Initial attempt at implementing Display Traits

Initial attempt to get display traits working with ASEnvironment.

To get proper ASDisplayTraits support, you must use an ASViewController. The ASViewController implements UITraitCollection-related methods (`traitCollectionDidChange:`, `willTransitionToTraitCollection:withTransitionCoordinator:`, viewWillTransitionToSize:withTransitionCoordinator`) to update the internal ASDisplayTraits and propagate them to subnodes.

ASTableNode and ASCollectionNode don't actually have their cells as subnodes, so a little bit of trickery is involved (on `setEnvironment:` the table/collection node gets its data controllers completedNodes and propagates the new traits. see `ASDisplayTraitsCollectionTableSetEnvironmentState`). The data controller also passes the current display traits when creating new cells.

ASViewController also supports the ability to return a custom set of display traits. So if you have a modal dialog that should always be told it is in a compact size class, you can set the override block before displaying the VC.

A new example, called Display Traits, has been added. It shows how display traits can be used in a ASViewController with a normal ASDisplayNode as its root, as well as in ASViewControllers hosting table nodes and collection nodes. There is also an example of overriding the default display traits of a VC.

Please provide feedback!

AsyncDisplayKit/ASTableNode.mm

@@ -0,0 +1,164 @@
+//
+//  ASTableNode.m
+//  AsyncDisplayKit
+//
+//  Created by Steven Ramkumar on 11/4/15.
+//  Copyright © 2015 Facebook. All rights reserved.
+//
+
+#import "ASEnvironmentInternal.h"
+#import "ASFlowLayoutController.h"
+#import "ASTableViewInternal.h"
+#import "ASDisplayNode+Subclasses.h"
+#import "ASRangeControllerUpdateRangeProtocol+Beta.h"
+
+@interface _ASTablePendingState : NSObject
+@property (weak, nonatomic) id <ASTableDelegate>   delegate;
+@property (weak, nonatomic) id <ASTableDataSource> dataSource;
+@end
+
+@implementation _ASTablePendingState
+@end
+
+@interface ASTableNode ()
+@property (nonatomic, strong) _ASTablePendingState *pendingState;
+@end
+
+@interface ASTableView ()
+- (instancetype)_initWithFrame:(CGRect)frame style:(UITableViewStyle)style dataControllerClass:(Class)dataControllerClass;
+@end
+
+@implementation ASTableNode
+
+- (instancetype)_initWithTableView:(ASTableView *)tableView
+{
+  // Avoid a retain cycle.  In this case, the ASTableView is creating us, and strongly retains us.
+  ASTableView * __weak weakTableView = tableView;
+  if (self = [super initWithViewBlock:^UIView *{ return weakTableView; }]) {
+    __unused __weak ASTableView *view = [self view];
+    return self;
+  }
+  return nil;
+}
+
+- (instancetype)_initWithFrame:(CGRect)frame style:(UITableViewStyle)style dataControllerClass:(Class)dataControllerClass
+{
+  ASDisplayNodeViewBlock tableViewBlock = ^UIView *{
+    return [[ASTableView alloc] _initWithFrame:frame style:style dataControllerClass:dataControllerClass ownedByNode:YES];
+  };
+
+  if (self = [super initWithViewBlock:tableViewBlock]) {
+    return self;
+  }
+  return nil;
+}
+
+- (instancetype)initWithStyle:(UITableViewStyle)style
+{
+  return [self _initWithFrame:CGRectZero style:style dataControllerClass:nil];
+}
+
+- (instancetype)init
+{
+  return [self _initWithFrame:CGRectZero style:UITableViewStylePlain dataControllerClass:nil];
+}
+
+- (void)didLoad
+{
+  [super didLoad];
+  
+  ASTableView *view = self.view;
+  view.tableNode    = self;
+
+  if (_pendingState) {
+    _ASTablePendingState *pendingState = _pendingState;
+    self.pendingState    = nil;
+    view.asyncDelegate   = pendingState.delegate;
+    view.asyncDataSource = pendingState.dataSource;
+  }
+}
+
+- (void)updateCurrentRangeWithMode:(ASLayoutRangeMode)rangeMode
+{
+    if (!self.isNodeLoaded) {
+        return;
+    }
+    
+    [self.view.rangeController updateCurrentRangeWithMode:rangeMode];
+}
+
+- (_ASTablePendingState *)pendingState
+{
+  if (!_pendingState && ![self isNodeLoaded]) {
+    self.pendingState = [[_ASTablePendingState alloc] init];
+  }
+  ASDisplayNodeAssert(![self isNodeLoaded] || !_pendingState, @"ASTableNode should not have a pendingState once it is loaded");
+  return _pendingState;
+}
+
+- (void)setDelegate:(id <ASTableDelegate>)delegate
+{
+  if ([self pendingState]) {
+    _pendingState.delegate = delegate;
+  } else {
+    ASDisplayNodeAssert([self isNodeLoaded], @"ASTableNode should be loaded if pendingState doesn't exist");
+    self.view.asyncDelegate = delegate;
+  }
+}
+
+- (id <ASTableDelegate>)delegate
+{
+  if ([self pendingState]) {
+    return _pendingState.delegate;
+  } else {
+    return self.view.asyncDelegate;
+  }
+}
+
+- (void)setDataSource:(id <ASTableDataSource>)dataSource
+{
+  if ([self pendingState]) {
+    _pendingState.dataSource = dataSource;
+  } else {
+    ASDisplayNodeAssert([self isNodeLoaded], @"ASTableNode should be loaded if pendingState doesn't exist");
+    self.view.asyncDataSource = dataSource;
+  }
+}
+
+- (id <ASTableDataSource>)dataSource
+{
+  if ([self pendingState]) {
+    return _pendingState.dataSource;
+  } else {
+    return self.view.asyncDataSource;
+  }
+}
+
+- (ASTableView *)view
+{
+  return (ASTableView *)[super view];
+}
+
+#if ASRangeControllerLoggingEnabled
+- (void)visibilityDidChange:(BOOL)isVisible
+{
+  [super visibilityDidChange:isVisible];
+  NSLog(@"%@ - visible: %d", self, isVisible);
+}
+#endif
+
+- (void)clearContents
+{
+  [super clearContents];
+  [self.view clearContents];
+}
+
+- (void)clearFetchedData
+{
+  [super clearFetchedData];
+  [self.view clearFetchedData];
+}
+
+ASDisplayTraitsCollectionTableSetEnvironmentState
+
+@end