[ASPrimitiveTraitCollection] Always treat preferredContentSize as a p…

…otential nil #trivial (#757)

* Fix ASPrimitiveTraitCollection initialization on iOS 9.
Add nil-value checks for preferredContentSizeCategory.

* * Mark -[ASTraitCollection init] as deprecated.
* Mark [ASViewController overrideDisplayTraitsWithWindowSize] as deprecated.

Code review changes:
* Remove unneeded nonnull annotations
* Add null check in ASTraitCollection constructor implementation
* Codestyle
* Add some documentation about ASPrimitiveTraitCollection vs ASTraitCollection usage

* Rename safeContentSizeCategory to AS_safeContentSizeCategory.

Remove safePrimitiveContentSizeCategory in favour of AS_safeContentSizeCategory.

Source/ASViewController.h

@@ -63,7 +63,7 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * Set this block to customize the ASDisplayTraits returned when the VC transitions to the given window size.
  */
-@property (nonatomic, copy) ASDisplayTraitsForTraitWindowSizeBlock overrideDisplayTraitsWithWindowSize;
+@property (nonatomic, copy) ASDisplayTraitsForTraitWindowSizeBlock overrideDisplayTraitsWithWindowSize ASDISPLAYNODE_DEPRECATED_MSG("This property is actually never accessed inside the framework");
 
 /**
  * @abstract Passthrough property to the the .interfaceState of the node.

Source/Details/ASTraitCollection.h

@@ -58,6 +58,14 @@ extern ASPrimitiveContentSizeCategory ASPrimitiveContentSizeCategoryMake(UIConte
 
 #pragma mark - ASPrimitiveTraitCollection
 
+/**
+ * @abstract This is an internal struct-representation of ASTraitCollection.
+ *
+ * @discussion This struct is for internal use only. Framework users should always use ASTraitCollection.
+ *
+ * If you use ASPrimitiveTraitCollection, please do make sure to initialize it with ASPrimitiveTraitCollectionMakeDefault()
+ * or ASPrimitiveTraitCollectionFromUITraitCollection(UITraitCollection*).
+ */
 typedef struct ASPrimitiveTraitCollection {
   UIUserInterfaceSizeClass horizontalSizeClass;
   UIUserInterfaceSizeClass verticalSizeClass;
@@ -112,17 +120,23 @@ ASDISPLAYNODE_EXTERN_C_END
 @protocol ASTraitEnvironment <NSObject>
 
 /**
- * Returns a struct-representation of the environment's ASEnvironmentDisplayTraits. This only exists as a internal
- * convenience method. Users should access the trait collections through the NSObject based asyncTraitCollection API
+ * @abstract Returns a struct-representation of the environment's ASEnvironmentDisplayTraits.
+ *
+ * @discussion This only exists as an internal convenience method. Users should access the trait collections through
+ * the NSObject based asyncTraitCollection API
  */
 - (ASPrimitiveTraitCollection)primitiveTraitCollection;
 
 /**
- * Sets a trait collection on this environment state.
+ * @abstract Sets a trait collection on this environment state.
+ *
+ * @discussion This only exists as an internal convenience method. Users should not override trait collection using it.
+ * Use [ASViewController overrideDisplayTraitsWithTraitCollection] block instead.
  */
 - (void)setPrimitiveTraitCollection:(ASPrimitiveTraitCollection)traitCollection;
 
 /**
+ * @abstract Returns the thread-safe UITraitCollection equivalent.
  */
 - (ASTraitCollection *)asyncTraitCollection;
 
@@ -179,8 +193,6 @@ AS_SUBCLASSING_RESTRICTED
 
 @property (nonatomic, assign, readonly) CGSize containerSize;
 
-+ (ASTraitCollection *)traitCollectionWithASPrimitiveTraitCollection:(ASPrimitiveTraitCollection)traits;
-
 + (ASTraitCollection *)traitCollectionWithUITraitCollection:(UITraitCollection *)traitCollection
                                               containerSize:(CGSize)windowSize;
 
@@ -211,13 +223,25 @@ AS_SUBCLASSING_RESTRICTED
                                                 containerSize:(CGSize)windowSize;
 #endif
 
-- (ASPrimitiveTraitCollection)primitiveTraitCollection;
 - (BOOL)isEqualToTraitCollection:(ASTraitCollection *)traitCollection;
 
 @end
 
+/**
+ * These are internal helper methods. Should never be called by the framework users.
+ */
+@interface ASTraitCollection (PrimitiveTraits)
+
++ (ASTraitCollection *)traitCollectionWithASPrimitiveTraitCollection:(ASPrimitiveTraitCollection)traits;
+
+- (ASPrimitiveTraitCollection)primitiveTraitCollection;
+
+@end
+
 @interface ASTraitCollection (Deprecated)
 
+- (instancetype)init ASDISPLAYNODE_DEPRECATED_MSG("The default constructor of this class is going to become unavailable. Use other constructors instead.");
+
 + (ASTraitCollection *)traitCollectionWithDisplayScale:(CGFloat)displayScale
                                     userInterfaceIdiom:(UIUserInterfaceIdiom)userInterfaceIdiom
                                    horizontalSizeClass:(UIUserInterfaceSizeClass)horizontalSizeClass