[Tabs] Update README for TabBarView. (#7799)

Updates the Tabs README for the latest changes to TabBarView.

Closes #7789

components/Tabs/README.md

@@ -345,11 +345,20 @@ MDCContainerScheme *containerScheme = [[MDCContainerScheme alloc] init];
 > **_NOTE:_** This is currently in Beta. Features may change without warning and without a change in the Material 
 > Components for iOS version number.
 
+<div class="article__asset article__asset--screenshot">
+    <img src="docs/assets/tabbarview-defaults-Fixed-Justified.png" alt="TabBarView showing only titles in a Justified Fixed Tabs layout." width="375">
+</div>
+<div class="article__asset article__asset--screenshot">
+    <img src="docs/assets/tabbarview-defaults-Scrollable-Leading.png" alt="TabBarView showing only titles in a Scrollable layout." width="375">
+</div>
+
+
 ### Importing MDCTabBarView
 
-`MDCTabBarView` is currently part of the MaterialComponentsBeta podspec. You may need to follow the [Material Components for iOS
-Beta integration guide](https://github.com/material-components/material-components-ios/blob/develop/contributing/beta_components.md) to configure
-your project to use `MDCTabBarView`.
+`MDCTabBarView` is currently part of the MaterialComponentsBeta podspec. You
+may need to follow the [Material Components for iOS Beta integration
+guide](https://github.com/material-components/material-components-ios/blob/develop/contributing/beta_components.md)
+to configure your project to use `MDCTabBarView`.
 
 <!--<div class="material-code-render" markdown="1">-->
 #### Swift
@@ -387,5 +396,84 @@ MDCTabBarView *tabBarView = [[MDCTabBarView alloc] init];
 
 ### Migrating from MDCTabBar
 
+#### Subclassing Restricted
+
+Subclassing is not supported by `MDCTabBarView`. Any requirements that you have
+for Material Tabs that are not met by the public APIs should be filed as a
+feature request or bug against `MDCTabBarView`.
+
+#### Colors, Fonts, and Theming
+
+To set the image tint colors, use `- setImageTintColor:forState:`. The
+`MDCTabBar` APIs, `- selectedItemTintColor` and `- unselectedItemTintColor` are
+unavailable. 
+
+To set the fonts of the labels, use `- setTitleFont:forState:`.
+The `MDCTabBar` APIs, `- selectedItemTitleFont` and `- unselectedItemTitleFont`
+are unavailable. Note that the tab bar may adjust the sizes of its views to
+account for changes in fonts, and that can result in unexpected changes from
+Fixed Tabs to Scrollable Tabs depending on font choices and title length.
+
+`MDCTabBarView` uses Material Ripple by default (`MDCRippleView`) and does not
+support Ink. You may configure the Ripple color for touch feedback by setting
+the `- rippleColor` property.
+
+#### Alignment and Item Rendering
+
+The `MDCTabBar` API `itemAppearance` has no equivalent on `MDCTabBarView`.
+Whatever relevant properties (*e.g.*, `title`, `image`) are provided on `UITabBarItem`
+should be used instead.
+
+The `MDCTabBar` APIs `displaysUppercaseTitles` and `titleTextTransform` have no
+equivalent in `MDCTabBarView`. Titles are rendered as set on `UITabBarItem` and
+`accessibilityLabel` should be set on the item if the title text is not
+correctly handled by UIAccessibility.
+
+#### UIBarPositioningDelegate
+
+`MDCTabBarView` no longer conforms to `UIBarPositioning`, since Material Tabs
+are always positioned above their related content views. As a result,
+`MDCTabBarViewDelegate` does not inherit from `UIBarPositioningDelegate`.
+
+#### Selection Indicator Template
+
+The Selection Indicator Template APIs and protocols are copied nearly verbatim
+from `MDCTabBar`, with the names changed to prevent collision. One difference
+is the expected behavior of `contentFrame` as used by the indicator template.
+In `MDCTabBar` the value of `contentFrame` was the union of the title and image
+frames. However, in `MDCTabBarView` the width of the `contentFrame` is always
+the width of the title (when present), and the height will include both the
+title and image. This change is necessary to support internal clients.
+
+#### Custom Views
+
+Features like badges and horizontal layout of titles and images are not
+supported on `MDCTabBarView`. Clients who need such behavior should implement
+their own custom `UIView` and assign it to the `mdc_customView` property of a
+`UITabBarItem` sublcass that conforms to `MDCTabBarItemCustomViewing`. A simple
+subclass conforming to the `MDCTabBarItemCustomViewing` protocol is provided as
+`MDCTabBarItem`.
+
+<!--<div class="material-code-render" markdown="1">-->
+##### Swift
+```swift
+let customView = MyCustomTabView()
+let customItem = MDCTabBarItem()
+customItem.mdc_customView = customView
+let tabBarView = MDCTabBarView()
+tabBarView.items = [ customItem ]
+```
+
+##### Objective-C
+
+```objc
+MyCustomTabView *customView = [[MyCustomTabView alloc] init];
+MDCTabBarItem *customItem = [[MDCTabBarItem alloc] init];
+customItem.mdc_customView = customView;
+MDCTabBarView *tabBarView = [[MDCTabBarView alloc] init];
+tabBarView.items = @[ customItem ]
+```
+<!--</div>-->
+
 > **_NOTE:_** This will be updated as APIs are added and migrations are defined.
 

components/Tabs/docs/tabbarview.md

@@ -1,11 +1,20 @@
 > **_NOTE:_** This is currently in Beta. Features may change without warning and without a change in the Material 
 > Components for iOS version number.
 
+<div class="article__asset article__asset--screenshot">
+    <img src="docs/assets/tabbarview-defaults-Fixed-Justified.png" alt="TabBarView showing only titles in a Justified Fixed Tabs layout." width="375">
+</div>
+<div class="article__asset article__asset--screenshot">
+    <img src="docs/assets/tabbarview-defaults-Scrollable-Leading.png" alt="TabBarView showing only titles in a Scrollable layout." width="375">
+</div>
+
+
 ### Importing MDCTabBarView
 
-`MDCTabBarView` is currently part of the MaterialComponentsBeta podspec. You may need to follow the [Material Components for iOS
-Beta integration guide](https://github.com/material-components/material-components-ios/blob/develop/contributing/beta_components.md) to configure
-your project to use `MDCTabBarView`.
+`MDCTabBarView` is currently part of the MaterialComponentsBeta podspec. You
+may need to follow the [Material Components for iOS Beta integration
+guide](https://github.com/material-components/material-components-ios/blob/develop/contributing/beta_components.md)
+to configure your project to use `MDCTabBarView`.
 
 <!--<div class="material-code-render" markdown="1">-->
 #### Swift
@@ -43,4 +52,83 @@ MDCTabBarView *tabBarView = [[MDCTabBarView alloc] init];
 
 ### Migrating from MDCTabBar
 
+#### Subclassing Restricted
+
+Subclassing is not supported by `MDCTabBarView`. Any requirements that you have
+for Material Tabs that are not met by the public APIs should be filed as a
+feature request or bug against `MDCTabBarView`.
+
+#### Colors, Fonts, and Theming
+
+To set the image tint colors, use `- setImageTintColor:forState:`. The
+`MDCTabBar` APIs, `- selectedItemTintColor` and `- unselectedItemTintColor` are
+unavailable. 
+
+To set the fonts of the labels, use `- setTitleFont:forState:`.
+The `MDCTabBar` APIs, `- selectedItemTitleFont` and `- unselectedItemTitleFont`
+are unavailable. Note that the tab bar may adjust the sizes of its views to
+account for changes in fonts, and that can result in unexpected changes from
+Fixed Tabs to Scrollable Tabs depending on font choices and title length.
+
+`MDCTabBarView` uses Material Ripple by default (`MDCRippleView`) and does not
+support Ink. You may configure the Ripple color for touch feedback by setting
+the `- rippleColor` property.
+
+#### Alignment and Item Rendering
+
+The `MDCTabBar` API `itemAppearance` has no equivalent on `MDCTabBarView`.
+Whatever relevant properties (*e.g.*, `title`, `image`) are provided on `UITabBarItem`
+should be used instead.
+
+The `MDCTabBar` APIs `displaysUppercaseTitles` and `titleTextTransform` have no
+equivalent in `MDCTabBarView`. Titles are rendered as set on `UITabBarItem` and
+`accessibilityLabel` should be set on the item if the title text is not
+correctly handled by UIAccessibility.
+
+#### UIBarPositioningDelegate
+
+`MDCTabBarView` no longer conforms to `UIBarPositioning`, since Material Tabs
+are always positioned above their related content views. As a result,
+`MDCTabBarViewDelegate` does not inherit from `UIBarPositioningDelegate`.
+
+#### Selection Indicator Template
+
+The Selection Indicator Template APIs and protocols are copied nearly verbatim
+from `MDCTabBar`, with the names changed to prevent collision. One difference
+is the expected behavior of `contentFrame` as used by the indicator template.
+In `MDCTabBar` the value of `contentFrame` was the union of the title and image
+frames. However, in `MDCTabBarView` the width of the `contentFrame` is always
+the width of the title (when present), and the height will include both the
+title and image. This change is necessary to support internal clients.
+
+#### Custom Views
+
+Features like badges and horizontal layout of titles and images are not
+supported on `MDCTabBarView`. Clients who need such behavior should implement
+their own custom `UIView` and assign it to the `mdc_customView` property of a
+`UITabBarItem` sublcass that conforms to `MDCTabBarItemCustomViewing`. A simple
+subclass conforming to the `MDCTabBarItemCustomViewing` protocol is provided as
+`MDCTabBarItem`.
+
+<!--<div class="material-code-render" markdown="1">-->
+##### Swift
+```swift
+let customView = MyCustomTabView()
+let customItem = MDCTabBarItem()
+customItem.mdc_customView = customView
+let tabBarView = MDCTabBarView()
+tabBarView.items = [ customItem ]
+```
+
+##### Objective-C
+
+```objc
+MyCustomTabView *customView = [[MyCustomTabView alloc] init];
+MDCTabBarItem *customItem = [[MDCTabBarItem alloc] init];
+customItem.mdc_customView = customView;
+MDCTabBarView *tabBarView = [[MDCTabBarView alloc] init];
+tabBarView.items = @[ customItem ]
+```
+<!--</div>-->
+
 > **_NOTE:_** This will be updated as APIs are added and migrations are defined.