[UE4]C++实现动态加载的问题：LoadClass<T>()和LoadObject<T>() 及静态加载问题：ConstructorHelpers::FClassFinder()和FObjectFinder()

Posted on 2017-10-25 12:13 Bill Yuan 阅读(5158) 评论(0) 编辑收藏举报

转自：http://aigo.iteye.com/blog/2281558

动态加载UObject和动态加载UClass分别用LoadObject<T>()，和LoadClass<T>() ，两者均在在UObjectGlobals.h中。

另外注意：LoadClass<T>的模版名称，不能直接写UBlueprint，例如：LoadClass<UBlueprint>是错误的，创建蓝图时选择的是什么父类，则写对应的父类名，假如是Actor，那么要写成：LoadClass<AActor>，否则无法加载成功。

路径名也必须带_C后缀（LoadObject不需要带_C后缀），例如，蓝图路径是：Blueprint'/Game/Blueprints/MyBP.MyBP'，

加后缀以后，则是：Blueprint'/Game/Blueprints/MyBP.MyBP_C'，

例子：

UClass* Test = LoadClass<AActor>(NULL, TEXT("Blueprint'/Game/Blueprints/MapPathBrush_BP.MapPathBrush_BP_C'"));

官方还没出文档，只能先看代码注释：

// Load an object.  
template< class T >   
inline T* LoadObject( UObject* Outer, const TCHAR* Name, const TCHAR* Filename=nullptr, uint32 LoadFlags=LOAD_None, UPackageMap* Sandbox=nullptr )  
{  
    return (T*)StaticLoadObject( T::StaticClass(), Outer, Name, Filename, LoadFlags, Sandbox );  
}

// Load a class object.  
template< class T >   
inline UClass* LoadClass( UObject* Outer, const TCHAR* Name, const TCHAR* Filename=nullptr, uint32 LoadFlags=LOAD_None, UPackageMap* Sandbox=nullptr )  
{  
    return StaticLoadClass( T::StaticClass(), Outer, Name, Filename, LoadFlags, Sandbox );  
}

/** 
 * Find or load an object by string name with optional outer and filename specifications. 
 * These are optional because the InName can contain all of the necessary information. 
 * 
 * @param ObjectClass   The class (or a superclass) of the object to be loaded. 
 * @param InOuter       An optional object to narrow where to find/load the object from 
 * @param InName        String name of the object. If it's not fully qualified, InOuter and/or Filename will be needed 
 * @param Filename      An optional file to load from (or find in the file's package object) 
 * @param LoadFlags     Flags controlling how to handle loading from disk 
 * @param Sandbox       A list of packages to restrict the search for the object 
 * @param bAllowObjectReconciliation    Whether to allow the object to be found via FindObject in the case of seek free loading 
 * 
 * @return The object that was loaded or found. NULL for a failure. 
 */  
COREUOBJECT_API UObject* StaticLoadObject( UClass* Class, UObject* InOuter, const TCHAR* Name, const TCHAR* Filename = NULL, uint32 LoadFlags = LOAD_None, UPackageMap* Sandbox = NULL, bool bAllowObjectReconciliation = true );  
COREUOBJECT_API UClass* StaticLoadClass(UClass* BaseClass, UObject* InOuter, const TCHAR* Name, const TCHAR* Filename = NULL, uint32 LoadFlags = LOAD_None, UPackageMap* Sandbox = NULL);

LoadObject加载例子，不需要添加后缀：

UTexture2D* Tex = LoadObject<UTexture2D>(NULL, TEXT("Texture2D'/Game/Textures/UI/tex_test001.tex_test001'"));

可以用LoadObject加载的文件包括：

Texture、Material、SoundWave、SoundCue、ParticlesSystem、AnimMontage、BlendSpace(1D，2D，3D)、AnimSequence、AnimBlueprint、SkeletalMesh等等。这些文件的父类都是UObject，所以也可以先加载为UObject*然后再强转为具体的类型，例如：

UObject* Obj = LoadObject<UObject>(NULL, TEXT("SkeletalMesh'/Game/MyMesh.MyMesh'"));  
USkeletalMesh* MyMesh = Cast<USkeletalMesh*>(Obj);

实现动态加载UObject：StaticLoadObject()；以Texture和Material为例

示例1：

动态加载Object的工具方法

UTexture2D* MyTextureLoader::LoadTextureFromPath(const FString& Path)  
{  
    if (Path.IsEmpty()) return NULL;  
  
    return Cast<UTexture2D>(StaticLoadObject(UTexture2D::StaticClass(), NULL, *(Path)));  
}

调用：

FString PathToLoad = "/Game/Textures/YourStructureHere";  
UTexture2D* tmpTexture = LoadTextureFromPath(PathToLoad);

示例2：
加载Material和Texture

struct FConstructorStatics  
 {  
     ConstructorHelpers::FObjectFinderOptional<UTexture> TextureFinder;  
     ConstructorHelpers::FObjectFinderOptional<UMaterial> MaterialFinder;  
     FConstructorStatics()  
         : TextureFinder(TEXT("Texture2D'/Game/Textures/2DBackground.2DBackground'"))  
         , MaterialFinder(TEXT("Material'/Game/Materials/DynamicTextureMaterial.DynamicTextureMaterial'"))  
     {  
     }  
 };  
 static FConstructorStatics ConstructorStatics;  
   
 Texture = ConstructorStatics.TextureFinder.Get();  
 UMaterial* Material = ConstructorStatics.MaterialFinder.Get();  
 DynamicMaterial = UMaterialInstanceDynamic::Create(Material, this);

设置调用加载好的Material和Texture：

DynamicMaterial->SetTextureParameterValue(FName("DynamicTexture"), Texture);  
Mesh->SetMaterial(0, DynamicMaterial);

如果资源永不再使用，想销毁资源对象，代码如下：

Texture2D* mytex; //这里假设mytex合法有效  
  
mytex->ConditionalBeginDestroy();  
mytex = NULL;  
GetWorld()->ForceGarbageCollection(true);

Dynamic Asset Loading with C++

https://www.youtube.com/watch?v=pJIAmSGxfmQ

Dynamic Load Object

https://wiki.unrealengine.com/Dynamic_Load_Object

[UE4]C++静态加载问题：ConstructorHelpers::FClassFinder()和FObjectFinder()

这里说的静态加载指的是必须在构造函数中完成的加载方式，动态加载值得是可以在Runtime期间加载的方式，UE4源码里面，前者其实是对后者的一层封装，即FObjectFinder()是对LoadObject()的封装。But，FClassFinder()不是对LoadClass()的封装，FClassFinder()内部调用的是LoadObject()。

如果要获取某个蓝图BP的类型class，可以通过ConstructorHelpers::FClassFinder()来获取，例如：

static ConstructorHelpers::FClassFinder<AActor> UnitSelector(TEXT("Blueprint'/Game/MyProject/MyBlueprint.MyBlueprint'"));  
TSubclassOf<AActor> UnitSelectorClass = UnitSelector.Class;

但是在启动游戏时会报错提示找不到文件，例如：

Default property warnings and errors:

Error: COD Constructor (MyGameMode): Failed to find /Game/MyProject/MyBlueprint.MyBlueprint

解决办法有两种（这是UE4的一个坑，浪费了我很长时间。。。）：

A，在copy reference出来的文件路径后面加_C，例如：Blueprint'/Game/Blueprints/MyBlueprint.MyBlueprint_C'

static ConstructorHelpers::FClassFinder<AActor> UnitSelector(TEXT("Blueprint'/Game/Blueprints/MyBlueprint.MyBlueprint_C'"));  
TSubclassOf<AActor> UnitSelectorClass = UnitSelector.Class;

B，去掉路径前缀：/Game/Blueprints/MyBlueprint

static ConstructorHelpers::FClassFinder<AActor> UnitSelector(TEXT("/Game/Blueprints/MyBlueprint"));  
TSubclassOf<AActor> UnitSelectorClass = UnitSelector.Class;

另外注意：FClassFinder<T>的模版名称，不能直接写UBlueprint，例如：FClassFinder<UBlueprint>是错误的。创建蓝图时选择的是什么父类，则写对应的父类名，假如是Actor，那么要写成：FClassFinder<AActor>，否则无法加载成功。

使用TSubclassOf<T>时模板名必须相同

另外， FClassFinder<T>()函数中的模版名必须和TSubclassOf<T>变量的模版名一样，例如上面的都是AActor，如果不一样，也会出现上面的错误。
再给个例子：

static ConstructorHelpers::FClassFinder<UUserWidget> TestBP(TEXT("/Game/Blueprints/MyWidget_BP"));  
TSubclassOf<UUserWidget> MyWidgetClass = TestBP.Class;

也可使用UClass*替换TSubclassOf<T>

例如：

static ConstructorHelpers::FClassFinder<UUserWidget> TestBP(TEXT("/Game/Blueprints/MyWidget_BP"));  
UClass* MyWidgetClass = TestBP.Class;

之前看到很多例子是通过FObjectFinder()来获取class，现在想想感觉是无奈之举，UE4的文档比较坑，不仅蓝图的文档更新不同步，C++的文档更是少得可怜。

static ConstructorHelpers::FObjectFinder<UBlueprint> UnitSelector(TEXT("Blueprint'/Game/MyProject/MyBlueprint.MyBlueprint'"));  
TSubclassOf<AActor> UnitSelectorClass = (UClass*)UnitSelector.Object->GeneratedClass;

其他参考：

CDO Constructor: Failed to find Blueprint

https://answers.unrealengine.com/questions/84880/cdo-constructor-failed-to-find-blueprint-ue-44.html