Do not reuse NSStringDrawingContexts (#2537)

Author: garrettmoon

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