ForceZoom: Popup Image Detail View Using 3D Touch Peek

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ForceZoom: Popup Image Detail View Using 3D Touch Peek

More experiments showing how useful 3D Touch can be for your apps

· Mobile Zone ·
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My experiments with 3D Touch on the iPhone 6s continue with ForceZoom, an extended UIImageView that displays a 1:1 peek detail view of a touched point on a large image.

The demo (above) contains three large images, forest (1600 x 1200), pharmacy (4535 x 1823) and petronas (3264 x 4896). An initial touch on the image displays the preview frame around the touch location and a deep press pops up a square preview of the image at that point at full resolution. The higher the resolution, the smaller the preview frame will be.

Installation & Implementation

ForceZoom consists of two files that need to be copied into a host application project:

To implement a ForceZoom component in an application, instantiate with a default image and view controller and add to a view:

    class ViewController: UIViewController
        var imageView: ForceZoomWidget!

        override func viewDidLoad()

            imageView = ForceZoomWidget(image: UIImage(named: "forest.jpg")!,
                viewController: self)


Displaying Preview Frame

Since the popup preview will be the largest square that can fit on the screen:

    var peekPreviewSize: CGFloat
        return min(UIScreen.mainScreen().bounds.size.width,

The white preview box, which is a CAShapeLayer, needs to be that size at the same scale as the image has been scaled on the screen. The maths to do this is in the displayPreviewFrame() method which is invoked by touchesBegan:

    let previewFrameSize = peekPreviewSize * imageScale

Where imageScale is simply the component's width or height divided by the image's width or height:

   var imageScale: CGFloat
        return min(bounds.size.width / image!.size.width, bounds.size.height / image!.size.height)


Launching the Peek Preview

When previewingContext(viewControllerForLocation) is invoked in response to the user's deep press, ForceZoomneeds to pass to the previewing component the normalised position of the touch. This is because I use the pop up image view's layer's contentsRect to position and clip the full resolution image and contentsRect uses normalised image coordinates.

There are a few steps in previewingContext(viewControllerForLocation) to do this. First off, I calculate the size of the preview frame as a normalised value. This will be used as an offset from the touch origin to form the clip rectangle's origin:

    let offset = ((peekPreviewSize * imageScale) / (imageWidth * imageScale)) / 2

Next, I calculate the distance between the edge of the component and the edge of the image it contains:

    let leftBorder = (bounds.width - (imageWidth * imageScale)) / 2

Then, with the location of the touch point and these two new values, I can create the normalised x origin of the clip rectangle:

    let normalisedXPosition = ((location.x - leftBorder) / (imageWidth * imageScale)) - offset

I do the same for and with those two normalised values create a preview point:

    let topBorder = (bounds.height - (imageHeight * imageScale)) / 2
    let normalisedYPosition = ((location.y - topBorder) / (imageHeight * imageScale)) - offset

    let normalisedPreviewPoint = CGPoint(x: normalisedXPosition, y: normalisedYPosition)

...which is passed to my ForceZoomPreview:

    let peek = ForceZoomPreview(normalisedPreviewPoint: normalisedPreviewPoint, image: image!)

The Peek Preview

The previewing component now has very little work to do. It's passed the normalised origin in its constructor (above), so all it needs to do is use those values to set the contentsRect of an image view:

    imageView.layer.contentsRect = CGRect(
        x: max(min(normalisedPreviewPoint.x, 1), 0),
        y: max(min(normalisedPreviewPoint.y, 1), 0),
        width: view.frame.width / image.size.width,

        height: view.frame.height / image.size.height)

Source Code

As always, the full source code for this project is available in my GitHub repository here. Enjoy!

swift ,ios

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