[Tabs] Create default selection indicator template. (#7751)

Adds the default (underline) selection indicator from Material.io. This is the same one as `MDCTabBar` and has been copied and renamed.

Part of #7744

components/Tabs/src/TabBarView/MDCTabBarViewUnderlineIndicatorTemplate.h

@@ -0,0 +1,23 @@
+// Copyright 2017-present the Material Components for iOS authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#import <UIKit/UIKit.h>
+
+#import "MDCTabBarViewIndicatorTemplate.h"
+
+/*
+ Default indicator template which produces a fixed-height solid line underneath the selected tab.
+ */
+@interface MDCTabBarViewUnderlineIndicatorTemplate : NSObject <MDCTabBarViewIndicatorTemplate>
+@end

components/Tabs/src/TabBarView/MDCTabBarViewUnderlineIndicatorTemplate.m

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+// Copyright 2017-present the Material Components for iOS authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#import "MDCTabBarViewUnderlineIndicatorTemplate.h"
+
+#import "MDCTabBarViewIndicatorAttributes.h"
+#import "MDCTabBarViewIndicatorContext.h"
+
+/// Height in points of the underline shown under selected items.
+static const CGFloat kUnderlineIndicatorHeight = 2;
+
+@implementation MDCTabBarViewUnderlineIndicatorTemplate
+
+- (MDCTabBarViewIndicatorAttributes *)indicatorAttributesForContext:
+    (id<MDCTabBarViewIndicatorContext>)context {
+  CGRect bounds = context.bounds;
+  MDCTabBarViewIndicatorAttributes *attributes = [[MDCTabBarViewIndicatorAttributes alloc] init];
+  CGRect underlineFrame =
+      CGRectMake(CGRectGetMinX(bounds), CGRectGetMaxY(bounds) - kUnderlineIndicatorHeight,
+                 CGRectGetWidth(bounds), kUnderlineIndicatorHeight);
+  attributes.path = [UIBezierPath bezierPathWithRect:underlineFrame];
+  return attributes;
+}
+
+@end