解决 Unity3D AssetBundle 异步加载与同步加载冲突问题

当调用AB包异步加载时，如果接下来的流程中需要同步加载相同的AB包，会出现什么情况呢？让我们做下实验。

我首先生成了一个叫img的AB包，里面放了一些图片和模型

接着我们编写了测试代码，看看会怎样输出

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
 
public class NewBehaviourScript : MonoBehaviour
{
    AssetBundleCreateRequest _abcr = null;
 
    void Start()
    {
 
        _abcr = AssetBundle.LoadFromFileAsync(Application.streamingAssetsPath + "/img");
        Debug.Log($"Start() isDone={_abcr.isDone} _abcr.assetBundle ={ (_abcr.assetBundle == null ? "null" : _abcr.assetBundle.name + " " + _abcr.assetBundle.GetHashCode())}");
 
        AssetBundle _assetBundle = AssetBundle.LoadFromFile(Application.streamingAssetsPath + "/img");
        Debug.Log($"Start() isDone={_abcr.isDone} _abcr.assetBundle ={ (_abcr.assetBundle == null ? "null" : _abcr.assetBundle.name + " " + _abcr.assetBundle.GetHashCode())}");
        Debug.Log($"Start() _assetBundle={(_assetBundle == null ? "null" : _assetBundle.name + " " + _assetBundle.GetHashCode())}");
    }
 
    void Update()
    {
        Debug.Log($"Update() isDone={_abcr.isDone.ToString()} _abcr.assetBundle ={(_abcr.assetBundle == null ? "null" : _abcr.assetBundle.name + " " + _abcr.assetBundle.GetHashCode())}");
 
    }
}

 我们先异步加载AB包，然后接着同步加载AB包，输出结果如下

 如图所示，异步加载后，再同步加载，直接抛出的异常提示不允许重复加载AB包，并且同步加载返回了null。 但是虽然同步加载返回了null，可调用同步加载后，异步加载还是被立即强制同步完成了。（笔者认为这个可能是个bug ），那么当我们需要处理这种冲突时如何处理呢？

我们这样修改代码，再看输出

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
 
public class NewBehaviourScript : MonoBehaviour
{
    AssetBundleCreateRequest _abcr = null;
 
 
    void Start()
    {
 
 
 
        _abcr = AssetBundle.LoadFromFileAsync(Application.streamingAssetsPath + "/img");
        Debug.Log($"Start() isDone={_abcr.isDone} _abcr.assetBundle ={ (_abcr.assetBundle == null ? "null" : _abcr.assetBundle.name + " " + _abcr.assetBundle.GetHashCode())}");
 
        _abcr.assetBundle.Unload(false);
        Debug.Log($"Start() isDone={_abcr.isDone} _abcr.assetBundle ={ (_abcr.assetBundle == null ? "null" : _abcr.assetBundle.name + " " + _abcr.assetBundle.GetHashCode())}");
 
        AssetBundle _assetBundle = AssetBundle.LoadFromFile(Application.streamingAssetsPath + "/img");
        Debug.Log($"Start() isDone={_abcr.isDone} _abcr.assetBundle ={ (_abcr.assetBundle == null ? "null" : _abcr.assetBundle.name + " " + _abcr.assetBundle.GetHashCode())}");
        Debug.Log($"Start() _assetBundle={(_assetBundle == null ? "null" : _assetBundle.name + " " + _assetBundle.GetHashCode())}");
 
    }
 
    void Update()
    {
        Debug.Log($"Update() isDone={_abcr.isDone.ToString()} _abcr.assetBundle ={(_abcr.assetBundle == null ? "null" : _abcr.assetBundle.name + " " + _abcr.assetBundle.GetHashCode())}");
 
 
     }
}

我们这次先异步加载，然后再使用请求中的AssetBundle对象的Unload(true)方法，在未加载完成时让其直接卸载，然后在同步加载。输出如下

可见，我们在异步加载后使用AssetBundleCreateRequest.assetBundle.Unload(true)直接卸载 . 使得异步加载立即完成且assetBundle=null.然后我们再发起同步加载，则成功的获得了AB包对象。但是我们注意到异步加载的AB包对象和同步加载的AB包对象的哈希不同，已经不是同一个对象了。

那么AB包的异步同步冲突我看测试完了，但是加载资源对象的异步同步冲突又会如何呢？

我们使用如下代码再做试验:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
 
public class NewBehaviourScript : MonoBehaviour
{
 
    AssetBundleRequest _abr = null;
 
    void Start()
    {
        AssetBundle _ab = AssetBundle.LoadFromFile(Application.streamingAssetsPath + "/img");
 
        _abr = _ab.LoadAllAssetsAsync();
        Debug.Log($"Start() _abr.isDone={_abr.isDone.ToString()} _abr.allAssets={(_abr.allAssets == null ? "null" : _abr.allAssets.Length.ToString() + " " + _abr.allAssets[0].GetHashCode())}");
 
        UnityEngine.Object[] _allAssets = _ab.LoadAllAssets();
        Debug.Log($"Start() _abr.isDone={_abr.isDone.ToString()} _abr.allAssets={(_abr.allAssets == null ? "null" : _abr.allAssets.Length.ToString() + " " + _abr.allAssets[0].GetHashCode())} ");
        Debug.Log($"Start() _allAssets={(_allAssets == null ? "null" : _allAssets.Length.ToString() + " " + _allAssets[0].GetHashCode())}");
    }
 
    void Update()
    {
        Debug.Log($"Update() _abr.isDone={_abr.isDone.ToString()} _abr.allAssets={(_abr.allAssets == null ? "null" : _abr.allAssets.Length.ToString() + " " + _abr.allAssets[0].GetHashCode())} ");
    }
}

我们先加载AB包，然后用AB包对象异步加载所有资源对象，之后我们在用此AB包对象同步加载所有资源对象。输出如下

和AB包的异步同步冲突抛出异常不同，当我们在异步加载后发起同步加载，异步请求立即完毕，且异步和同步都返回了资源对象，而且我们可以看到返回的资源对象哈希值相同是同一个对象。总之，资源加载异步同步之间没有冲突，如果先发起异步后发起同步，则异步会立即完成。