gecko/gfx/thebes/gfxCachedTempSurface.h

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 *   Karl Tomlinson <karlt+@karlt.net>
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#ifndef GFX_CACHED_TEMP_SURFACE_H
#define GFX_CACHED_TEMP_SURFACE_H

#include "gfxASurface.h"
#include "nsExpirationTracker.h"

class gfxContext;

/**
 * This class can be used to cache double-buffering back surfaces.
 *
 * Large resource allocations may have an overhead that can be avoided by
 * caching.  Caching also alows the system to use history in deciding whether
 * to manage the surfaces in video or system memory.
 *
 * However, because we don't want to set aside megabytes of unused resources
 * unncessarily, these surfaces are released on a timer.
 */

class THEBES_API gfxCachedTempSurface {
public:
  /**
   * Returns a context for a surface that can be efficiently copied to
   * |aSimilarTo|.
   *
   * When |aContentType| has an alpha component, the surface will be cleared.
   * For opaque surfaces, the initial surface contents are undefined.
   * When |aContentType| differs in different invocations this is handled
   * appropriately, creating a new surface if necessary.
   * 
   * |aSimilarTo| should be of the same gfxSurfaceType in each invocation.
   * Because the cached surface may have been created during a previous
   * invocation, this will not be efficient if the new |aSimilarTo| has a
   * different format.
   */
  already_AddRefed<gfxContext> Get(gfxASurface::gfxContentType aContentType,
                                   const gfxIntSize& aSize,
                                   gfxASurface* aSimilarTo);

  void Expire() { mSurface = nsnull; }
  nsExpirationState* GetExpirationState() { return &mExpirationState; }
  ~gfxCachedTempSurface();

private:
  nsRefPtr<gfxASurface> mSurface;
  gfxIntSize mSize;
  nsExpirationState mExpirationState;
#ifdef DEBUG
  gfxASurface::gfxSurfaceType mType;
#endif 
};

#endif /* GFX_CACHED_TEMP_SURFACE_H */