Center zooming view in scroll view -layoutSubview.

center zooming view.m

- (void)layoutSubviews {
    [super layoutSubviews];
    // center the image as it becomes smaller than the size of the screen
    CGSize boundsSize = self.bounds.size;
    CGRect frameToCenter = self.imageView.frame;
    // center horizontally
    if (frameToCenter.size.width < boundsSize.width) {
        frameToCenter.origin.x = (boundsSize.width - frameToCenter.size.width) / 2;
    } else {
        frameToCenter.origin.x = 0;
    }
    
    // center vertically
    if (frameToCenter.size.height < boundsSize.height) {
        frameToCenter.origin.y = (boundsSize.height - frameToCenter.size.height) / 2;
    } else {
        frameToCenter.origin.y = 0;
    }
    
    self.imageView.frame = frameToCenter;
  // to handle the interaction between CATiledLayer and high resolution screens, we need to manually set the
	// tiling view's contentScaleFactor to 1.0. (If we omitted this, it would be 2.0 on high resolution screens,
	// which would cause the CATiledLayer to ask us for tiles of the wrong scales.)
	self.imageView.contentScaleFactor = 1.0;
}