3rd party (or even 1st party) modules are currently very restricted in the way that they can interact with the Output Cache functionality.

I propose adding an event handler interface to allow developers to hook into specific events offered by the Output Cache filter.

This could look something along the lines of this:

public interface IOutputCacheEventHandler : IEventHandler
{
    void Caching(string cacheKey, CacheItem cacheItem);
    void Cached(string cacheKey, CacheItem cacheItem);
    void Retrieving(string cacheKey);
    void Retrieved(string cacheKey, CacheItem cacheItem);
}

Example use cases would be:

• Altering markup immediately before the item is cached (for example to remove the Glimpse console)
• Adding additional logging or diagnostics