gecko/gfx/layers/ThebesLayerBuffer.h

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#ifndef THEBESLAYERBUFFER_H_
#define THEBESLAYERBUFFER_H_

#include "gfxContext.h"
#include "gfxASurface.h"
#include "nsRegion.h"

namespace mozilla {
namespace layers {

class ThebesLayer;

/**
 * This class encapsulates the buffer used to retain ThebesLayer contents,
 * i.e., the contents of the layer's GetVisibleRegion().
 * 
 * This is a cairo/Thebes surface, but with a literal twist. Scrolling
 * causes the layer's visible region to move. We want to keep
 * reusing the same surface if the region size hasn't changed, but we don't
 * want to keep moving the contents of the surface around in memory. So
 * we use a trick.
 * Consider just the vertical case, and suppose the buffer is H pixels
 * high and we're scrolling down by N pixels. Instead of copying the
 * buffer contents up by N pixels, we leave the buffer contents in place,
 * and paint content rows H to H+N-1 into rows 0 to N-1 of the buffer.
 * Then we can refresh the screen by painting rows N to H-1 of the buffer
 * at row 0 on the screen, and then painting rows 0 to N-1 of the buffer
 * at row H-N on the screen.
 * mBufferRotation.y would be N in this example.
 */
class ThebesLayerBuffer {
public:
  typedef gfxASurface::gfxContentType ContentType;

  /**
   * Controls the size of the backing buffer of this.
   * - SizedToVisibleBounds: the backing buffer is exactly the same
   *   size as the bounds of ThebesLayer's visible region
   * - ContainsVisibleBounds: the backing buffer is large enough to
   *   fit visible bounds.  May be larger.
   */
  enum BufferSizePolicy {
    SizedToVisibleBounds,
    ContainsVisibleBounds
  };

  ThebesLayerBuffer(BufferSizePolicy aBufferSizePolicy)
    : mBufferRotation(0,0)
    , mBufferSizePolicy(aBufferSizePolicy)
  {
    MOZ_COUNT_CTOR(ThebesLayerBuffer);
  }
  virtual ~ThebesLayerBuffer()
  {
    MOZ_COUNT_DTOR(ThebesLayerBuffer);
  }

  /**
   * Wipe out all retained contents. Call this when the entire
   * buffer becomes invalid.
   */
  void Clear()
  {
    mBuffer = nsnull;
    mBufferRect.Empty();
  }

  /**
   * This is returned by BeginPaint. The caller should draw into mContext.
   * mRegionToDraw must be drawn. mRegionToInvalidate has been invalidated
   * by ThebesLayerBuffer and must be redrawn on the screen.
   * mRegionToInvalidate is set when the buffer has changed from
   * opaque to transparent or vice versa, since the details of rendering can
   * depend on the buffer type.
   */
  struct PaintState {
    nsRefPtr<gfxContext> mContext;
    nsIntRegion mRegionToDraw;
    nsIntRegion mRegionToInvalidate;
  };

  /**
   * Start a drawing operation. This returns a PaintState describing what
   * needs to be drawn to bring the buffer up to date in the visible region.
   * This queries aLayer to get the currently valid and visible regions.
   * The returned mContext may be null if mRegionToDraw is empty.
   * Otherwise it must not be null.
   * mRegionToInvalidate will contain mRegionToDraw.
   */
  PaintState BeginPaint(ThebesLayer* aLayer, ContentType aContentType);

  /**
   * Return a new surface of |aSize| and |aType|.
   */
  virtual already_AddRefed<gfxASurface>
  CreateBuffer(ContentType aType, const nsIntSize& aSize) = 0;

  /**
   * Get the underlying buffer, if any. This is useful because we can pass
   * in the buffer as the default "reference surface" if there is one.
   * Don't use it for anything else!
   */
  gfxASurface* GetBuffer() { return mBuffer; }

protected:
  enum XSide {
    LEFT, RIGHT
  };
  enum YSide {
    TOP, BOTTOM
  };
  nsIntRect GetQuadrantRectangle(XSide aXSide, YSide aYSide);
  void DrawBufferQuadrant(gfxContext* aTarget, XSide aXSide, YSide aYSide, float aOpacity);
  void DrawBufferWithRotation(gfxContext* aTarget, float aOpacity);

  const nsIntRect& BufferRect() const { return mBufferRect; }
  const nsIntPoint& BufferRotation() const { return mBufferRotation; }

  already_AddRefed<gfxASurface>
  SetBuffer(gfxASurface* aBuffer,
            const nsIntRect& aBufferRect, const nsIntPoint& aBufferRotation)
  {
    gfxASurface* tmp = mBuffer;
    mBuffer = aBuffer;
    mBufferRect = aBufferRect;
    mBufferRotation = aBufferRotation;
    return tmp;
  }

private:
  PRBool BufferSizeOkFor(const nsIntSize& aSize)
  {
    return (aSize == mBufferRect.Size() ||
            (SizedToVisibleBounds != mBufferSizePolicy &&
             aSize < mBufferRect.Size()));
  }

  nsRefPtr<gfxASurface> mBuffer;
  /** The area of the ThebesLayer that is covered by the buffer as a whole */
  nsIntRect             mBufferRect;
  /**
   * The x and y rotation of the buffer. Conceptually the buffer
   * has its origin translated to mBufferRect.TopLeft() - mBufferRotation,
   * is tiled to fill the plane, and the result is clipped to mBufferRect.
   * So the pixel at mBufferRotation within the buffer is what gets painted at
   * mBufferRect.TopLeft().
   * This is "rotation" in the sense of rotating items in a linear buffer,
   * where items falling off the end of the buffer are returned to the
   * buffer at the other end, not 2D rotation!
   */
  nsIntPoint            mBufferRotation;
  BufferSizePolicy      mBufferSizePolicy;
};

}
}

#endif /* THEBESLAYERBUFFER_H_ */