[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.mm

@@ -0,0 +1,117 @@
+//
+//  ASRelativeLayoutSpec.mm
+//  AsyncDisplayKit
+//
+//  Created by Samuel Stow on 12/31/15.
+//
+
+#import "ASRelativeLayoutSpec.h"
+
+#import "ASInternalHelpers.h"
+#import "ASLayout.h"
+
+@implementation ASRelativeLayoutSpec
+
+- (instancetype)initWithHorizontalPosition:(ASRelativeLayoutSpecPosition)horizontalPosition verticalPosition:(ASRelativeLayoutSpecPosition)verticalPosition sizingOption:(ASRelativeLayoutSpecSizingOption)sizingOption child:(id<ASLayoutable>)child
+{
+  if (!(self = [super init])) {
+    return nil;
+  }
+  ASDisplayNodeAssertNotNil(child, @"Child cannot be nil");
+  _horizontalPosition = horizontalPosition;
+  _verticalPosition = verticalPosition;
+  _sizingOption = sizingOption;
+  [self setChild:child];
+  return self;
+}
+
++ (instancetype)relativePositionLayoutSpecWithHorizontalPosition:(ASRelativeLayoutSpecPosition)horizontalPosition verticalPosition:(ASRelativeLayoutSpecPosition)verticalPosition sizingOption:(ASRelativeLayoutSpecSizingOption)sizingOption child:(id<ASLayoutable>)child
+{
+  return [[self alloc] initWithHorizontalPosition:horizontalPosition verticalPosition:verticalPosition sizingOption:sizingOption child:child];
+}
+
+- (void)setHorizontalPosition:(ASRelativeLayoutSpecPosition)horizontalPosition
+{
+  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
+  _horizontalPosition = horizontalPosition;
+}
+
+- (void)setVerticalPosition:(ASRelativeLayoutSpecPosition)verticalPosition {
+  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
+  _verticalPosition = verticalPosition;
+}
+
+- (void)setSizingOption:(ASRelativeLayoutSpecSizingOption)sizingOption
+{
+  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
+  _sizingOption = sizingOption;
+}
+
+- (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
+{
+  CGSize size = {
+    constrainedSize.max.width,
+    constrainedSize.max.height
+  };
+  
+  BOOL reduceWidth = (_horizontalPosition & ASRelativeLayoutSpecPositionCenter) != 0 ||
+  (_horizontalPosition & ASRelativeLayoutSpecPositionEnd) != 0;
+  
+  BOOL reduceHeight = (_verticalPosition & ASRelativeLayoutSpecPositionCenter) != 0 ||
+  (_verticalPosition & ASRelativeLayoutSpecPositionEnd) != 0;
+  
+  // Layout the child
+  const CGSize minChildSize = {
+    reduceWidth ? 0 : constrainedSize.min.width,
+    reduceHeight ? 0 : constrainedSize.min.height,
+  };
+  ASLayout *sublayout = [self.child measureWithSizeRange:ASSizeRangeMake(minChildSize, constrainedSize.max)];
+  
+  // If we have an undetermined height or width, use the child size to define the layout
+  // size
+  size = ASSizeRangeClamp(constrainedSize, {
+    isfinite(size.width) == NO ? sublayout.size.width : size.width,
+    isfinite(size.height) == NO ? sublayout.size.height : size.height
+  });
+  
+  // If minimum size options are set, attempt to shrink the size to the size of the child
+  size = ASSizeRangeClamp(constrainedSize, {
+    MIN(size.width, (_sizingOption & ASRelativeLayoutSpecSizingOptionMinimumWidth) != 0 ? sublayout.size.width : size.width),
+    MIN(size.height, (_sizingOption & ASRelativeLayoutSpecSizingOptionMinimumHeight) != 0 ? sublayout.size.height : size.height)
+  });
+  
+  // Compute the position for the child on each axis according to layout parameters
+  CGFloat xPosition = [self proportionOfAxisForAxisPosition:_horizontalPosition];
+  CGFloat yPosition = [self proportionOfAxisForAxisPosition:_verticalPosition];
+  
+  sublayout.position = {
+    ASRoundPixelValue((size.width - sublayout.size.width) * xPosition),
+    ASRoundPixelValue((size.height - sublayout.size.height) * yPosition)
+  };
+  
+  return [ASLayout layoutWithLayoutableObject:self size:size sublayouts:@[sublayout]];
+}
+
+- (void)setChildren:(NSArray *)children
+{
+  ASDisplayNodeAssert(NO, @"not supported by this layout spec");
+}
+
+- (NSArray *)children
+{
+  ASDisplayNodeAssert(NO, @"not supported by this layout spec");
+  return nil;
+}
+
+- (CGFloat)proportionOfAxisForAxisPosition:(ASRelativeLayoutSpecPosition)position
+{
+  if ((position & ASRelativeLayoutSpecPositionCenter) != 0) {
+    return 0.5f;
+  } else if ((position & ASRelativeLayoutSpecPositionEnd) != 0) {
+    return 1.0f;
+  } else {
+    return 0.0f;
+  }
+}
+
+@end