[Banner] Add intrinsicContentSize support with an example. (#7620)

Previously, `intrinsicContentSize` was not supported in MDCBannerView, which means users have to specify layout constraints for width and height manually. After adding `intrinsicContentSize` support, users will only need to specify width constraints for banner to show it properly. This implementation and behavior matches UITextView.

Screenshot:
![Simulator Screen Shot - iPhone 7 - 2019-06-17 at 17 18 30](https://user-images.githubusercontent.com/8836258/59637507-f3b32e80-9123-11e9-8133-7108c1a7824e.png)

components/Banner/BUILD

@@ -40,7 +40,10 @@ mdc_examples_objc_library(
     name = "ObjcExamples",
     deps = [
         ":Banner",
+        "//components/Buttons",
+        "//components/Buttons:Theming",
         "//components/schemes/Color",
+        "//components/schemes/Container",
     ],
 )
 

components/Banner/examples/BannerAutolayoutExampleViewController.m

@@ -0,0 +1,101 @@
+// Copyright 2019-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 "MaterialBanner.h"
+#import "MaterialButtons+Theming.h"
+#import "MaterialButtons.h"
+#import "MaterialColorScheme.h"
+#import "MaterialContainerScheme.h"
+
+static NSString *const exampleText = @"Lorem ipsum dolor";
+
+@interface BannerAutolayoutExampleViewController : UIViewController
+
+@property(nonatomic, readwrite, strong) MDCContainerScheme *containerScheme;
+@property(nonatomic, readwrite, strong) MDCBannerView *bannerView;
+
+@end
+
+@implementation BannerAutolayoutExampleViewController
+
+- (void)viewDidLoad {
+  [super viewDidLoad];
+
+  self.containerScheme = [[MDCContainerScheme alloc] init];
+  self.view.backgroundColor = self.containerScheme.colorScheme.backgroundColor;
+
+  // Action Button
+  MDCButton *button = [[MDCButton alloc] init];
+  button.translatesAutoresizingMaskIntoConstraints = NO;
+  [button applyTextThemeWithScheme:self.containerScheme];
+  [button setTitle:@"Material Banner" forState:UIControlStateNormal];
+  [button addTarget:self
+                action:@selector(didTapButton)
+      forControlEvents:UIControlEventTouchUpInside];
+  [self.view addSubview:button];
+  NSLayoutConstraint *buttonConstraintCenterX =
+      [button.centerXAnchor constraintEqualToAnchor:self.view.centerXAnchor];
+  buttonConstraintCenterX.active = YES;
+  NSLayoutConstraint *buttonConstraintCenterY =
+      [button.centerYAnchor constraintEqualToAnchor:self.view.centerYAnchor];
+  buttonConstraintCenterY.active = YES;
+
+  // Prepare Banner
+  MDCBannerView *bannerView = [[MDCBannerView alloc] init];
+  bannerView.translatesAutoresizingMaskIntoConstraints = NO;
+  bannerView.textLabel.text = exampleText;
+  bannerView.trailingButton.hidden = YES;
+  bannerView.showsDivider = YES;
+  bannerView.layoutMargins = UIEdgeInsetsZero;
+  MDCButton *actionButton = bannerView.leadingButton;
+  [actionButton applyTextThemeWithScheme:self.containerScheme];
+  [actionButton setTitle:@"Dismiss" forState:UIControlStateNormal];
+  [actionButton addTarget:self
+                   action:@selector(didTapDismissOnBannerView)
+         forControlEvents:UIControlEventTouchUpInside];
+  [self.view addSubview:bannerView];
+  NSLayoutConstraint *bannerViewConstraintTop =
+      [bannerView.topAnchor constraintEqualToAnchor:self.view.layoutMarginsGuide.topAnchor];
+  bannerViewConstraintTop.active = YES;
+  NSLayoutConstraint *bannerViewConstraintLeft =
+      [bannerView.leftAnchor constraintEqualToAnchor:self.view.leftAnchor];
+  bannerViewConstraintLeft.active = YES;
+  NSLayoutConstraint *bannerViewConstraintRight =
+      [bannerView.rightAnchor constraintEqualToAnchor:self.view.rightAnchor];
+  bannerViewConstraintRight.active = YES;
+  bannerView.hidden = YES;
+  self.bannerView = bannerView;
+}
+
+- (void)didTapButton {
+  self.bannerView.hidden = NO;
+}
+
+- (void)didTapDismissOnBannerView {
+  self.bannerView.hidden = YES;
+}
+
+#pragma mark - CBC
+
++ (NSDictionary *)catalogMetadata {
+  return @{
+    @"breadcrumbs" : @[ @"Banner", @"Banner (Autolayout)" ],
+    @"primaryDemo" : @NO,
+    @"presentable" : @YES,
+  };
+}
+
+@end

components/Banner/src/MDCBannerView.m

@@ -374,13 +374,23 @@ - (CGSize)sizeThatFits:(CGSize)size {
   return CGSizeMake(size.width, frameHeight);
 }
 
+- (CGSize)intrinsicContentSize {
+  CGFloat intrinsicContentHeight = [self sizeThatFits:self.bounds.size].height;
+  return CGSizeMake(UIViewNoIntrinsicMetric, intrinsicContentHeight);
+}
+
 - (void)updateConstraints {
   MDCBannerViewLayoutStyle layoutStyle = [self layoutStyleForSizeToFit:self.bounds.size];
   [self updateConstraintsWithLayoutStyle:layoutStyle];
 
   [super updateConstraints];
 }
 
+- (void)layoutSubviews {
+  [super layoutSubviews];
+  [self invalidateIntrinsicContentSize];
+}
+
 #pragma mark - Layout methods
 
 - (void)updateConstraintsWithLayoutStyle:(MDCBannerViewLayoutStyle)layoutStyle {