Asset Bundle .mainAsset is null (WWW object only)

[jasonFHH]

I dabbled with Asset Bundles in Unity about a year ago and was disappointed to find that it introduced a lot of crashes and mem leaks into my game. Ultimately, I wasn’t able to use it for this reason. I recently say a video from Unite 2012 ( http://video.unity3d.com/video/6944412/unite-2012-asset-bundles ) and I figured now it was safe enough to go back to Asset Bundles. I even downloaded their tool for managing multiple asset bundles.

My problem now is that asset bundles downloaded via WWW.LoadFromCacheOrDownload (url, version) return an Asset Bundle object with a null .mainAsset property. Loading the prefab that it’s based on using Resources.Load seems to be just fine, however using WWW to fetch a compressed prefab returns null .mainAssets (regardless of .unity3d stored locally or remotely).

I don’t think this is a classic “Did you drag the prefab to the variable” since the asset bundles aren’t supposed to be available at build-time and are instead dynamically fetched at runtime. I’m not sure what’s going on though.

// Wait for the Caching system to be ready
			
			while (!Caching.ready)
				yield return null;
			
			// Load the AssetBundle file from Cache if it exists with the same version or download and store it in the cache
			string url = "http://www.example.com/ab/chars/" + charName + ".unity3d";
			Debug.Log ("Url-d: " + url);
			using(WWW www = WWW.LoadFromCacheOrDownload (url, version)){
				yield return www;
				if (www.error != null)
				{
					//throw new Exception("WWW download had an error:" + www.error);
					Debug.LogError("WWW download (" + url + ") had an error:" + www.error);
				}
				if(www == null)
				{
					Debug.LogError ("www object is null!");
				}
				AssetBundle bundle = www.assetBundle;
				if(bundle == null)
				{
					Main.LogMessage ("[error] bundle is null!");
					Debug.LogError ("null bundle");
				}
				
				
				GameObject newObj = null;
				
					if(bundle.mainAsset == null)
					{
						Debug.LogError ("Main asset in url (" + url + ") is null");	
					}
					
					try
					{
						newObj = Instantiate(bundle.mainAsset) as GameObject;
						if(newObj == null)
						{
							Debug.Log ("Could not instantiate main asset in asset bundle.");
						}
					}
					catch(System.Exception ex)
					{
						Debug.LogError(ex.Message);
					}
	            // Unload the AssetBundles compressed contents to conserve memory
	            bundle.Unload(false);
				
				
				
			}

[horsman1]

As @whydoidoit said above the issue is that not all Asset Bundles are built with a main asset (including the ones built with our tool). The solution is use AssetBundle.Load, LoadAsync, or LoadAll with a string reference to the asset.