How to implement Magnifier [duplicate] - objective-c

Question

I would like be able to create a movable magnifier (like the one you have when you copy and paste) in a custom view, for zooming a part of my view.
I have no idea on how to start, do you have any idea?
Thanks in advance for your help :)

Answer 1

We do this in Crosswords. In your drawRect method, mask off a circle (using a monochrome bitmap containing the 'mask' of your magnifying glass) and draw your subject view in there with a 2x scale transform. Then draw a magnifying glass image over that and you're done.
- (void) drawRect: (CGRect) rect {
CGContextRef context = UIGraphicsGetCurrentContext();
CGRect bounds = self.bounds;
CGImageRef mask = [UIImage imageNamed: #"loupeMask"].CGImage;
UIImage *glass = [UIImage imageNamed: #"loupeImage"];
CGContextSaveGState(context);
CGContextClipToMask(context, bounds, mask);
CGContextFillRect(context, bounds);
CGContextScaleCTM(context, 2.0, 2.0);
//draw your subject view here
CGContextRestoreGState(context);
[glass drawInRect: bounds];
}

Answer 2

There is a complete example over here. There is a minor error in the downloaded project but otherwise it works great and does exactly what you need.

Answer 3

I use this code in Swift 3 :
class MagnifyingGlassView: UIView {
var zoom: CGFloat = 2 {
didSet {
setNeedsDisplay()
}
}
weak var readView: UIView?
// MARK: - UIVIew
override init(frame: CGRect) {
super.init(frame: frame)
setupView()
}
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
setupView()
}
override func draw(_ rect: CGRect) {
guard let readView = readView else { return }
let magnifiedBounds = magnifyBounds(of: readView, zoom: zoom)
readView.drawHierarchy(in: magnifiedBounds, afterScreenUpdates: false)
}
// MARK: - Private
private func setupView() {
isOpaque = false
backgroundColor = UIColor.clear
}
private func magnifyBounds(of view: UIView, zoom: CGFloat) -> CGRect {
let transform = CGAffineTransform(scaleX: zoom, y: zoom)
var bounds = view.bounds.applying(transform)
bounds.center = view.bounds.center
return view.convert(bounds, to: self)
}
}
extension CGRect {
var center: CGPoint {
get {
return CGPoint(x: origin.x + width / 2, y: origin.y + height / 2)
}
set {
origin.x = newValue.x - width / 2
origin.y = newValue.y - height / 2
}
}
}
You need to call setNeedsDisplay in scrollViewDidScroll: if your read view is a scrollView.