[ASCornerLayoutSpec] New layout spec class for declarative corner element layout. (#657)

* Add new layout spec class with snapshot testing. Update examples and CHANGELOG.md

* Code review updates.

* Open curly bracket in a new line.

examples/LayoutSpecExamples-Swift/Sample/LayoutExampleNode.swift

@@ -143,3 +143,140 @@ class FlexibleSeparatorSurroundingContent : LayoutExampleNode {
     return "try rotating me!"
   }
 }
+
+class CornerLayoutSample : PhotoWithOutsetIconOverlay {
+  let photoNode1 = ASImageNode()
+  let photoNode2 = ASImageNode()
+  let dotNode = ASImageNode()
+  let badgeTextNode = ASTextNode()
+  let badgeImageNode = ASImageNode()
+  
+  struct ImageSize {
+    static let avatar = CGSize(width: 100, height: 100)
+    static let icon = CGSize(width: 26, height: 26)
+  }
+  
+  struct ImageColor {
+    static let avatar = UIColor.lightGray
+    static let icon = UIColor.red
+  }
+  
+  required init() {
+    super.init()
+    
+    let avatarImage = UIImage.draw(size: ImageSize.avatar, fillColor: ImageColor.avatar) { () -> UIBezierPath in
+      return UIBezierPath(roundedRect: CGRect(origin: CGPoint.zero, size: ImageSize.avatar), cornerRadius: ImageSize.avatar.width / 20)
+    }
+    
+    let iconImage = UIImage.draw(size: ImageSize.icon, fillColor: ImageColor.icon) { () -> UIBezierPath in
+      return UIBezierPath(ovalIn: CGRect(origin: CGPoint.zero, size: ImageSize.icon))
+    }
+    
+    photoNode1.image = avatarImage
+    photoNode2.image = avatarImage
+    dotNode.image = iconImage
+    
+    badgeTextNode.attributedText = NSAttributedString.attributedString(string: " 999+ ", fontSize: 20, color: .white)
+    
+    badgeImageNode.image = UIImage.as_resizableRoundedImage(withCornerRadius: 12, cornerColor: .clear, fill: .red)
+  }
+  
+  override func layoutSpecThatFits(_ constrainedSize: ASSizeRange) -> ASLayoutSpec {
+    photoNode.style.preferredSize = ImageSize.avatar
+    iconNode.style.preferredSize = ImageSize.icon
+    
+    let badgeSpec = ASBackgroundLayoutSpec(child: badgeTextNode, background: badgeImageNode)
+    let cornerSpec1 = ASCornerLayoutSpec(child: photoNode1, corner: dotNode, location: .topRight)
+    let cornerSpec2 = ASCornerLayoutSpec(child: photoNode2, corner: badgeSpec, location: .topRight)
+    let cornerSpec3 = ASCornerLayoutSpec(child: photoNode, corner: iconNode, location: .topRight)
+    
+    cornerSpec1.offset = CGPoint(x: -3, y: 3)
+
+    let stackSpec = ASStackLayoutSpec.vertical()
+    stackSpec.spacing = 40
+    stackSpec.children = [cornerSpec1, cornerSpec2, cornerSpec3]
+    
+    return stackSpec
+  }
+  
+  override class func title() -> String {
+    return "Declarative way for Corner image Layout"
+  }
+  
+  override class func descriptionTitle() -> String? {
+    return nil
+  }
+}
+
+class UserProfileSample : LayoutExampleNode {
+  
+  let badgeNode = ASImageNode()
+  let avatarNode = ASImageNode()
+  let usernameNode = ASTextNode()
+  let subtitleNode = ASTextNode()
+  
+  struct ImageSize {
+    static let avatar = CGSize(width: 44, height: 44)
+    static let badge = CGSize(width: 15, height: 15)
+  }
+  
+  struct ImageColor {
+    static let avatar = UIColor.lightGray
+    static let badge = UIColor.red
+  }
+  
+  required init() {
+    super.init()
+    
+    avatarNode.image = UIImage.draw(size: ImageSize.avatar, fillColor: ImageColor.avatar) { () -> UIBezierPath in
+      return UIBezierPath(ovalIn: CGRect(origin: CGPoint.zero, size: ImageSize.avatar))
+    }
+    
+    badgeNode.image = UIImage.draw(size: ImageSize.badge, fillColor: ImageColor.badge) { () -> UIBezierPath in
+      return UIBezierPath(ovalIn: CGRect(origin: CGPoint.zero, size: ImageSize.badge))
+    }
+    
+    makeSingleLine(for: usernameNode, with: "Hello world", fontSize: 17, textColor: .black)
+    makeSingleLine(for: subtitleNode, with: "This is a long long subtitle, with a long long appended string.", fontSize: 14, textColor: .lightGray)
+  }
+  
+  private func makeSingleLine(for node: ASTextNode, with text: String, fontSize: CGFloat, textColor: UIColor) {
+    node.attributedText = NSAttributedString.attributedString(string: text, fontSize: fontSize, color: textColor)
+    node.maximumNumberOfLines = 1
+  }
+  
+  override func layoutSpecThatFits(_ constrainedSize: ASSizeRange) -> ASLayoutSpec {
+    let avatarBox = ASCornerLayoutSpec(child: avatarNode, corner: badgeNode, location: .bottomRight)
+    avatarBox.offset = CGPoint(x: -6, y: -6)
+    
+    let textBox = ASStackLayoutSpec.vertical()
+    textBox.justifyContent = .spaceAround
+    textBox.children = [usernameNode, subtitleNode]
+    
+    let profileBox = ASStackLayoutSpec.horizontal()
+    profileBox.spacing = 10
+    profileBox.children = [avatarBox, textBox]
+    
+    // Apply text truncation
+    let elems: [ASLayoutElement] = [usernameNode, subtitleNode, textBox, profileBox]
+    for elem in elems {
+      elem.style.flexShrink = 1
+    }
+    
+    let insetBox = ASInsetLayoutSpec(
+      insets: UIEdgeInsets(top: 120, left: 20, bottom: CGFloat.infinity, right: 20),
+      child: profileBox
+    )
+    
+    return insetBox
+  }
+  
+  override class func title() -> String {
+    return "Common user profile layout."
+  }
+  
+  override class func descriptionTitle() -> String? {
+    return "For corner image layout and text truncation."
+  }
+  
+}