMethod.SelfReturn.ParamProp && InFunc->HasMetaData(PyGenUtil::ScriptMethodMutableMetaDataKey))
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{
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if (!SelfParam.ParamProp->IsA<FStructProperty>())
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{
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REPORT_PYTHON_GENERATION_ISSUE(Error, TEXT("Function '%s.%s' is marked as 'ScriptMethodMutable' but the 'self' argument is not a struct."), *InFunc->GetOwnerClass()->GetName(), *InFunc->GetName());
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return;
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}
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GeneratedWrappedDynamicMethod.SelfReturn = SelfParam;
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}
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```
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