Collapse & uncollapse views

CollapseViews.swift

extension UIView {
    
    private var previousHeightConstraint: NSLayoutConstraint? {
        return constraints.filterFirst { $0.firstAttribute == .Height }
    }
    
}

/**
 Collapse a view by adding/modifying constraint height.
 
 **Warning:** It is intended to be used to collapse a view *without* subviews.
 
 - Parameter view: The view to collapse
 - Parameter animated: Indicates if the collapse should be animated.
 - Parameter animationDuration: The animationDuration of the collapse.
 */
func collapse(view: UIView, animated: Bool = true, animationDuration: NSTimeInterval = 1) {
    if let previousHeightConstraint = view.previousHeightConstraint {
        // We save the previous height value
        setAssociatedObject(view, value: previousHeightConstraint.constant, key: &hideableUIViewConstraintKey, policy: .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        previousHeightConstraint.constant = 0
    } else {
        // We create a new height constraint with constant 0
        let zeroheightConstraint = NSLayoutConstraint(item: view, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 0)
        view.addConstraint(zeroheightConstraint)
    }
    
    view.setNeedsLayout()
    
    if animated {
        UIView.animateWithDuration(animationDuration) {
            view.layoutIfNeeded()
        }
    } else {
        view.layoutIfNeeded()
    }
}

/**
 Uncollapse a view by removing/modifying constraint height.
 
 **Warning:** It is intended to be used to uncollapse a view *without* subviews.
 
 - Parameter view: The view to collapse
 - Parameter animated: Indicates if the collapse should be animated.
 - Parameter animationDuration: The animationDuration of the collapse.
 */
func uncollapse(view: UIView, animated: Bool, animationDuration: NSTimeInterval = 1) {
    view.constraints.forEach { constraint in
        // We should find a height constraint
        if constraint.firstAttribute == .Height {
            // If we have a previous height, it means that before collapsing it had another height constraint.
            if let previousHeight: CGFloat = getAssociatedObject(view, key: &hideableUIViewConstraintKey) {
                constraint.constant = previousHeight
            } else {
                // We remove the height constraint added because it didn't have a height before collapsing.
                view.removeConstraint(constraint)
            }
        }
    }

    view.setNeedsLayout()
    if animated {
        UIView.animateWithDuration(animationDuration) {
            view.layoutIfNeeded()
        }
    } else {
        view.layoutIfNeeded()
    }
}

private final class AssociatedObjectBox<T> {
    let value: T
    
    init(_ value: T) {
        self.value = value
    }
}

private func lift<T>(value: T) -> AssociatedObjectBox<T> {
    return AssociatedObjectBox(value)
}

private func setAssociatedObject<T>(object: AnyObject, value: T, key: UnsafePointer<Void>, policy: objc_AssociationPolicy) {
    if let v: AnyObject = value as? AnyObject {
        objc_setAssociatedObject(object, key, v,  policy)
    } else {
        objc_setAssociatedObject(object, key, lift(value), policy)
    }
}

private func getAssociatedObject<T>(object: AnyObject, key: UnsafePointer<Void>) -> T? {
    if let v = objc_getAssociatedObject(object, key) as? T {
        return v
    } else if let v = objc_getAssociatedObject(object, key) as? AssociatedObjectBox<T> {
        return v.value
    } else { return nil }
}

private var hideableUIViewConstraintKey: UInt8 = 0