NSDictionaryOfVariableBindings for Swift

ZzViewHolder.swift

/**
 * NSDictionaryOfVariableBindings for Swift
 *
 * In Objective-C you could use macros to quickly get a Dictionary of views,
 * keyed on their names in the sourcecode. Like so:
 *
 *     UILabel      *label = [UILabel autolayoutView];
 *     NSDictionary *views = NSDictionaryOfVariableBindings(label);
 *
 * Which you could then use in, for example, VFL:
 *
 *     H:|-[label]-|
 *
 * In Swift `NSDictionaryOfVariableBindings` is not available anymore.
 *
 * But we can use reflection to accomplish something similar in Swift. It
 * happens at runtime, so it is much slower, but yet. So how does this work?
 * In custom views or VCs you usually have a bunch of instance variables for
 * the subviews you use.
 *
 *     class MyTableViewCell : UITableViewCell, ZzViewHolder { // <===
 *       let label  = UILabel()
 *       let button = UIButton()
 *
 *       override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
 *         super.init(style: style, reuseIdentifier: reuseIdentifier)
 *
 *         allViews.forEach {                                  // <===
 *           $0.translatesAutoresizingMaskIntoConstraints = false
 *           addSubview($0)
 *         }
 *
 *         NSLayoutConstraint.activate(NSLayoutConstraint.constraints(
 *               withVisualFormat: "H:|-[label]-[button]-|",
 *               options: [], metrics: nil,
 *               views: idToViews // <===
 *         ))
 *       }
 *     }
 */
public protocol ZzViewHolder {
  // just a mixin protocol
}

public extension ZzViewHolder {
  
  public var allViews : [ ZzView ] {
    return Mirror(reflecting: self).children.flatMap { $0.1 as? ZzView }
  }
  
  public var idToView : [ String : ZzView ] {
    // Unfortunately in Swift, we cannot return a trampoline because VFL API
    // expects a [:], not an arbitrary Map protocol.
    var idToView = [ String : ZzView ]()
    for ( key, value ) in Mirror(reflecting: self).children {
      guard let key = key, let view = value as? ZzView else { continue }
      idToView[key] = view
    }
    return idToView
  }
  
}

#if os(macOS)
  public typealias ZzView = NSView
#else
  public typealias ZzView = UIView
#endif