Let ASLayout determine if a node should measure

Author: Levi McCallum

AsyncDisplayKit/ASDisplayNode.mm

@@ -650,10 +650,15 @@ - (BOOL)shouldMeasureWithSizeRange:(ASSizeRange)constrainedSize
     }
   }
 
-  // only calculate the size if
-  //  - we haven't already
-  //  - the constrained size range is different
-  return (!_flags.isMeasured || !ASSizeRangeEqualToSizeRange(constrainedSize, _layout.constrainedSizeRange));
+  // Only generate a new layout if:
+  // - The current layout is dirty
+  // - The passed constrained size is different than the layout's constrained size
+  return ([self _dirtyLayout] || !ASSizeRangeEqualToSizeRange(constrainedSize, _layout.constrainedSizeRange));
+}
+
+- (BOOL)_dirtyLayout
+{
+  return _layout == nil || _layout.isDirty;
 }
 
 #pragma mark - Layout Transition
@@ -1019,7 +1024,7 @@ - (void)__setNeedsLayout
   ASDisplayNodeAssertThreadAffinity(self);
   ASDN::MutexLocker l(_propertyLock);
 
-  if (!_flags.isMeasured) {
+  if ([self _dirtyLayout]) {
     return;
   }
 
@@ -1095,7 +1100,7 @@ - (void)measureNodeWithBoundsIfNecessary:(CGRect)bounds
   // Normally measure will be called before layout occurs. If this doesn't happen, nothing is going to call it at all.
   // We simply call measureWithSizeRange: using a size range equal to whatever bounds were provided to that element or
   // try to measure the node with the largest size as possible
-  if (self.supernode == nil && !self.supportsRangeManagedInterfaceState && !_flags.isMeasured) {
+  if (self.supernode == nil && !self.supportsRangeManagedInterfaceState && [self _dirtyLayout] == NO) {
     if (CGRectEqualToRect(bounds, CGRectZero)) {
       LOG(@"Warning: No size given for node before node was trying to layout itself: %@. Please provide a frame for the node.", self);
     } else {
@@ -1108,7 +1113,7 @@ - (void)layout
 {
   ASDisplayNodeAssertMainThread();
 
-  if (!_flags.isMeasured) {
+  if ([self _dirtyLayout]) {
     return;
   }
 
@@ -2058,8 +2063,10 @@ - (UIImage *)placeholderImage
 - (void)invalidateCalculatedLayout
 {
   ASDN::MutexLocker l(_propertyLock);
-  // This will cause -measureWithSizeRange: to actually compute the size instead of returning the previously cached size
-  _flags.isMeasured = NO;
+
+  // This will cause the next call to -measureWithSizeRange: to actually compute a new layout
+  // instead of returning the current layout
+  _layout.dirty = YES;
 }
 
 - (void)__didLoad
@@ -2395,17 +2402,14 @@ - (void)applyPendingLayoutContext
   }
 }
 
-- (void)applyLayout:(ASLayout *)layout
-      layoutContext:(ASLayoutTransition *)layoutContext
+- (void)applyLayout:(ASLayout *)layout layoutContext:(ASLayoutTransition *)layoutContext
 {
   ASDN::MutexLocker l(_propertyLock);
   _layout = layout;
 
   ASDisplayNodeAssertTrue(layout.layoutableObject == self);
   ASDisplayNodeAssertTrue(layout.size.width >= 0.0);
   ASDisplayNodeAssertTrue(layout.size.height >= 0.0);
-
-  _flags.isMeasured = YES;
 
   if (self.usesImplicitHierarchyManagement && layoutContext != nil) {
     [layoutContext applySubnodeInsertions];

AsyncDisplayKit/Layout/ASLayout.h

@@ -21,7 +21,9 @@ extern CGPoint const CGPointNull;
 
 extern BOOL CGPointIsNull(CGPoint point);
 
-/** Represents a computed immutable layout tree. */
+/**
+ * A node in the layout tree that represents the size and position of the object that created it (ASLayoutable).
+ */
 @interface ASLayout : NSObject
 
 /**
@@ -56,6 +58,11 @@ extern BOOL CGPointIsNull(CGPoint point);
  */
 @property (nonatomic, readonly) NSArray<ASLayout *> *immediateSublayouts;
 
+/**
+ * Mark the layout dirty for future regeneration.
+ */
+@property (nonatomic, getter=isDirty) BOOL dirty;
+
 /**
  * A boolean describing if the current layout has been flattened.
  */

AsyncDisplayKit/Layout/ASLayout.mm

@@ -50,6 +50,7 @@ + (instancetype)layoutWithLayoutableObject:(id<ASLayoutable>)layoutableObject
     }
     l->_constrainedSizeRange = sizeRange;
     l->_size = size;
+    l->_dirty = NO;
 
     if (CGPointIsNull(position) == NO) {
       l->_position = CGPointMake(ASCeilPixelValue(position.x), ASCeilPixelValue(position.y));

AsyncDisplayKit/Private/ASDisplayNodeInternal.h

@@ -82,7 +82,6 @@ FOUNDATION_EXPORT NSString * const ASRenderingEngineDidDisplayNodesScheduledBefo
     unsigned implementsDrawParameters:1;
 
     // internal state
-    unsigned isMeasured:1;
     unsigned isEnteringHierarchy:1;
     unsigned isExitingHierarchy:1;
     unsigned isInHierarchy:1;