AssetManager

请先阅读官方文档:资源管理

/** 
 * A singleton UObject that is responsible for loading and unloading PrimaryAssets, and maintaining game-specific asset references
 * Games should override this class and change the class reference
 */

突然发现内容有点少，既然提到了asset manager，那我们把资源加载、资源引用也一并学了吧。
参考官方文档：

• 异步加载资源
• 引用资源

阅读之后，你应该知道：

• 什么是SoftObject、软引用
• 资源加载：LoadObject<> 和 FStreamableManager的作用
  （FStreamableManager实例可以通过AssetManager::GetStreamableManager()来获得。与AssetManager不同，在引擎中可以有多个FStreamableManager。）
• 你可以利用资源注册表和对象库检索资源

RPGAssetManager.h/cpp

// Copyright 1998-2019 Epic Games, Inc. All Rights Reserved.

#pragma once

#include "ActionRPG.h"
#include "Engine/AssetManager.h"
#include "RPGAssetManager.generated.h"

class URPGItem;

/**
 * Game implementation of asset manager, overrides functionality and stores game-specific types
 * It is expected that most games will want to override AssetManager as it provides a good place for game-specific loading logic
 * This is used by setting AssetManagerClassName in DefaultEngine.ini
 */
UCLASS()
class ACTIONRPG_API URPGAssetManager : public UAssetManager
{
    GENERATED_BODY()

public:
    // Constructor and overrides
    URPGAssetManager() {}
    virtual void StartInitialLoading() override;

    /** Static types for items */
    static const FPrimaryAssetType  PotionItemType;
    static const FPrimaryAssetType  SkillItemType;
    static const FPrimaryAssetType  TokenItemType;
    static const FPrimaryAssetType  WeaponItemType;

    /** Returns the current AssetManager object */
    static URPGAssetManager& Get();

    /**
     * Synchronously loads an RPGItem subclass, this can hitch but is useful when you cannot wait for an async load
     * This does not maintain a reference to the item so it will garbage collect if not loaded some other way
     *
     * @param PrimaryAssetId The asset identifier to load
     * @param bDisplayWarning If true, this will log a warning if the item failed to load
     */
    URPGItem* ForceLoadItem(const FPrimaryAssetId& PrimaryAssetId, bool bLogWarning = true);
};

源文件：

// Copyright 1998-2019 Epic Games, Inc. All Rights Reserved.

#include "RPGAssetManager.h"
#include "Items/RPGItem.h"
#include "AbilitySystemGlobals.h"

const FPrimaryAssetType URPGAssetManager::PotionItemType = TEXT("Potion");
const FPrimaryAssetType URPGAssetManager::SkillItemType = TEXT("Skill");
const FPrimaryAssetType URPGAssetManager::TokenItemType = TEXT("Token");
const FPrimaryAssetType URPGAssetManager::WeaponItemType = TEXT("Weapon");

/**  获得asset manager单例*/
URPGAssetManager& URPGAssetManager::Get()
{
    URPGAssetManager* This = Cast<URPGAssetManager>(GEngine->AssetManager);

    if (This)
    {
        return *This;
    }
    else
    {
        UE_LOG(LogActionRPG, Fatal, TEXT("Invalid AssetManager in DefaultEngine.ini, must be RPGAssetManager!"));
        return *NewObject<URPGAssetManager>(); // never calls this
    }
}

/** Starts initial load, gets called from InitializeObjectReferences */
void URPGAssetManager::StartInitialLoading()
{
    Super::StartInitialLoading();

// 加载GAS中的全局数据，如tag等
    UAbilitySystemGlobals::Get().InitGlobalData();
}

/**  同步加载指定资源  ， TryLoad() 会调用 LoadObject()*/
URPGItem* URPGAssetManager::ForceLoadItem(const FPrimaryAssetId& PrimaryAssetId, bool bLogWarning)
{   
    FSoftObjectPath ItemPath = GetPrimaryAssetPath(PrimaryAssetId);

    // This does a synchronous load and may hitch
    URPGItem* LoadedItem = Cast<URPGItem>(ItemPath.TryLoad());

    if (bLogWarning && LoadedItem == nullptr)
    {
        UE_LOG(LogActionRPG, Warning, TEXT("Failed to load item for identifier %s!"), *PrimaryAssetId.ToString());
    }

    return LoadedItem;
}