Do not reuse NSStringDrawingContexts (facebookarchive#2537)

AsyncDisplayKit/TextKit/ASTextKitRenderer.mm

@@ -35,20 +35,11 @@
  return truncationCharacterSet;
 }
 
-@interface ASTextKitRenderer()
-/**
- * This object is lazily created. It is provided to the NSAttributedString
- * drawing methods used by the fast-paths in the size calculation and drawing
- * instance methods.
- */
-@property (nonatomic, strong, readonly) NSStringDrawingContext *stringDrawingContext;
-@end
-
 @implementation ASTextKitRenderer {
  CGSize _calculatedSize;
  BOOL _sizeIsCalculated;
 }
-@synthesize attributes = _attributes, context = _context, shadower = _shadower, truncater = _truncater, fontSizeAdjuster = _fontSizeAdjuster, stringDrawingContext = _stringDrawingContext;
+@synthesize attributes = _attributes, context = _context, shadower = _shadower, truncater = _truncater, fontSizeAdjuster = _fontSizeAdjuster;
 
 #pragma mark - Initialization
 
@@ -118,15 +109,18 @@ - (ASTextKitContext *)context
 
 - (NSStringDrawingContext *)stringDrawingContext
 {
- if (_stringDrawingContext == nil) {
- _stringDrawingContext = [[NSStringDrawingContext alloc] init];
-
- if (isinf(_constrainedSize.width) == NO && _attributes.maximumNumberOfLines > 0) {
- ASDisplayNodeAssert(_attributes.maximumNumberOfLines != 1, @"Max line count 1 is not supported in fast-path.");
- [_stringDrawingContext setValue:@(_attributes.maximumNumberOfLines) forKey:@"maximumNumberOfLines"];
- }
+ // String drawing contexts are not safe to use from more than one thread.
+ // i.e. if they are created on one thread, it is unsafe to use them on another.
+ // Therefore we always need to create a new one.
+ //
+ // http://web.archive.org/web/20140703122636/https://developer.apple.com/library/ios/documentation/uikit/reference/NSAttributedString_UIKit_Additions/Reference/Reference.html
+ NSStringDrawingContext *stringDrawingContext = [[NSStringDrawingContext alloc] init];
+
+ if (isinf(_constrainedSize.width) == NO && _attributes.maximumNumberOfLines > 0) {
+ ASDisplayNodeAssert(_attributes.maximumNumberOfLines != 1, @"Max line count 1 is not supported in fast-path.");
+ [stringDrawingContext setValue:@(_attributes.maximumNumberOfLines) forKey:@"maximumNumberOfLines"];
  }
- return _stringDrawingContext;
+ return stringDrawingContext;
 }
 
 #pragma mark - Sizing