Add preferredSize as getter to ASLayoutElementStyle (#2427)

AsyncDisplayKit/Layout/ASLayoutElement.h

@@ -250,10 +250,11 @@ extern NSString * const ASLayoutElementStyleLayoutPositionProperty;
  * @discussion This method is optional, but one of either preferredSize or preferredLayoutSize is required
  * for nodes that either have no intrinsic content size or 
  * should be laid out at a different size than its intrinsic content size. For example, this property could be 
- * set on an ASImageNode to display at a size different from the underlying image size. 
+ * set on an ASImageNode to display at a size different from the underlying image size.
+ *
+ * @warning Calling the getter when the size's width or height are relative will cause an assert.
  */
 @property (nonatomic, assign) CGSize preferredSize;
-- (CGSize)preferredSize UNAVAILABLE_ATTRIBUTE;
 
  /**
  * @abstract An optional property that provides a minimum size bound for a layout element. If provided, this restriction will 

AsyncDisplayKit/Layout/ASLayoutElement.mm

@@ -254,6 +254,22 @@ do {\
   ASLayoutElementStyleCallDelegate(ASLayoutElementStyleHeightProperty);
 }
 
+- (CGSize)preferredSize
+{
+  ASDN::MutexLocker l(__instanceLock__);
+  if (_size.width.unit != ASDimensionUnitPoints) {
+    NSCAssert(NO, @"Cannot get preferredSize of element with fractional width. Width: %@.", NSStringFromASDimension(_size.width));
+    return CGSizeZero;
+  }
+  
+  if (_size.height.unit != ASDimensionUnitPoints) {
+    NSCAssert(NO, @"Cannot get preferredSize of element with fractional height. Height: %@.", NSStringFromASDimension(_size.height));
+    return CGSizeZero;
+  }
+  
+  return CGSizeMake(_size.width.value, _size.height.value);
+}
+
 - (void)setMinSize:(CGSize)minSize
 {
   ASDN::MutexLocker l(__instanceLock__);

AsyncDisplayKitTests/ASLayoutElementStyleTests.m

@@ -8,7 +8,7 @@
 //  of patent rights can be found in the PATENTS file in the same directory.
 //
 
-#import <XCTest/XCTest.h>
+#import "ASXCTExtensions.h"
 #import "ASLayoutElement.h"
 
 #pragma mark - ASLayoutElementStyleTestsDelegate
@@ -61,6 +61,7 @@
   CGSize size = CGSizeMake(100, 100);
   
   style.preferredSize = size;
+  ASXCTAssertEqualSizes(style.preferredSize, size);
   XCTAssertTrue(ASDimensionEqualToDimension(style.width, ASDimensionMakeWithPoints(size.width)));
   XCTAssertTrue(ASDimensionEqualToDimension(style.height, ASDimensionMakeWithPoints(size.height)));
   
@@ -73,6 +74,19 @@
   XCTAssertTrue(ASDimensionEqualToDimension(style.maxHeight, ASDimensionMakeWithPoints(size.height)));
 }
 
+- (void)testReadingInvalidSizeForPreferredSize
+{
+  ASLayoutElementStyle *style = [ASLayoutElementStyle new];
+  
+  XCTAssertThrows(style.preferredSize);
+  
+  style.width = ASDimensionMake(ASDimensionUnitFraction, 0.5);
+  XCTAssertThrows(style.preferredSize);
+  
+  style.preferredSize = CGSizeMake(100, 100);
+  XCTAssertNoThrow(style.preferredSize);
+}
+
 - (void)testSettingSizeViaLayoutSize
 {
   ASLayoutElementStyle *style = [ASLayoutElementStyle new];