TilingView

TilingView.h

#import <UIKit/UIKit.h>

typedef UIImage*(^getTile)(CGFloat scale, int row, int col);

@interface TilingView : UIView {
    getTile getTile;
}

- (id)initWithSize:(CGSize)size levelsOfDetail:(NSInteger)levelsOfDetail tileSize:(CGSize)tileSize;
@property (nonatomic,copy) getTile getTile;

@end

TilingView.m

#import "TilingView.h"
#import <QuartzCore/CATiledLayer.h>

@implementation TilingView

@synthesize getTile;

+ (Class)layerClass {
	return [CATiledLayer class];
}

- (id)initWithSize:(CGSize)size levelsOfDetail:(NSInteger)levelsOfDetail tileSize:(CGSize)tileSize {
    self = [super initWithFrame:CGRectMake(0, 0, size.width, size.height)];
    if (self) {
        CATiledLayer *tiledLayer = (CATiledLayer *)[self layer];
        tiledLayer.levelsOfDetail = levelsOfDetail;
        tiledLayer.tileSize = tileSize;
    }
    return self;
}

- (void)drawRect:(CGRect)rect {
 	CGContextRef context = UIGraphicsGetCurrentContext();
    
    // get the scale from the context by getting the current transform matrix, then asking for
    // its "a" component, which is one of the two scale components. We could also ask for "d".
    // This assumes (safely) that the view is being scaled equally in both dimensions.
    CGFloat scale = CGContextGetCTM(context).a;
    
    CATiledLayer *tiledLayer = (CATiledLayer *)[self layer];
    CGSize tileSize = tiledLayer.tileSize;
    
    // Even at scales lower than 100%, we are drawing into a rect in the coordinate system of the full
    // image. One tile at 50% covers the width (in original image coordinates) of two tiles at 100%. 
    // So at 50% we need to stretch our tiles to double the width and height; at 25% we need to stretch 
    // them to quadruple the width and height; and so on.
    // (Note that this means that we are drawing very blurry images as the scale gets low. At 12.5%, 
    // our lowest scale, we are stretching about 6 small tiles to fill the entire original image area. 
    // But this is okay, because the big blurry image we're drawing here will be scaled way down before 
    // it is displayed.)
    tileSize.width /= scale;
    tileSize.height /= scale;
    
    // calculate the rows and columns of tiles that intersect the rect we have been asked to draw
    int firstCol = floorf(CGRectGetMinX(rect) / tileSize.width);
    int lastCol = floorf((CGRectGetMaxX(rect)-1) / tileSize.width);
    int firstRow = floorf(CGRectGetMinY(rect) / tileSize.height);
    int lastRow = floorf((CGRectGetMaxY(rect)-1) / tileSize.height);

    for (int row = firstRow; row <= lastRow; row++) {
        for (int col = firstCol; col <= lastCol; col++) {
            UIImage *tile = self.getTile(1/scale,row,col);
            CGRect tileRect = CGRectMake(tileSize.width * col, tileSize.height * row,
                                         tileSize.width, tileSize.height);

            // if the tile would stick outside of our bounds, we need to truncate it so as to avoid
            // stretching out the partial tiles at the right and bottom edges
            tileRect = CGRectIntersection(self.bounds, tileRect);

            [tile drawInRect:tileRect];
        }
    }
}

@end