31 Preventing Eviction

Applications have a mechanism for preventing the DGS from evicting particular nodes (for example a GUI widget that is active within a window). This might be important because they represent transient state that cannot be calculated from the persistent models. For example a widget may record a scroll bar position.

Before evicting an independent or dependent node, the DGS evictor calls IsEvictable()

    virtual bool IsEvictable() const = 0;

If this return false then the node is not evicted. It is instead moved to the back of the eviction queue.

The xc++ lamguage supports evictable{...} blocks on $dep variables which provide the implementation of IsEvictable() on that node. For example:

    $dep void selfCleaningAgent_
        evictable  { return false; }
        invalidate { PostTaskToCleanAgent(); }
        calc       { CalcAgent(); };

If an evictable{...} block isn't specified on a $dep then an implementation of IsEvictable() is generated which calls CacheValueIsEvictable() on the dependent variable.

$cache functions cannot define an 'evictable' block. Instead they can specialise/overload the function CacheValueIsEvictable for the mapped value type. The default implementation returns true:

    template <typename T>
    bool CacheValueIsEvictable(const T&)
    {
        // By default we assume cached values in maps are evictable.
        return true;
    }

The CedaGui IView system provides an overload as follows:

    $interface+ IView : IObject
    {
        bool IsEvictable() const;
        ...
    };

    inline bool CacheValueIsEvictable(ptr<IView> v)
    {
        return v ? v->IsEvictable() : true;
    }