Open Radar

// 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 need to also ask for the "d" component as it might not be precisely the same as the "a" component, even at the "same" scale.
CGFloat _scaleX = CGContextGetCTM(context).a;
CGFloat _scaleY = CGContextGetCTM(context).d;

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 /= _scaleX;
tileSize.height /= -_scaleY;

// 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 tileForScale:_scaleX row:row col: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];

        /// change this to yes to annotate
        if (NO) {
            [[UIColor whiteColor] set];
            CGContextSetLineWidth(context, 6.0 / _scaleX);
            CGContextStrokeRect(context, tileRect);
        }
    }
}

_scale = (1 / (float)((int)(1/scale))) * 1000;  //This gets scale numbers: 1000, 500, 250 etc. which is what Apple uses in PhotoScroller

//_scale = (int)(1/_scale); // this gets scale numbers 1, 2, 4, 8 etc.

 // we use "imageWithContentsOfFile:" instead of "imageNamed:" here because we don't want UIImage to cache our tiles
NSString *tileName = [NSString stringWithFormat:@"%@_%d_%d_%d", imageName, (int)_scale, col, row];
NSString *path = [[NSBundle mainBundle] pathForResource:tileName ofType:imageNameExtension];
UIImage *image = [UIImage imageWithContentsOfFile:path];
return image;