[ASRelativeLayoutSpec] New core layout spec type: Relative Position, now powers Center as well.

This spec allows positioning a child at any 9-part box position (corners, edges, or center).

AsyncDisplayKit/Layout/ASRelativeLayoutSpec.h

@@ -0,0 +1,73 @@
+//
+//  ASRelativeLayoutSpec.h
+//  AsyncDisplayKit
+//
+//  Created by Samuel Stow on 12/31/15.
+//
+
+#import <AsyncDisplayKit/ASLayoutSpec.h>
+
+/** How the child is positioned within the spec. */
+typedef NS_OPTIONS(NSUInteger, ASRelativeLayoutSpecPosition) {
+    /** The child is positioned at point 0 relatively to the layout axis (ie left / top most) */
+    ASRelativeLayoutSpecPositionStart = 0,
+    /** The child is centered along the specified axis */
+    ASRelativeLayoutSpecPositionCenter = 1 << 0,
+    /** The child is positioned at the maximum point of the layout axis (ie right / bottom most) */
+    ASRelativeLayoutSpecPositionEnd = 1 << 1,
+};
+
+/** How much space the spec will take up. */
+typedef NS_OPTIONS(NSUInteger, ASRelativeLayoutSpecSizingOption) {
+    /** The spec will take up the maximum size possible */
+    ASRelativeLayoutSpecSizingOptionDefault,
+    /** The spec will take up the minimum size possible along the X axis */
+    ASRelativeLayoutSpecSizingOptionMinimumWidth = 1 << 0,
+    /** The spec will take up the minimum size possible along the Y axis */
+    ASRelativeLayoutSpecSizingOptionMinimumHeight = 1 << 1,
+    /** Convenience option to take up the minimum size along both the X and Y axis */
+    ASRelativeLayoutSpecSizingOptionMinimumSize = ASRelativeLayoutSpecSizingOptionMinimumWidth | ASRelativeLayoutSpecSizingOptionMinimumHeight,
+};
+
+NS_ASSUME_NONNULL_BEGIN
+
+/** Lays out a single layoutable child and positions it within the layout bounds according to vertical and horizontal positional specifiers.
+ *  Can position the child at any of the 4 corners, or the middle of any of the 4 edges, as well as the center - similar to "9-part" image areas.
+ */
+@interface ASRelativeLayoutSpec : ASLayoutSpec
+
+// You may create a spec with alloc / init, then set any non-default properties; or use a convenience initialize that accepts all properties.
+@property (nonatomic, assign) ASRelativeLayoutSpecPosition horizontalPosition;
+@property (nonatomic, assign) ASRelativeLayoutSpecPosition verticalPosition;
+@property (nonatomic, assign) ASRelativeLayoutSpecSizingOption sizingOption;
+
+/*!
+ * @discussion convenience constructor for a ASRelativeLayoutSpec
+ * @param horizontalPosition how to position the item on the horizontal (x) axis
+ * @param verticalPosition how to position the item on the vertical (y) axis
+ * @param sizingOption how much size to take up
+ * @param child the child to layout
+ * @return a configured ASRelativeLayoutSpec
+ */
++ (instancetype)relativePositionLayoutSpecWithHorizontalPosition:(ASRelativeLayoutSpecPosition)horizontalPosition
+                                                verticalPosition:(ASRelativeLayoutSpecPosition)verticalPosition
+                                                    sizingOption:(ASRelativeLayoutSpecSizingOption)sizingOption
+                                                           child:(id<ASLayoutable>)child;
+
+/*!
+ * @discussion convenience initializer for a ASRelativeLayoutSpec
+ * @param horizontalPosition how to position the item on the horizontal (x) axis
+ * @param verticalPosition how to position the item on the vertical (y) axis
+ * @param sizingOption how much size to take up
+ * @param child the child to layout
+ * @return a configured ASRelativeLayoutSpec
+ */
+- (instancetype)initWithHorizontalPosition:(ASRelativeLayoutSpecPosition)horizontalPosition
+                          verticalPosition:(ASRelativeLayoutSpecPosition)verticalPosition
+                              sizingOption:(ASRelativeLayoutSpecSizingOption)sizingOption
+                                     child:(id<ASLayoutable>)child;
+
+@end
+
+NS_ASSUME_NONNULL_END
+