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How to measure touch strength using 3D Touch

Paul Hudson       @twostraws

You can read a user's 3D Touch strength using the force property of a UITouch, which is best used when compared against the touch.maximumPossibleForce. For example, you can divide one into the other to see how much relative strength is applied, or do a straight comparison to check to see whether the user is pressing as hard as possible.

Before you try to make use of 3D Touch, make sure it's available by checking the forceTouchCapability of your current trait collection. Here's an example touchesMoved() implementation that checks whether 3D Touch is available and the user is pressing hard:

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    super.touchesMoved(touches, with: event)

    if let touch = touches.first {
        if view.traitCollection.forceTouchCapability == .available {
            if touch.force == touch.maximumPossibleForce {
                // user pressed hard – do something!
            }
        }
    }
}

Available from iOS 7.0 – see Hacking with Swift tutorial 37

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