[Tabs] Adds support for rendering custom views. (#7755)

Clients can provide their own custom views if MDCTabBarView doesn't provide
sufficient behavior by default. An example custom view (containing a UISwitch)
is added to the Dragons example and a test.

![Simulator Screen Shot - iPhone 7 - 2019-06-28 at 22 49 36](https://user-images.githubusercontent.com/1753199/60379003-26e59f80-99f9-11e9-8d68-49d775c7b3c8.png)


Closes #7722

components/Tabs/examples/MDCTabBarViewTypicalExampleViewController.m

@@ -19,6 +19,42 @@
 
 static NSString *const kExampleTitle = @"TabBarView";
 
+/** A custom view to place in an MDCTabBarView. */
+@interface MDCTabBarViewTypicalExampleViewControllerCustomView
+    : UIView <MDCTabBarViewIndicatorSupporting>
+/** A switch shown in the view. */
+@property(nonatomic, strong) UISwitch *aSwitch;
+@end
+
+@implementation MDCTabBarViewTypicalExampleViewControllerCustomView
+
+- (CGRect)contentFrame {
+  return CGRectStandardize(self.aSwitch.frame);
+}
+
+- (UISwitch *)aSwitch {
+  if (!_aSwitch) {
+    _aSwitch = [[UISwitch alloc] init];
+    [self addSubview:_aSwitch];
+  }
+  return _aSwitch;
+}
+
+- (void)layoutSubviews {
+  [super layoutSubviews];
+  self.aSwitch.center = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
+}
+
+- (CGSize)intrinsicContentSize {
+  return self.aSwitch.intrinsicContentSize;
+}
+
+- (CGSize)sizeThatFits:(CGSize)size {
+  return [self.aSwitch sizeThatFits:size];
+}
+
+@end
+
 /**
  Typical use example showing how to place an @c MDCTabBarView within another view.
  */
@@ -70,10 +106,15 @@ - (void)viewDidLoad {
               image:[[UIImage imageNamed:@"Search"]
                         imageWithRenderingMode:UIImageRenderingModeAlwaysTemplate]
                 tag:4];
+  MDCTabBarItem *item6 = [[MDCTabBarItem alloc] initWithTitle:@"A switch" image:nil tag:5];
+  MDCTabBarViewTypicalExampleViewControllerCustomView *switchView =
+      [[MDCTabBarViewTypicalExampleViewControllerCustomView alloc] init];
+  item6.mdc_customView = switchView;
+  switchView.aSwitch.onTintColor = self.containerScheme.colorScheme.primaryColor;
 
   self.tabBar = [[MDCTabBarView alloc] init];
   self.tabBar.tabBarDelegate = self;
-  self.tabBar.items = @[ item1, item2, item3, item4, item5 ];
+  self.tabBar.items = @[ item1, item2, item3, item4, item5, item6 ];
   self.tabBar.barTintColor = self.containerScheme.colorScheme.secondaryColor;
   [self.tabBar setTitleColor:self.containerScheme.colorScheme.onSecondaryColor
                     forState:UIControlStateNormal];

components/Tabs/src/TabBarView/MDCTabBarView.m

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 #import "MDCTabBarView.h"
+#import "MDCTabBarItemCustomViewing.h"
 #import "MDCTabBarViewDelegate.h"
 #import "private/MDCTabBarViewItemView.h"
 
@@ -120,21 +121,32 @@ - (void)setItems:(NSArray<UITabBarItem *> *)items {
   _items = [items copy];
 
   for (UITabBarItem *item in self.items) {
-    MDCTabBarViewItemView *itemView = [[MDCTabBarViewItemView alloc] init];
-    itemView.translatesAutoresizingMaskIntoConstraints = NO;
-    itemView.titleLabel.text = item.title;
-    itemView.accessibilityLabel = item.accessibilityLabel;
-    itemView.accessibilityHint = item.accessibilityHint;
-    itemView.accessibilityIdentifier = item.accessibilityIdentifier;
-    itemView.accessibilityTraits = item.accessibilityTraits == UIAccessibilityTraitNone
-                                       ? UIAccessibilityTraitButton
-                                       : item.accessibilityTraits;
-    itemView.titleLabel.textColor = [self titleColorForState:UIControlStateNormal];
-    itemView.iconImageView.image = item.image;
+    UIView *itemView;
+    if ([item conformsToProtocol:@protocol(MDCTabBarItemCustomViewing)]) {
+      UITabBarItem<MDCTabBarItemCustomViewing> *customItem =
+          (UITabBarItem<MDCTabBarItemCustomViewing> *)item;
+      if (customItem.mdc_customView) {
+        itemView = customItem.mdc_customView;
+      }
+    }
+    if (!itemView) {
+      MDCTabBarViewItemView *mdcItemView = [[MDCTabBarViewItemView alloc] init];
+      mdcItemView.titleLabel.text = item.title;
+      mdcItemView.accessibilityLabel = item.accessibilityLabel;
+      mdcItemView.accessibilityHint = item.accessibilityHint;
+      mdcItemView.accessibilityIdentifier = item.accessibilityIdentifier;
+      mdcItemView.accessibilityTraits = item.accessibilityTraits == UIAccessibilityTraitNone
+                                            ? UIAccessibilityTraitButton
+                                            : item.accessibilityTraits;
+      mdcItemView.titleLabel.textColor = [self titleColorForState:UIControlStateNormal];
+      mdcItemView.iconImageView.image = item.image;
+      itemView = mdcItemView;
+    }
     [itemView setContentCompressionResistancePriority:UILayoutPriorityRequired
                                               forAxis:UILayoutConstraintAxisHorizontal];
     [itemView setContentCompressionResistancePriority:UILayoutPriorityRequired
                                               forAxis:UILayoutConstraintAxisVertical];
+    itemView.translatesAutoresizingMaskIntoConstraints = NO;
     UITapGestureRecognizer *tapGesture =
         [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(didTapItemView:)];
     [itemView addGestureRecognizer:tapGesture];

components/Tabs/tests/snapshot/TabBarView/MDCTabBarViewSnapshotTests.m

@@ -14,7 +14,9 @@
 
 #import "MaterialSnapshot.h"
 
+#import "MDCTabBarItem.h"
 #import "MDCTabBarView.h"
+#import "MDCTabBarViewIndicatorSupporting.h"
 
 /** The typical size of an image in a Tab bar. */
 static const CGSize kTypicalImageSize = (CGSize){24, 24};
@@ -31,6 +33,41 @@
 /** The maximum width of a tab bar item. */
 static const CGFloat kMaxItemWidth = 360;
 
+/** A custom view to place in an MDCTabBarView. */
+@interface MDCTabBarViewSnapshotTestsCustomView : UIView <MDCTabBarViewIndicatorSupporting>
+/** A switch shown in the view. */
+@property(nonatomic, strong) UISwitch *aSwitch;
+@end
+
+@implementation MDCTabBarViewSnapshotTestsCustomView
+
+- (CGRect)contentFrame {
+  return CGRectStandardize(self.aSwitch.frame);
+}
+
+- (UISwitch *)aSwitch {
+  if (!_aSwitch) {
+    _aSwitch = [[UISwitch alloc] init];
+    [self addSubview:_aSwitch];
+  }
+  return _aSwitch;
+}
+
+- (void)layoutSubviews {
+  [super layoutSubviews];
+  self.aSwitch.center = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
+}
+
+- (CGSize)intrinsicContentSize {
+  return self.aSwitch.intrinsicContentSize;
+}
+
+- (CGSize)sizeThatFits:(CGSize)size {
+  return [self.aSwitch sizeThatFits:size];
+}
+
+@end
+
 static NSString *const kItemTitleShort1Latin = @"Quando";
 static NSString *const kItemTitleShort2Latin = @"No";
 static NSString *const kItemTitleShort3Latin = @"Facer";
@@ -873,4 +910,24 @@ - (void)testBarTintColor {
   // Then
   [self generateSnapshotAndVerifyForView:self.tabBarView];
 }
+
+#pragma mark - Custom View support
+
+- (void)testCustomView {
+  // Given
+  MDCTabBarViewSnapshotTestsCustomView *customView =
+      [[MDCTabBarViewSnapshotTestsCustomView alloc] init];
+  MDCTabBarItem *customViewItem = [[MDCTabBarItem alloc] initWithTitle:@"Title" image:nil tag:0];
+  customViewItem.mdc_customView = customView;
+  UITabBarItem *item1 = [[UITabBarItem alloc] initWithTitle:@"One" image:self.typicalIcon1 tag:0];
+  UITabBarItem *item3 = [[UITabBarItem alloc] initWithTitle:@"Three" image:self.typicalIcon3 tag:3];
+
+  // When
+  self.tabBarView.items = @[ item1, customViewItem, item3 ];
+  [self.tabBarView sizeToFit];
+
+  // Then
+  [self generateSnapshotAndVerifyForView:self.tabBarView];
+}
+
 @end