fix retain circle, add .distribution support

Example/StackLayoutExample/UI/Examples/LayoutModes/LayoutModesView.swift

@@ -33,6 +33,7 @@ class LayoutModesView: BaseView {
         super.init()
 
         stackView.direction(.column).define { (stack) in
+            stack.spacing(4) // only for .distribution
             stack.addItem(BasicView(text: "View 1"))
             stack.addItem(BasicView(text: "View 2"))
             stack.addItem(BasicView(text: "View 3"))
@@ -114,7 +115,8 @@ class LayoutModesView: BaseView {
         case .end:          stackView.justifyContent(.spaceBetween)
         case .spaceBetween: stackView.justifyContent(.spaceAround)
         case .spaceAround:  stackView.justifyContent(.spaceEvenly)
-        case .spaceEvenly:  stackView.justifyContent(.start)
+        case .spaceEvenly:  stackView.justifyContent(.distribution)
+        case .distribution: stackView.justifyContent(.start)
         }
 
         applyChange()
@@ -152,6 +154,7 @@ class LayoutModesView: BaseView {
         case .spaceBetween: justifyLabel.text = ".spaceBetween"
         case .spaceAround:  justifyLabel.text = ".spaceAround"
         case .spaceEvenly:  justifyLabel.text = ".spaceEvenly"
+        case .distribution: justifyLabel.text = ".distribution"
         }
 
         switch stackView.getAlignItems() {
@@ -161,6 +164,7 @@ class LayoutModesView: BaseView {
         case .end:     alignItemsLabel.text = ".end"
         }
 
+        justifyLabel.sizeToFit()
         buttonsStackView.markDirty()
     }
 }

Example/StackLayoutExample/UI/Examples/UnitTests/UnitTestsView.swift

@@ -300,6 +300,7 @@ class UnitTestsView: BaseView {
         case .spaceBetween: stackView.justifyContent(.spaceAround)
         case .spaceAround:  stackView.justifyContent(.spaceEvenly)
         case .spaceEvenly:  stackView.justifyContent(.start)
+        case .distribution: stackView.justifyContent(.distribution)
         }
 
         setNeedsLayout()

Sources/Impl/Container.swift

@@ -21,7 +21,7 @@
 import UIKit
 
 class Container {
-    fileprivate let stackView: StackView
+    unowned fileprivate let stackView: StackView
     var width: CGFloat?
     var height: CGFloat?
     var innerWidth: CGFloat?
@@ -75,6 +75,8 @@ class Container {
 
     var mainAxisTotalItemsLength: CGFloat = 0
 
+    var mainAxisTotalItemsMargin: CGFloat = 0
+
     init(_ stackView: StackView) {
         self.stackView = stackView
     }
@@ -128,6 +130,7 @@ class Container {
 
         items.forEach({ (item) in
             mainAxisTotalItemsLength += item.mainAxisStartMargin
+            mainAxisTotalItemsMargin += item.mainAxisStartMargin
 
             if direction == .column {
                 mainAxisTotalItemsLength += (item.height != nil ? item.height! : 0)
@@ -136,6 +139,7 @@ class Container {
             }
 
             mainAxisTotalItemsLength += item.mainAxisEndMargin
+            mainAxisTotalItemsMargin += item.mainAxisEndMargin
         })
     }
 

Sources/Impl/ItemInfo.swift

@@ -20,9 +20,9 @@
 import UIKit
 
 class ItemInfo {
-    var view: UIView
-    var stackItem: StackItemImpl
-    var container: Container
+    unowned let view: UIView
+    unowned let stackItem: StackItemImpl
+    unowned let container: Container
 
     private var _width: CGFloat?
     var width: CGFloat? {

Sources/Impl/StackItemImpl.swift

@@ -30,7 +30,7 @@ enum MeasureType {
 
 
 class StackItemImpl: NSObject, StackItem {
-    internal let view: UIView
+    unowned internal let view: UIView
 
     internal var width: Value?
     internal var minWidth: Value?

Sources/Impl/StackView+Layout.swift

@@ -64,6 +64,20 @@ extension StackView {
 
     private func adjustItemsSizeToContainer(container: Container) {
         guard let mainAxisInnerLength = container.mainAxisInnerLength else { return }
+
+        if self.justifyContent == .distribution {
+            let count = CGFloat(container.items.count)
+            let gap = max(count - 1, 0) * (_spacing?.value ?? 0) + container.mainAxisTotalItemsMargin
+            let len = (mainAxisInnerLength - gap) / count
+            for item in container.items {
+                if self.direction == .column {
+                    item.height = len
+                } else {
+                    item.width = len
+                }
+            }
+            return
+        }
 
         var previousLength: CGFloat?
         var lengthDiff = mainAxisInnerLength - container.mainAxisTotalItemsLength
@@ -164,6 +178,8 @@ extension StackView {
             case .spaceEvenly:
                 betweenSpacing = (containerMainAxisInnner - mainAxisTotalItemsLength) / CGFloat(container.items.count + 1)
                 startEndSpacing = betweenSpacing
+            case .distribution:
+                betweenSpacing = _spacing?.value ?? 0
             }
         }
 

Sources/StackView.swift

@@ -33,6 +33,8 @@ public class StackView: UIView {
     internal var _paddingBottom: Value?
     internal var _paddingRight: Value?
     internal var _paddingEnd: Value?
+
+    internal var _spacing: Value?
 
     public override init(frame: CGRect) {
         super.init(frame: frame)
@@ -322,6 +324,25 @@ public class StackView: UIView {
         return self
     }
 
+    //
+    // MARK: Spacing
+    //
+
+    /**
+     The `spacing` property only for justify .distribution in the StackView.
+
+     - Parameter value: Default value is 0
+     */
+    @discardableResult
+    public func spacing(_ spacing: CGFloat) -> StackView {
+        _spacing = Value(spacing)
+        return self
+    }
+
+    public func getSpacing() -> CGFloat {
+        return _spacing?.value ?? 0
+    }
+
     //
     // Layout view
     //

Sources/Types.swift

@@ -47,6 +47,8 @@ public enum SJustifyContent {
     case spaceAround
     /// Items are positioned with space before, between, and after items
     case spaceEvenly
+    /// Items are adjusted to same length in main axis, with space and margin between them
+    case distribution
 }
 
 /**