How to setup Pinch to Zoom for an image in RubyMotion

In this post we will see how to build "pinch to zoom" functionality to zoom in an image in RubyMotion.

First let's add a UIViewController that is initialized with an image.

class ImageViewController < UIViewController
  def initWithImage(image)
    @image = image
  end
end

UIScrollView and UIImageView

Now, we will add a UIScrollView with frame size set to full screen size and some other properties as listed below.

scrollView = UIScrollView.alloc.initWithFrame(UIScreen.mainScreen.bounds)
scrollView.scrollEnabled = false
scrollView.clipsToBounds = true
scrollView.contentSize = @image.size
scrollView.minimumZoomScale = 1.0
scrollView.maximumZoomScale = 4.0
scrollView.zoomScale = 0.3

Create a new UIImageView and add it to the scrollView created above.

imageView = UIImageView.alloc.initWithImage(@image)
imageView.contentMode = UIViewContentModeScaleAspectFit
imageView.userInteractionEnabled = true
imageView.frame = scrollView.bounds

We are setting the image view's content mode to UIViewContentModeScaleAspectFit. Content mode can be set to either UIViewContentModeScaleToFill, UIViewContentModeAspectFill or UIViewContentModeScaleAspectFit depending on what suits your app. By default, contentMode property for most views is set to UIViewContentModeScaleToFill, which causes the view’s contents to be scaled to fit the new frame size. This Apple doc explains this behavior.

We need to add the above imageView as a subview to our scrollView.

scrollView.addSubview(imageView)
self.view.addSubview(@scrollView)

This is how our controller looks with all the above additions.

class ImageViewController < UIViewController

  def initWithImage(image)
    @image = image
    scrollView = UIScrollView.alloc.initWithFrame(UIScreen.mainScreen.bounds)
    scrollView.scrollEnabled = false
    scrollView.clipsToBounds = true
    scrollView.contentSize = @image.size
    scrollView.minimumZoomScale = 1.0
    scrollView.maximumZoomScale = 4.0
    scrollView.zoomScale = 0.3
    scrollView.delegate = self

    imageView = UIImageView.alloc.initWithImage(@image)
    imageView.contentMode = UIViewContentModeScaleToFill
    imageView.userInteractionEnabled = true
    imageView.frame = scrollView.bounds
    init
  end

end

ScrollView delegate

We must set a delegate for our scroll view to support zooming. The delegate object must conform to the UIScrollViewDelegate protocol. This is the reason we are setting scrollView.delegate = self above. The delegate class must implement viewForZoomingInScrollView and scrollViewDidZoom methods.

def viewForZoomingInScrollView(scrollView)
  scrollView.subviews.first
end

def scrollViewDidZoom(scrollView)
  if scrollView.zoomScale != 1.0
    scrollView.scrollEnabled = true
  else
    scrollView.scrollEnabled = false
  end
end

These two methods added above allow the scrollView to support pinch to zoom.

Supporting orientation changes

There is one more thing to do if we want to support orientations changes. We need to add the following methods:

def shouldAutorotateToInterfaceOrientation(*)
  true
end

def viewDidLayoutSubviews
  @scrollView.frame = self.view.bounds
end

We have to set the scrollView's frame to view bounds in viewDidLayoutSubviews so that the scrollView frame is resized when the device orientation changes.

That's it. With all those changes now our app supports orientation change and now we are able to pinch and zoom images.