How to I create a resource manager?

Question

Currently I'm using XNA's ContentManager to load my content. For the unfamiliar the ContentManager deserializes classes from disk and keeps a reference for future requests.

I'm currently facing a problem with management, where for some resources I don't want to deserialize them repeatedly (e.g. a texture) but for others I do (e.g. destructible level geometry). As it is now when I replay a level I get back the original class with the player's changes.

For this reason I decided to inherit from ContentManager to provide some way of re-deserializing a resource to get a new copy (probably using the protected ReadAsset<T> method).

Now I'm thinking about how I should manage this. Ideally I want some way of saying you can re-use Textures, but not Levels (i.e. when a Level is requested it always deserializes a new instance).

While I think about that I'd also like to create my custom ContentManager with a view to get other niceties such as automatic unloading, on-demand or in advance loading of assets, pre-loaded place-holder assets etc.

How is this sort of class typically designed and how can I overcome my immediate problem (reloading particular types and re-using instances of others)?

Answer 1

It sounds to me like ContentTracker has what you require. It allows for disposing and reloading of individual items, among many other things.

It should be what you're looking for, I think.

(Do note, that it's for XNA 3.1 only, however there is a link at the bottom of the Codeplex homepage where someone has kindly ported it to 4.0)

Answer 2

If I rembemer correctly, caching assets is done in Load method. So all you need to do is override this metod, and make custom caching (store loaded asset in dictionary with assetName as key). (This will help you later in unloading only certain items).

And then make new method LoadNew which will bypass caching (or create clone)

Answer 3

update

After a bit of a chat with Kikaimaru, I realized the deep copy wont work as seamlessly as I thought. Too bad, I thought it was very elegant. The methods below will still work though.

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If you are still set on doing it, try something like this. Just overload the method telling it to cache or not. I pretty much snagged this from this blog, and he goes into more detail there. You might find it useful should you decide to go this way.

public override T Load<T>(string assetName)
{
    return Load<T>(assetName, true);
}

public T Load<T>(strng assetName, bool cache)
{
    if (loadedAssets.ContainsKey(assetName))
        return (T)loadedAssets[assetName];

    T asset = ReadAsset<T>(assetName, null);

    if(cache)
        loadedAssets.Add(assetName, asset);

    return asset;
}

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I had another thought. What you could do it something along the lines of creating an interface to make all your non-cacheable object implement. Have the load manager look for that, and don't cache if it is found. You could accomplish the same thing with an attribute if you wanted.

public interface INoCache{} //no member here, just empty...Maybe a bool "ShouldCache" if you want...

public class Destructible : INoCache

//In your ContentManager
public T Load<T>(string assetName)
{
    if (loadedAssets.ContainsKey(assetName))
        return (T)loadedAssets[assetName];

    T asset = ReadAsset<T>(assetName, null);

    //this logic seems a bit backwards, but 
    //it makes for a little cleaner code IMHO
    if(!(asset is INoCache))
        loadedAssets.Add(assetName, asset);

    return asset;
}