components/BottomNavigation/src/MDCBottomNavigationBarController.m - external/github.com/material-components/material-components-ios - Git at Google

 // Copyright 2018-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 "MDCBottomNavigationBarController.h"

 // A context for Key Value Observing
 static void *const kObservationContext = (void *)&kObservationContext;

 @interface MDCBottomNavigationBarController ()

 /** The view that hosts the content for the selected view controller **/
 @property(nonatomic, strong) UIView *content;

 @end

 @implementation MDCBottomNavigationBarController

 - (instancetype)init {
   self = [super init];
   if (self) {
     _navigationBar = [[MDCBottomNavigationBar alloc] init];
     _content = [[UIView alloc] init];
     _viewControllers = @[];
     _selectedIndex = NSNotFound;

     [_navigationBar addObserver:self
                      forKeyPath:NSStringFromSelector(@selector(items))
                         options:NSKeyValueObservingOptionNew
                         context:kObservationContext];
   }

   return self;
 }

 - (void)dealloc {
   [_navigationBar removeObserver:self forKeyPath:NSStringFromSelector(@selector(items))];
 }

 - (void)viewDidLoad {
   [super viewDidLoad];

   self.navigationBar.delegate = self;

   // Add subviews and create their constraints
   [self.view addSubview:self.content];
   [self.view addSubview:self.navigationBar];
   [self loadConstraints];
 }

 - (void)setSelectedViewController:(nullable UIViewController *)selectedViewController {
   // Assert that the given VC is one of our view controllers or it is nil (we are unselecting)
   NSAssert(
       selectedViewController == nil || [self.viewControllers containsObject:selectedViewController],
       @"Attempting to set BottomBarViewControllers to a view controller it does not contain");

   // Early return if we are already set to the given VC
   if (self.selectedViewController == selectedViewController) {
     return;
   }

   // Remove current VC and add new one.
   [self removeContentViewController:self.selectedViewController];
   [self addContentViewController:selectedViewController];

   // Set the iVar and update selected index
   _selectedViewController = selectedViewController;
   self.selectedIndex = [self.viewControllers indexOfObject:selectedViewController];
 }

 - (void)setSelectedIndex:(NSUInteger)selectedIndex {
   // If we are setting to NSNotFound deselect the items
   if (selectedIndex == NSNotFound) {
     [self deselectCurrentItem];
     return;
   }

   BOOL outOfBounds = selectedIndex >= self.viewControllers.count ||
                      selectedIndex >= self.navigationBar.items.count;

   NSAssert(!outOfBounds,
            @"Attempting to set BottomBarViewController's selectedIndex to %li. This"
             " value is not within the bounds of the navigation bar's items and/or view controllers",
            (unsigned long)selectedIndex);

   // Early return if we are out of bounds or if the the index is already selected.
   if (outOfBounds || selectedIndex == _selectedIndex) {
     return;
   }

   // Update the selected index value and views.
   _selectedIndex = selectedIndex;
   [self updateViewsForSelectedIndex:selectedIndex];
 }

 - (void)setViewControllers:(NSArray<UIViewController *> *)viewControllers {
   [self deselectCurrentItem];
   NSArray *viewControllersCopy = [viewControllers copy];
   _viewControllers = viewControllersCopy;
   self.navigationBar.items = [self tabBarItemsForViewControllers:viewControllersCopy];

   self.selectedViewController = viewControllersCopy.firstObject;
 }

 - (UIViewController *)childViewControllerForStatusBarStyle {
   return self.selectedViewController;
 }

 - (UIViewController *)childViewControllerForStatusBarHidden {
   return self.selectedViewController;
 }

 #pragma mark - MDCBottomNavigationBarDelegate

 - (void)bottomNavigationBar:(MDCBottomNavigationBar *)bottomNavigationBar
               didSelectItem:(UITabBarItem *)item {
   // Early return if we cannot find the view controller.
   NSUInteger index = [self.navigationBar.items indexOfObject:item];
   if (index >= [self.viewControllers count] || index == NSNotFound) {
     return;
   }

   // Update selected view controller
   UIViewController *selectedViewController = [self.viewControllers objectAtIndex:index];
   self.selectedViewController = selectedViewController;

   // Notify the delegate.
   if ([self.delegate respondsToSelector:@selector(bottomNavigationBarController:
                                                         didSelectViewController:)]) {
     [self.delegate bottomNavigationBarController:self
                          didSelectViewController:selectedViewController];
   }
 }

 - (BOOL)bottomNavigationBar:(MDCBottomNavigationBar *)bottomNavigationBar
            shouldSelectItem:(UITabBarItem *)item {
   NSUInteger index = [self.navigationBar.items indexOfObject:item];
   if (index >= [self.viewControllers count] || index == NSNotFound) {
     return NO;
   }

   // Pass the response to the delegate if they want to handle this request.
   if ([self.delegate respondsToSelector:@selector(bottomNavigationBarController:
                                                         didSelectViewController:)]) {
     UIViewController *viewControllerToSelect = [self.viewControllers objectAtIndex:index];
     return [self.delegate bottomNavigationBarController:self
                              shouldSelectViewController:viewControllerToSelect];
   }

   return YES;
 }

 #pragma mark - Key Value Observation Methods

 - (void)observeValueForKeyPath:(NSString *)keyPath
                       ofObject:(id)object
                         change:(NSDictionary<NSKeyValueChangeKey, id> *)change
                        context:(void *)context {
   if (context != kObservationContext) {
     [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
     return;
   }

   id newValue = [change objectForKey:NSKeyValueChangeNewKey];
   if (object == self.navigationBar &&
       [keyPath isEqualToString:NSStringFromSelector(@selector(items))] &&
       [newValue isKindOfClass:[NSArray class]]) {
     [self didUpdateNavigationBarItemsWithNewValue:(NSArray *)newValue];
   }
 }

 - (void)didUpdateNavigationBarItemsWithNewValue:(NSArray *)items {
   // Verify tab bar items correspond with the view controllers tab bar items.
   if (items.count != self.viewControllers.count) {
     [[self unauthorizedItemsChangedException] raise];
   }

   // Verify each new and the view controller's tab bar items are equal.
   for (NSUInteger i = 0; i < self.viewControllers.count; i++) {
     UITabBarItem *viewControllerTabBarItem = [self.viewControllers objectAtIndex:i].tabBarItem;
     UITabBarItem *newTabBarItem = [items objectAtIndex:i];
     if (![viewControllerTabBarItem isEqual:newTabBarItem]) {
       [[self unauthorizedItemsChangedException] raise];
     }
   }
 }

 #pragma mark - Private Methods

 /**
  * Removes the given view controller from its parent view controller and its view from its superview
  */
 - (void)removeContentViewController:(UIViewController *)viewController {
   [viewController removeFromParentViewController];
   [viewController.view removeFromSuperview];
 }

 /**
  * Adds the given view controller to the view controller hierarchy and its view to the content
  * view.
  */
 - (void)addContentViewController:(UIViewController *)viewController {
   BOOL doesNotContainViewController = ![self.childViewControllers containsObject:viewController];
   if (viewController && doesNotContainViewController) {
     [self addChildViewController:viewController];
     [self.content addSubview:viewController.view];
     [self addConstraintsForContentView:viewController.view];
     [viewController didMoveToParentViewController:self];
   }
 }

 /**
  * Deselects the currently set item.  Sets the selectedIndex to NSNotFound, the naviagation bar's
  * selected item to nil, and the selectedViewController to nil.
  */
 - (void)deselectCurrentItem {
   _selectedIndex = NSNotFound;
   self.navigationBar.selectedItem = nil;

   // Force removal of the currently selected viewcontroller if there is one.
   self.selectedViewController = nil;
 }

 /**
  * Sets the selected view controller to the corresponding index and updates the navigation bar's
  * selected item.
  */
 - (void)updateViewsForSelectedIndex:(NSUInteger)index {
   // Update the selected view controller
   UIViewController *selectedViewController = [self.viewControllers objectAtIndex:index];
   self.selectedViewController = selectedViewController;

   // Update the navigation bar's selected item.
   self.navigationBar.selectedItem = selectedViewController.tabBarItem;
   [self setNeedsStatusBarAppearanceUpdate];
 }

 /**
  * Hooks up the constraints for the subviews of this controller.  Namely the content view and the
  * navigation bar.
  */
 - (void)loadConstraints {
   self.content.translatesAutoresizingMaskIntoConstraints = NO;
   self.navigationBar.translatesAutoresizingMaskIntoConstraints = NO;

   if (@available(iOS 9.0, *)) {
     [self loadiOS9PlusConstraints];
   } else {
     [self loadPreiOS9Constraints];
   }
 }

 - (void)loadPreiOS9Constraints {
   // Navigation Bar Constraints
   NSArray<NSLayoutConstraint *> *navigationBarConstraints = @[
     [NSLayoutConstraint constraintWithItem:self.navigationBar
                                  attribute:NSLayoutAttributeLeading
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:self.view
                                  attribute:NSLayoutAttributeLeading
                                 multiplier:1
                                   constant:0],
     [NSLayoutConstraint constraintWithItem:self.navigationBar
                                  attribute:NSLayoutAttributeTrailing
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:self.view
                                  attribute:NSLayoutAttributeTrailing
                                 multiplier:1
                                   constant:0],
     [NSLayoutConstraint constraintWithItem:self.navigationBar
                                  attribute:NSLayoutAttributeBottom
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:self.view
                                  attribute:NSLayoutAttributeBottom
                                 multiplier:1
                                   constant:0],
     [NSLayoutConstraint constraintWithItem:self.navigationBar
                                  attribute:NSLayoutAttributeTop
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:self.content
                                  attribute:NSLayoutAttributeBottom
                                 multiplier:1
                                   constant:0]
   ];

   // Content View Constraints
   NSArray<NSLayoutConstraint *> *contentConstraints = @[
     [NSLayoutConstraint constraintWithItem:self.content
                                  attribute:NSLayoutAttributeLeading
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:self.view
                                  attribute:NSLayoutAttributeLeading
                                 multiplier:1
                                   constant:0],
     [NSLayoutConstraint constraintWithItem:self.content
                                  attribute:NSLayoutAttributeTrailing
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:self.view
                                  attribute:NSLayoutAttributeTrailing
                                 multiplier:1
                                   constant:0],
     [NSLayoutConstraint constraintWithItem:self.content
                                  attribute:NSLayoutAttributeTop
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:self.view
                                  attribute:NSLayoutAttributeTop
                                 multiplier:1
                                   constant:0]
   ];

   [NSLayoutConstraint activateConstraints:navigationBarConstraints];
   [NSLayoutConstraint activateConstraints:contentConstraints];
 }

 - (void)loadiOS9PlusConstraints {
   if (@available(iOS 9.0, *)) {
     // Navigation Bar Constraints
     [self.view.leftAnchor constraintEqualToAnchor:self.navigationBar.leftAnchor].active = YES;
     [self.view.rightAnchor constraintEqualToAnchor:self.navigationBar.rightAnchor].active = YES;
     [self.navigationBar.topAnchor constraintEqualToAnchor:self.content.bottomAnchor].active = YES;
     [self.navigationBar.bottomAnchor constraintEqualToAnchor:self.view.bottomAnchor].active = YES;
   }

   if (@available(iOS 11.0, *)) {
     [self.navigationBar.barItemsBottomAnchor
         constraintEqualToAnchor:self.view.safeAreaLayoutGuide.bottomAnchor]
         .active = YES;
   }

   if (@available(iOS 9.0, *)) {
     // Content View Constraints
     [self.view.leftAnchor constraintEqualToAnchor:self.content.leftAnchor].active = YES;
     [self.view.rightAnchor constraintEqualToAnchor:self.content.rightAnchor].active = YES;

     [self.view.topAnchor constraintEqualToAnchor:self.content.topAnchor].active = YES;
   }
 }

 /**
  * Pins the given view to the edges of the content view.
  */
 - (void)addConstraintsForContentView:(UIView *)view {
   view.translatesAutoresizingMaskIntoConstraints = NO;
   if (@available(iOS 9.0, *)) {
     [view.leadingAnchor constraintEqualToAnchor:self.content.leadingAnchor].active = YES;
     [view.trailingAnchor constraintEqualToAnchor:self.content.trailingAnchor].active = YES;
     [view.topAnchor constraintEqualToAnchor:self.content.topAnchor].active = YES;
     [view.bottomAnchor constraintEqualToAnchor:self.content.bottomAnchor].active = YES;
   } else {
     [NSLayoutConstraint constraintWithItem:self.content
                                  attribute:NSLayoutAttributeLeading
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:view
                                  attribute:NSLayoutAttributeLeading
                                 multiplier:1
                                   constant:0]
         .active = YES;
     [NSLayoutConstraint constraintWithItem:self.content
                                  attribute:NSLayoutAttributeTrailing
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:view
                                  attribute:NSLayoutAttributeTrailing
                                 multiplier:1
                                   constant:0]
         .active = YES;
     [NSLayoutConstraint constraintWithItem:self.content
                                  attribute:NSLayoutAttributeTop
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:view
                                  attribute:NSLayoutAttributeTop
                                 multiplier:1
                                   constant:0]
         .active = YES;
     [NSLayoutConstraint constraintWithItem:self.content
                                  attribute:NSLayoutAttributeBottom
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:view
                                  attribute:NSLayoutAttributeBottom
                                 multiplier:1
                                   constant:0]
         .active = YES;
   }
 }

 /** Maps an array of view controllers to their corrisponding tab bar items **/
 - (NSArray<UITabBarItem *> *)tabBarItemsForViewControllers:
     (NSArray<UIViewController *> *)viewControllers {
   NSMutableArray<UITabBarItem *> *tabBarItems = [NSMutableArray array];
   for (UIViewController *viewController in viewControllers) {
     UITabBarItem *tabBarItem = viewController.tabBarItem;
     NSAssert(tabBarItem != nil,
              @"%@'s tabBarItem is nil. Please ensure that each view controller "
               "added to %@ has set its tab bar item property",
              viewController, NSStringFromClass([self class]));

     if (tabBarItem) {
       [tabBarItems addObject:tabBarItem];
     }
   }

   return tabBarItems;
 }

 /**
  * Returns an exception for when the navigation bar's items are changed from outside of this class.
  */
 - (NSException *)unauthorizedItemsChangedException {
   NSString *reason = [NSString
       stringWithFormat:
           @"Attempting to set %@'s navigation bar items.  Please instead use setViewControllers:",
           NSStringFromClass([self class])];
   return [NSException exceptionWithName:NSInternalInconsistencyException
                                  reason:reason
                                userInfo:nil];
 }

 @end
	// Copyright 2018-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 "MDCBottomNavigationBarController.h"

	// A context for Key Value Observing
	static void const kObservationContext = (void )&kObservationContext;

	@interface MDCBottomNavigationBarController ()

	/ The view that hosts the content for the selected view controller /
	@property(nonatomic, strong) UIView *content;

	@end

	@implementation MDCBottomNavigationBarController

	- (instancetype)init {
	self = [super init];
	if (self) {
	_navigationBar = [[MDCBottomNavigationBar alloc] init];
	_content = [[UIView alloc] init];
	_viewControllers = @[];
	_selectedIndex = NSNotFound;

	[_navigationBar addObserver:self
	forKeyPath:NSStringFromSelector(@selector(items))
	options:NSKeyValueObservingOptionNew
	context:kObservationContext];
	}

	return self;
	}

	- (void)dealloc {
	[_navigationBar removeObserver:self forKeyPath:NSStringFromSelector(@selector(items))];
	}

	- (void)viewDidLoad {
	[super viewDidLoad];

	self.navigationBar.delegate = self;

	// Add subviews and create their constraints
	[self.view addSubview:self.content];
	[self.view addSubview:self.navigationBar];
	[self loadConstraints];
	}

	- (void)setSelectedViewController:(nullable UIViewController *)selectedViewController {
	// Assert that the given VC is one of our view controllers or it is nil (we are unselecting)
	NSAssert(
	selectedViewController == nil \|\| [self.viewControllers containsObject:selectedViewController],
	@"Attempting to set BottomBarViewControllers to a view controller it does not contain");

	// Early return if we are already set to the given VC
	if (self.selectedViewController == selectedViewController) {
	return;
	}

	// Remove current VC and add new one.
	[self removeContentViewController:self.selectedViewController];
	[self addContentViewController:selectedViewController];

	// Set the iVar and update selected index
	_selectedViewController = selectedViewController;
	self.selectedIndex = [self.viewControllers indexOfObject:selectedViewController];
	}

	- (void)setSelectedIndex:(NSUInteger)selectedIndex {
	// If we are setting to NSNotFound deselect the items
	if (selectedIndex == NSNotFound) {
	[self deselectCurrentItem];
	return;
	}

	BOOL outOfBounds = selectedIndex >= self.viewControllers.count \|\|
	selectedIndex >= self.navigationBar.items.count;

	NSAssert(!outOfBounds,
	@"Attempting to set BottomBarViewController's selectedIndex to %li. This"
	" value is not within the bounds of the navigation bar's items and/or view controllers",
	(unsigned long)selectedIndex);

	// Early return if we are out of bounds or if the the index is already selected.
	if (outOfBounds \|\| selectedIndex == _selectedIndex) {
	return;
	}

	// Update the selected index value and views.
	_selectedIndex = selectedIndex;
	[self updateViewsForSelectedIndex:selectedIndex];
	}

	- (void)setViewControllers:(NSArray<UIViewController > )viewControllers {
	[self deselectCurrentItem];
	NSArray *viewControllersCopy = [viewControllers copy];
	_viewControllers = viewControllersCopy;
	self.navigationBar.items = [self tabBarItemsForViewControllers:viewControllersCopy];

	self.selectedViewController = viewControllersCopy.firstObject;
	}

	- (UIViewController *)childViewControllerForStatusBarStyle {
	return self.selectedViewController;
	}

	- (UIViewController *)childViewControllerForStatusBarHidden {
	return self.selectedViewController;
	}

	#pragma mark - MDCBottomNavigationBarDelegate

	- (void)bottomNavigationBar:(MDCBottomNavigationBar *)bottomNavigationBar
	didSelectItem:(UITabBarItem *)item {
	// Early return if we cannot find the view controller.
	NSUInteger index = [self.navigationBar.items indexOfObject:item];
	if (index >= [self.viewControllers count] \|\| index == NSNotFound) {
	return;
	}

	// Update selected view controller
	UIViewController *selectedViewController = [self.viewControllers objectAtIndex:index];
	self.selectedViewController = selectedViewController;

	// Notify the delegate.
	if ([self.delegate respondsToSelector:@selector(bottomNavigationBarController:
	didSelectViewController:)]) {
	[self.delegate bottomNavigationBarController:self
	didSelectViewController:selectedViewController];
	}
	}

	- (BOOL)bottomNavigationBar:(MDCBottomNavigationBar *)bottomNavigationBar
	shouldSelectItem:(UITabBarItem *)item {
	NSUInteger index = [self.navigationBar.items indexOfObject:item];
	if (index >= [self.viewControllers count] \|\| index == NSNotFound) {
	return NO;
	}

	// Pass the response to the delegate if they want to handle this request.
	if ([self.delegate respondsToSelector:@selector(bottomNavigationBarController:
	didSelectViewController:)]) {
	UIViewController *viewControllerToSelect = [self.viewControllers objectAtIndex:index];
	return [self.delegate bottomNavigationBarController:self
	shouldSelectViewController:viewControllerToSelect];
	}

	return YES;
	}

	#pragma mark - Key Value Observation Methods

	- (void)observeValueForKeyPath:(NSString *)keyPath
	ofObject:(id)object
	change:(NSDictionary<NSKeyValueChangeKey, id> *)change
	context:(void *)context {
	if (context != kObservationContext) {
	[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
	return;
	}

	id newValue = [change objectForKey:NSKeyValueChangeNewKey];
	if (object == self.navigationBar &&
	[keyPath isEqualToString:NSStringFromSelector(@selector(items))] &&
	[newValue isKindOfClass:[NSArray class]]) {
	[self didUpdateNavigationBarItemsWithNewValue:(NSArray *)newValue];
	}
	}

	- (void)didUpdateNavigationBarItemsWithNewValue:(NSArray *)items {
	// Verify tab bar items correspond with the view controllers tab bar items.
	if (items.count != self.viewControllers.count) {
	[[self unauthorizedItemsChangedException] raise];
	}

	// Verify each new and the view controller's tab bar items are equal.
	for (NSUInteger i = 0; i < self.viewControllers.count; i++) {
	UITabBarItem *viewControllerTabBarItem = [self.viewControllers objectAtIndex:i].tabBarItem;
	UITabBarItem *newTabBarItem = [items objectAtIndex:i];
	if (![viewControllerTabBarItem isEqual:newTabBarItem]) {
	[[self unauthorizedItemsChangedException] raise];
	}
	}
	}

	#pragma mark - Private Methods

	/**
	* Removes the given view controller from its parent view controller and its view from its superview
	*/
	- (void)removeContentViewController:(UIViewController *)viewController {
	[viewController removeFromParentViewController];
	[viewController.view removeFromSuperview];
	}

	/**
	* Adds the given view controller to the view controller hierarchy and its view to the content
	* view.
	*/
	- (void)addContentViewController:(UIViewController *)viewController {
	BOOL doesNotContainViewController = ![self.childViewControllers containsObject:viewController];
	if (viewController && doesNotContainViewController) {
	[self addChildViewController:viewController];
	[self.content addSubview:viewController.view];
	[self addConstraintsForContentView:viewController.view];
	[viewController didMoveToParentViewController:self];
	}
	}

	/**
	* Deselects the currently set item. Sets the selectedIndex to NSNotFound, the naviagation bar's
	* selected item to nil, and the selectedViewController to nil.
	*/
	- (void)deselectCurrentItem {
	_selectedIndex = NSNotFound;
	self.navigationBar.selectedItem = nil;

	// Force removal of the currently selected viewcontroller if there is one.
	self.selectedViewController = nil;
	}

	/**
	* Sets the selected view controller to the corresponding index and updates the navigation bar's
	* selected item.
	*/
	- (void)updateViewsForSelectedIndex:(NSUInteger)index {
	// Update the selected view controller
	UIViewController *selectedViewController = [self.viewControllers objectAtIndex:index];
	self.selectedViewController = selectedViewController;

	// Update the navigation bar's selected item.
	self.navigationBar.selectedItem = selectedViewController.tabBarItem;
	[self setNeedsStatusBarAppearanceUpdate];
	}

	/**
	* Hooks up the constraints for the subviews of this controller. Namely the content view and the
	* navigation bar.
	*/
	- (void)loadConstraints {
	self.content.translatesAutoresizingMaskIntoConstraints = NO;
	self.navigationBar.translatesAutoresizingMaskIntoConstraints = NO;

	if (@available(iOS 9.0, *)) {
	[self loadiOS9PlusConstraints];
	} else {
	[self loadPreiOS9Constraints];
	}
	}

	- (void)loadPreiOS9Constraints {
	// Navigation Bar Constraints
	NSArray<NSLayoutConstraint > navigationBarConstraints = @[
	[NSLayoutConstraint constraintWithItem:self.navigationBar
	attribute:NSLayoutAttributeLeading
	relatedBy:NSLayoutRelationEqual
	toItem:self.view
	attribute:NSLayoutAttributeLeading
	multiplier:1
	constant:0],
	[NSLayoutConstraint constraintWithItem:self.navigationBar
	attribute:NSLayoutAttributeTrailing
	relatedBy:NSLayoutRelationEqual
	toItem:self.view
	attribute:NSLayoutAttributeTrailing
	multiplier:1
	constant:0],
	[NSLayoutConstraint constraintWithItem:self.navigationBar
	attribute:NSLayoutAttributeBottom
	relatedBy:NSLayoutRelationEqual
	toItem:self.view
	attribute:NSLayoutAttributeBottom
	multiplier:1
	constant:0],
	[NSLayoutConstraint constraintWithItem:self.navigationBar
	attribute:NSLayoutAttributeTop
	relatedBy:NSLayoutRelationEqual
	toItem:self.content
	attribute:NSLayoutAttributeBottom
	multiplier:1
	constant:0]
	];

	// Content View Constraints
	NSArray<NSLayoutConstraint > contentConstraints = @[
	[NSLayoutConstraint constraintWithItem:self.content
	attribute:NSLayoutAttributeLeading
	relatedBy:NSLayoutRelationEqual
	toItem:self.view
	attribute:NSLayoutAttributeLeading
	multiplier:1
	constant:0],
	[NSLayoutConstraint constraintWithItem:self.content
	attribute:NSLayoutAttributeTrailing
	relatedBy:NSLayoutRelationEqual
	toItem:self.view
	attribute:NSLayoutAttributeTrailing
	multiplier:1
	constant:0],
	[NSLayoutConstraint constraintWithItem:self.content
	attribute:NSLayoutAttributeTop
	relatedBy:NSLayoutRelationEqual
	toItem:self.view
	attribute:NSLayoutAttributeTop
	multiplier:1
	constant:0]
	];

	[NSLayoutConstraint activateConstraints:navigationBarConstraints];
	[NSLayoutConstraint activateConstraints:contentConstraints];
	}

	- (void)loadiOS9PlusConstraints {
	if (@available(iOS 9.0, *)) {
	// Navigation Bar Constraints
	[self.view.leftAnchor constraintEqualToAnchor:self.navigationBar.leftAnchor].active = YES;
	[self.view.rightAnchor constraintEqualToAnchor:self.navigationBar.rightAnchor].active = YES;
	[self.navigationBar.topAnchor constraintEqualToAnchor:self.content.bottomAnchor].active = YES;
	[self.navigationBar.bottomAnchor constraintEqualToAnchor:self.view.bottomAnchor].active = YES;
	}

	if (@available(iOS 11.0, *)) {
	[self.navigationBar.barItemsBottomAnchor
	constraintEqualToAnchor:self.view.safeAreaLayoutGuide.bottomAnchor]
	.active = YES;
	}

	if (@available(iOS 9.0, *)) {
	// Content View Constraints
	[self.view.leftAnchor constraintEqualToAnchor:self.content.leftAnchor].active = YES;
	[self.view.rightAnchor constraintEqualToAnchor:self.content.rightAnchor].active = YES;

	[self.view.topAnchor constraintEqualToAnchor:self.content.topAnchor].active = YES;
	}
	}

	/**
	* Pins the given view to the edges of the content view.
	*/
	- (void)addConstraintsForContentView:(UIView *)view {
	view.translatesAutoresizingMaskIntoConstraints = NO;
	if (@available(iOS 9.0, *)) {
	[view.leadingAnchor constraintEqualToAnchor:self.content.leadingAnchor].active = YES;
	[view.trailingAnchor constraintEqualToAnchor:self.content.trailingAnchor].active = YES;
	[view.topAnchor constraintEqualToAnchor:self.content.topAnchor].active = YES;
	[view.bottomAnchor constraintEqualToAnchor:self.content.bottomAnchor].active = YES;
	} else {
	[NSLayoutConstraint constraintWithItem:self.content
	attribute:NSLayoutAttributeLeading
	relatedBy:NSLayoutRelationEqual
	toItem:view
	attribute:NSLayoutAttributeLeading
	multiplier:1
	constant:0]
	.active = YES;
	[NSLayoutConstraint constraintWithItem:self.content
	attribute:NSLayoutAttributeTrailing
	relatedBy:NSLayoutRelationEqual
	toItem:view
	attribute:NSLayoutAttributeTrailing
	multiplier:1
	constant:0]
	.active = YES;
	[NSLayoutConstraint constraintWithItem:self.content
	attribute:NSLayoutAttributeTop
	relatedBy:NSLayoutRelationEqual
	toItem:view
	attribute:NSLayoutAttributeTop
	multiplier:1
	constant:0]
	.active = YES;
	[NSLayoutConstraint constraintWithItem:self.content
	attribute:NSLayoutAttributeBottom
	relatedBy:NSLayoutRelationEqual
	toItem:view
	attribute:NSLayoutAttributeBottom
	multiplier:1
	constant:0]
	.active = YES;
	}
	}

	/ Maps an array of view controllers to their corrisponding tab bar items /
	- (NSArray<UITabBarItem > )tabBarItemsForViewControllers:
	(NSArray<UIViewController > )viewControllers {
	NSMutableArray<UITabBarItem > tabBarItems = [NSMutableArray array];
	for (UIViewController *viewController in viewControllers) {
	UITabBarItem *tabBarItem = viewController.tabBarItem;
	NSAssert(tabBarItem != nil,
	@"%@'s tabBarItem is nil. Please ensure that each view controller "
	"added to %@ has set its tab bar item property",
	viewController, NSStringFromClass([self class]));

	if (tabBarItem) {
	[tabBarItems addObject:tabBarItem];
	}
	}

	return tabBarItems;
	}

	/**
	* Returns an exception for when the navigation bar's items are changed from outside of this class.
	*/
	- (NSException *)unauthorizedItemsChangedException {
	NSString *reason = [NSString
	stringWithFormat:
	@"Attempting to set %@'s navigation bar items. Please instead use setViewControllers:",
	NSStringFromClass([self class])];
	return [NSException exceptionWithName:NSInternalInconsistencyException
	reason:reason
	userInfo:nil];
	}

	@end