167 lines
9.7 KiB
Markdown
167 lines
9.7 KiB
Markdown
### TAssetPtr
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因为在wiki上已经有介绍TAssetPtr的内容了，所以我就直接翻译了，并且在末尾给予一定的补充。以下是翻译自wiki的内容：
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一般情况下，我们无需将这个场景中所有Asset都加载了再进入游戏，我们可以先加载必要Asset，在进入场景后，异步加载剩余的非必要资源。而TAssetPtr可以解决这个问题
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TAssetPtr类似于标准指针，其区别在于TAssetPtr指向的资产可能已经加载，也可能还没有加载，如果资产没有加载，则它包含加载该资产所需的信息。（TAssetPtr属于弱指针）
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一个简单的引用Asset的方法就是创建带有UProperty标记的TAssetPtr成员变量，并且在编辑器中指定想要加载的Asset。
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本质上是TSoftClassPtr，包含了FSoftObjectPtr对象。可以通过FSoftObjectPath构造，并**用其监视Asset的状态**。
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#### 类型
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变量 | 描述
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---|---
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TAssetPtr<T> | 指向尚未加载但可以根据请求加载的资产的指针
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TAssetSubclassOf<T> | 指向已定义的基类的子类的指针，该子类尚未加载，但可以根据请求加载。用于指向蓝图而不是基本component。（大概是因为蓝图是Asset）
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#### 关键功能
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.h
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```
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/** 定义Asset指针. 别忘记添加UPROPERTY标记 */
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UPROPERTY(EditAnywhere)
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TAssetPtr<MyClass> MyAssetPointer;
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/** 定义子类版本. */
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UPROPERTY(EditAnywhere)
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TAssetSubclassOf<MyBaseClass> MyAssetSubclassOfPointer;
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```
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.cpp
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```
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// 调用IsValid()去测试这个AssetPtr是不是指向一个有效的UObject
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MyAssetPointer.IsValid();
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//调用Get()来返回其指向的UObject（在UObject存在的情况下）
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MyAssetPointer.Get();
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/** 特别注意TAssetSubclassOf的Get()，它返回的是UClass的指针！！ */
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MyAssetSubclassOfPointer.Get()
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/** 要正确使用UClass指针，必须使用GetDefaultObject<T>()来获得指向UObject或派生类的指针 */
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MyAssetSubclassOfPointer.Get()->GetDefaultObject<MyBaseClass>()
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// 调用ToStringReference()返回希望加载的Asset的FStringAssetReference
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MyAssetPointer.ToStringReference();
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```
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#### 如何使用
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变量 | 描述
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FStreamableManager | 运行时的Asset流控制管理器，这是用户定义的对象，应该被定义在类似GameInstance之类的方便访问的对象中。
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FStringAssetReference | 一个包含Asset应用字符串的结构体，能对Asset进行弱引用。
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##### Asset载入器
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FStreamableManager是异步资源加载器，最好定义在类似GameInstance之类持久性对象中，原因有下：
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1. 访问方便，这使得在需要时加载资产变得很容易。
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2. 具备持久性，因为你永远不想在加载对象时丢失或销毁对FStreamableManager的引用。
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##### 使用方法
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###### 简单异步载入
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允许您加载单个资产并获得它的**强引用**。这意味着在您使用unload手动卸载它之前，它永远不会被垃圾回收。(这个 方法已经被废弃，请使用RequestAsyncLoad，并设置bManageActiveHandle为true)
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```
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// the .h
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TAssetPtr<ABaseItem> MyItem;
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// the .cpp
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FStringAssetReference AssetToLoad
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AssetToLoad = MyItem.ToStringReference();
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AssetLoader.SimpleAsyncLoad(AssetToLoad);
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```
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###### 请求式异步载入
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```
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//the .h
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TArray< TAssetPtr<ABaseItem> > MyItems;
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// the .cpp
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TArray<FStringAssetReference> AssetsToLoad
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for(TAssetPtr<ABaseItem>& AssetPtr : MyItems) // C++11 ranged loop
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{
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AssetsToLoad.AddUnique(AssetPtr.ToStringReference());
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}
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AssetLoader.RequestAsyncLoad(AssetsToLoad, FStreamableDelegate::CreateUObject(this, &MyClass::MyFunctionToBeCalledAfterAssetsAreLoaded));
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```
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PS.实际看过代码之后发现这个RequestAsyncLoad还有一个回调版本的。
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```
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/**
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* This is the primary streamable operation. Requests streaming of one or more target objects. When complete, a delegate function is called. Returns a Streamable Handle.
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*
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* @param TargetsToStream Assets to load off disk
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* @param DelegateToCall Delegate to call when load finishes. Will be called on the next tick if asset is already loaded, or many seconds later
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* @param Priority Priority to pass to the streaming system, higher priority will be loaded first
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* @param bManageActiveHandle If true, the manager will keep the streamable handle active until explicitly released
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* @param bStartStalled If true, the handle will start in a stalled state and will not attempt to actually async load until StartStalledHandle is called on it
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* @param DebugName Name of this handle, will be reported in debug tools
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*/
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TSharedPtr<FStreamableHandle> RequestAsyncLoad(const TArray<FSoftObjectPath>& TargetsToStream, FStreamableDelegate DelegateToCall = FStreamableDelegate(), TAsyncLoadPriority Priority = DefaultAsyncLoadPriority, bool bManageActiveHandle = false, bool bStartStalled = false, const FString& DebugName = TEXT("RequestAsyncLoad ArrayDelegate"));
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TSharedPtr<FStreamableHandle> RequestAsyncLoad(const FSoftObjectPath& TargetToStream, FStreamableDelegate DelegateToCall = FStreamableDelegate(), TAsyncLoadPriority Priority = DefaultAsyncLoadPriority, bool bManageActiveHandle = false, bool bStartStalled = false, const FString& DebugName = TEXT("RequestAsyncLoad SingleDelegate"));
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/** Lambda Wrappers. Be aware that Callback may go off multiple seconds in the future. */
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TSharedPtr<FStreamableHandle> RequestAsyncLoad(const TArray<FSoftObjectPath>& TargetsToStream, TFunction<void()>&& Callback, TAsyncLoadPriority Priority = DefaultAsyncLoadPriority, bool bManageActiveHandle = false, bool bStartStalled = false, const FString& DebugName = TEXT("RequestAsyncLoad ArrayLambda"));
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TSharedPtr<FStreamableHandle> RequestAsyncLoad(const FSoftObjectPath& TargetToStream, TFunction<void()>&& Callback, TAsyncLoadPriority Priority = DefaultAsyncLoadPriority, bool bManageActiveHandle = false, bool bStartStalled = false, const FString& DebugName = TEXT("RequestAsyncLoad SingleLambda"));
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```
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###### 使用Asset
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当你的Asset加载完成，别忘记调用Get()来取得它。
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```
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MyItem.Get(); // returns a pointer to the LIVE UObject
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```
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### 本人额外添加的内容（一些有用的东西）
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#### FStreamableManager
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```
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/**
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* 同步版本的载入函数，用于载入多个（一组）资源，返回一个handle。
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*
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* @param TargetsToStream Assets to load off disk
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* @param bManageActiveHandle If true, the manager will keep the streamable handle active until explicitly released
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* @param DebugName Name of this handle, will be reported in debug tools
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*/
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TSharedPtr<FStreamableHandle> RequestSyncLoad(const TArray<FSoftObjectPath>& TargetsToStream, bool bManageActiveHandle = false, const FString& DebugName = TEXT("RequestSyncLoad Array"));
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TSharedPtr<FStreamableHandle> RequestSyncLoad(const FSoftObjectPath& TargetToStream, bool bManageActiveHandle = false, const FString& DebugName = TEXT("RequestSyncLoad Single"));
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/**
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* 同步版本的载入函数，用于载入单个资源，返回UObject的指针。如果没有找到则返回空指针。
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*
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* @param Target Specific asset to load off disk
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* @param bManageActiveHandle If true, the manager will keep the streamable handle active until explicitly released
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* @param RequestHandlePointer If non-null, this will set the handle to the handle used to make this request. This useful for later releasing the handle
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*/
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UObject* LoadSynchronous(const FSoftObjectPath& Target, bool bManageActiveHandle = false, TSharedPtr<FStreamableHandle>* RequestHandlePointer = nullptr);
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```
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#### FStreamableHandle
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同步或者异步加载的句柄，只要句柄处于激活状态，那么Asset就不会被回收。你可以句柄来控制Asset，以及绑定相应的委托。
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```
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/** Bind delegate that is called when load completes, only works if loading is in progress. This will overwrite any already bound delegate! */
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bool BindCompleteDelegate(FStreamableDelegate NewDelegate);
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/** Bind delegate that is called if handle is canceled, only works if loading is in progress. This will overwrite any already bound delegate! */
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bool BindCancelDelegate(FStreamableDelegate NewDelegate);
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/** Bind delegate that is called periodically as delegate updates, only works if loading is in progress. This will overwrite any already bound delegate! */
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bool BindUpdateDelegate(FStreamableUpdateDelegate NewDelegate);
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/**
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* Blocks until the requested assets have loaded. This pushes the requested asset to the top of the priority list,
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* but does not flush all async loading, usually resulting in faster completion than a LoadObject call
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*
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* @param Timeout Maximum time to wait, if this is 0 it will wait forever
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* @param StartStalledHandles If true it will force all handles waiting on external resources to try and load right now
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*/
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EAsyncPackageState::Type WaitUntilComplete(float Timeout = 0.0f, bool bStartStalledHandles = true);
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/** Gets list of assets references this load was started with. This will be the paths before redirectors, and not all of these are guaranteed to be loaded */
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void GetRequestedAssets(TArray<FSoftObjectPath>& AssetList) const;
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/** Adds all loaded assets if load has succeeded. Some entries will be null if loading failed */
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void GetLoadedAssets(TArray<UObject *>& LoadedAssets) const;
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/** Returns first asset in requested asset list, if it's been successfully loaded. This will fail if the asset failed to load */
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UObject* GetLoadedAsset() const;
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/** Returns number of assets that have completed loading out of initial list, failed loads will count as loaded */
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void GetLoadedCount(int32& LoadedCount, int32& RequestedCount) const;
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/** Returns progress as a value between 0.0 and 1.0. */
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float GetProgress() const;
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```
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#### FSoftObjectPath
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一个包含对象的引用字符串的结构体。它可以对按需加载的资产进行弱引用。可以使用UProperties进行标记，使得它可以在编辑器中显示并且可以指定资源。
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可以使用TryLoad进行Asset载入，返回UObject指针，为空则表示载入失败。
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