CATiledLayer

levelsOfDetail和levelsOfDetailBias
http://cocoabob.net/?p=120

http://www.cocoachina.com/ask/questions/show/1748/CATiledLayer%E9%97%AE%E9%A2%98

@interface CATiledLayer : CALayer

/* The time in seconds that newly added images take to "fade-in" to the
 * rendered representation of the tiled layer. The default implementation
 * returns 0.25 seconds. */

+ (CFTimeInterval)fadeDuration;

/* The number of levels of detail maintained by this layer. Defaults to
 * one. Each LOD is half the resolution of the previous level. If too
 * many levels are specified for the current size of the layer, then
 * the number of levels is clamped to the maximum value (the bottom
 * most LOD must contain at least a single pixel in each dimension). */

@property size_t levelsOfDetail;

/* The number of magnified levels of detail for this layer. Defaults to
 * zero. Each previous LOD is twice the resolution of the later. E.g.
 * specifying 'levelBias' of two means the layer has two extra levels:
 * 2x and 4x. */

@property size_t levelsOfDetailBias;

/* The maximum size of each tile used to create the layer's content.
 * Defaults to (256, 256). Note that there is a maximum tile size, and
 * requests for tiles larger than that limit will cause a suitable
 * value to be substituted. */

@property CGSize tileSize;

@end