add setCustomSpacing support

Classes/Extensions/View+Spacing.swift

@@ -0,0 +1,48 @@
+#if os(macOS)
+import AppKit
+#else
+import UIKit
+#endif
+
+private var spacingAssociationKey: UInt8 = 0
+
+extension BaseView {
+    @available(iOS 11.0, *)
+    var spacing: State<CGFloat>? {
+        get {
+            return objc_getAssociatedObject(self, &spacingAssociationKey) as? State<CGFloat>
+        }
+        set(newValue) {
+            objc_setAssociatedObject(self, &spacingAssociationKey, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
+        }
+    }
+
+    @available(iOS 11.0, *)
+    @discardableResult
+    public func customSpacing(_ spacing: CGFloat) -> Self {
+        if self.spacing == nil {
+            self.spacing = State.init(wrappedValue: spacing)
+        }
+        else {
+            self.spacing?.wrappedValue = spacing
+        }
+
+        if let superView = self.superview as? _STV {
+            superView.setCustomSpacing(spacing, after: self)
+        }
+        return self
+    }
+
+    @available(iOS 11.0, *)
+    @discardableResult
+    public func customSpacing(_ spacing: State<CGFloat>) -> Self {
+        self.spacing = spacing
+        spacing.listen { [weak self] new in
+            guard let self = self else { return }
+            if let superView = self.superview as? _STV {
+                superView.setCustomSpacing(new, after: self)
+            }
+        }
+        return self
+    }
+}

Classes/Views/Universal/StackView.swift

@@ -182,7 +182,16 @@ open class _StackView: _STV, AnyDeclarativeProtocol, DeclarativeProtocolInternal
         self.spacing = state.wrappedValue
         return self
     }
-
+
+    open override func addArrangedSubview(_ view: UIView) {
+        super.addArrangedSubview(view)
+        if #available(iOS 11.0, *) {
+            if let spacing = view.spacing {
+                self.setCustomSpacing(spacing.wrappedValue, after: view)
+            }
+        }
+    }
+
     func add(item: BodyBuilderItemable) {
         switch item.bodyBuilderItem {
             case .single(let view):