[ASTextKitComponents] Make sure Main Thread Checker isn't triggered during background calculations #trivial (#612)

* Add unit test

* Make sure TextKit components can calculate size in background without upsetting Main Thread Checker
- Add a new thread-safe text view bounds.
- Temporary components stack doesn't have a text view so it can be safely deallocated off main.

* Add ASTextKitComponentsTextView

* Remove unnecessary change

* Fix minor mistake

* ASTextKitComponentsTextView has only 1 initializer

* Minor change

* Switch to atomic property

* Remove manual synthesization

Source/TextKit/ASTextKitComponents.mm

@@ -20,6 +20,37 @@
 
 #import <tgmath.h>
 
+@interface ASTextKitComponentsTextView ()
+@property (atomic, assign) CGRect threadSafeBounds;
+@end
+
+@implementation ASTextKitComponentsTextView
+
+- (instancetype)initWithFrame:(CGRect)frame textContainer:(NSTextContainer *)textContainer
+{
+  self = [super initWithFrame:frame textContainer:textContainer];
+  if (self) {
+    _threadSafeBounds = self.bounds;
+  }
+  return self;
+}
+
+- (void)setFrame:(CGRect)frame
+{
+  ASDisplayNodeAssertMainThread();
+  [super setFrame:frame];
+  self.threadSafeBounds = self.bounds;
+}
+
+- (void)setBounds:(CGRect)bounds
+{
+  ASDisplayNodeAssertMainThread();
+  [super setBounds:bounds];
+  self.threadSafeBounds = bounds;
+}
+
+@end
+
 @interface ASTextKitComponents ()
 
 // read-write redeclarations
@@ -27,9 +58,6 @@
 @property (nonatomic, strong, readwrite) NSTextContainer *textContainer;
 @property (nonatomic, strong, readwrite) NSLayoutManager *layoutManager;
 
-// Indicates whether or not this object must be deallocated on main thread. Defaults to YES.
-@property (nonatomic, assign) BOOL requiresMainThreadDeallocation;
-
 @end
 
 @implementation ASTextKitComponents
@@ -61,8 +89,6 @@
   components.textContainer.lineFragmentPadding = 0.0; // We want the text laid out up to the very edges of the text-view.
   [components.layoutManager addTextContainer:components.textContainer];
 
-  components.requiresMainThreadDeallocation = YES;
-
   return components;
 }
 
@@ -70,11 +96,9 @@
 
 - (void)dealloc
 {
-  if (_requiresMainThreadDeallocation) {
-    ASDisplayNodeAssertMainThread();
-  }
   // Nil out all delegates to prevent crash
   if (_textView) {
+    ASDisplayNodeAssertMainThread();
     _textView.delegate = nil;
   }
   _layoutManager.delegate = nil;
@@ -88,10 +112,8 @@
 
   // If our text-view's width is already the constrained width, we can use our existing TextKit stack for this sizing calculation.
   // Otherwise, we create a temporary stack to size for `constrainedWidth`.
-  if (CGRectGetWidth(components.textView.bounds) != constrainedWidth) {
+  if (CGRectGetWidth(components.textView.threadSafeBounds) != constrainedWidth) {
     components = [ASTextKitComponents componentsWithAttributedSeedString:components.textStorage textContainerSize:CGSizeMake(constrainedWidth, CGFLOAT_MAX)];
-    // The temporary stack can be deallocated off main
-    components.requiresMainThreadDeallocation = NO;
   }
 
   // Force glyph generation and layout, which may not have happened yet (and isn't triggered by -usedRectForTextContainer:).
@@ -112,9 +134,7 @@
   
   // Always use temporary stack in case of threading issues
   components = [ASTextKitComponents componentsWithAttributedSeedString:components.textStorage textContainerSize:CGSizeMake(constrainedWidth, CGFLOAT_MAX)];
-  // The temporary stack can be deallocated off main
-  components.requiresMainThreadDeallocation = NO;
-  
+
   // Force glyph generation and layout, which may not have happened yet (and isn't triggered by - usedRectForTextContainer:).
   [components.layoutManager ensureLayoutForTextContainer:components.textContainer];