components/Tabs/src/MDCTabBarViewController.m

// 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 "MDCTabBarViewController.h"

#import "MaterialShadowElevations.h"
#import "MaterialShadowLayer.h"
#import "MDCTabBar.h"
#import "MDCTabBarAlignment.h"
#import "MDCTabBarControllerDelegate.h"
#import "MDCTabBarItemAppearance.h"

const CGFloat MDCTabBarViewControllerAnimationDuration = (CGFloat)0.3;

/**
 * View to host shadow for the tab bar.
 */
@interface MDCTabBarShadowView : UIView
@end

@implementation MDCTabBarShadowView

+ (Class)layerClass {
  return [MDCShadowLayer class];
}

- (instancetype)initWithFrame:(CGRect)frame {
  self = [super initWithFrame:frame];
  if (self) {
    [self commonMDCTabBarShadowViewInitialization];
  }
  return self;
}

- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder {
  self = [super initWithCoder:aDecoder];
  if (self) {
    [self commonMDCTabBarShadowViewInitialization];
  }
  return self;
}

- (void)commonMDCTabBarShadowViewInitialization {
  MDCShadowLayer *shadowLayer = (MDCShadowLayer *)self.layer;
  [shadowLayer setElevation:MDCShadowElevationMenu];
}

@end

@interface MDCTabBarViewController ()

@property(nonatomic, readwrite, nonnull) MDCTabBar *tabBar;

@end

@implementation MDCTabBarViewController {
  /** For showing/hiding, Animation needs to know where it wants to end up. */
  BOOL _tabBarWantsToBeHidden;
  MDCTabBarShadowView *_tabBarShadow;
}

- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder {
  self = [super initWithCoder:aDecoder];
  if (self) {
    [self commonInit];
  }
  return self;
}

- (instancetype)init {
  self = [super init];
  if (self) {
    [self commonInit];
  }
  return self;
}

- (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
  self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
  if (self) {
    [self commonInit];
  }
  return self;
}

- (void)commonInit {
  if (self.tabBar) {
    return;
  }
  MDCTabBar *tabBar = [[MDCTabBar alloc] initWithFrame:CGRectZero];
  tabBar.alignment = MDCTabBarAlignmentJustified;
  tabBar.delegate = self;
  self.tabBar = tabBar;
  _tabBarShadow = [[MDCTabBarShadowView alloc] initWithFrame:CGRectZero];
}

- (void)viewDidLoad {
  [super viewDidLoad];
  UIView *view = self.view;
  view.clipsToBounds = YES;
  view.autoresizingMask = UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleWidth |
                          UIViewAutoresizingFlexibleRightMargin |
                          UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleHeight |
                          UIViewAutoresizingFlexibleBottomMargin;
  [view addSubview:_tabBarShadow];
  [view addSubview:self.tabBar];
  [self updateOldViewControllers:nil to:_viewControllers];
  [self updateOldSelectedViewController:nil to:_selectedViewController];
}

- (void)viewDidLayoutSubviews {
  [super viewDidLayoutSubviews];
  [self updateLayout];
}

- (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection {
  [super traitCollectionDidChange:previousTraitCollection];

  if (self.traitCollectionDidChangeBlock) {
    self.traitCollectionDidChangeBlock(self, previousTraitCollection);
  }
}

#pragma mark - Properties

- (BOOL)tabBarHidden {
  return self.tabBar.hidden;
}

- (void)setTabBarHidden:(BOOL)hidden {
  [self setTabBarHidden:hidden animated:NO];
}

- (void)setTabBarHidden:(BOOL)hidden animated:(BOOL)animated {
  if (_tabBar.hidden != hidden) {
    if (animated) {
      // Before entering our animation block put the view's layout in a good state.
      [self.view layoutIfNeeded];
    }
    _tabBarWantsToBeHidden = hidden;
    // Hiding the tab bar has the side effect of growing the current viewController to use that
    // space. This does that in its layoutSubViews.
    [self.view setNeedsLayout];
    if (animated) {
      if (!hidden) {
        // If we are showing, set the state before the animation.
        _tabBar.hidden = hidden;
        _tabBarShadow.hidden = hidden;
      }
      [UIView animateWithDuration:MDCTabBarViewControllerAnimationDuration
          animations:^{
            [self.view layoutIfNeeded];
          }
          completion:^(__unused BOOL finished) {
            // If we are hiding, set the state after the animation.
            self.tabBar.hidden = hidden;
            self->_tabBarShadow.hidden = hidden;
          }];
    } else {
      _tabBar.hidden = hidden;
      _tabBarShadow.hidden = hidden;
    }
  }
}

- (void)setViewControllers:(NSArray<UIViewController *> *)viewControllers {
  NSArray<UIViewController *> *oldViewControllers = _viewControllers;
  _viewControllers = [viewControllers copy];
  [self updateOldViewControllers:oldViewControllers to:viewControllers];
}

- (void)updateOldViewControllers:(NSArray<UIViewController *> *)oldViewControllers
                              to:(NSArray<UIViewController *> *)viewControllers {
  if (!self.isViewLoaded || [oldViewControllers isEqual:viewControllers]) {
    return;
  }

  if (![oldViewControllers isEqual:viewControllers]) {
    // For all view controllers that this is removing, follow UIViewController.h's rules for
    // for removing a child view controller. See the comments in UIViewController.h for more
    // information.
    for (UIViewController *viewController in oldViewControllers) {
      if (![viewControllers containsObject:viewController]) {
        [viewController willMoveToParentViewController:nil];
        if (viewController.isViewLoaded) {
          [viewController.view removeFromSuperview];
        }
        [viewController removeFromParentViewController];
      }
    }
    // Show the newly-visible view controller.
    [self updateTabBarItems];
  }
}

- (void)updateOldSelectedViewController:(nullable UIViewController *)oldSelectedViewController
                                     to:(nullable UIViewController *)selectedViewController {
  if (!self.isViewLoaded || oldSelectedViewController == selectedViewController) {
    return;
  }
  if (selectedViewController) {
    NSAssert([_viewControllers containsObject:selectedViewController], @"not one of us.");
  }

  [oldSelectedViewController willMoveToParentViewController:nil];
  [oldSelectedViewController.view removeFromSuperview];
  [oldSelectedViewController removeFromParentViewController];

  if (![self.childViewControllers containsObject:selectedViewController]) {
    [self addChildViewController:selectedViewController];
    UIView *view = selectedViewController.view;
    [self.view addSubview:view];
    [selectedViewController didMoveToParentViewController:self];
  }
  [self updateTabBarItems];
  BOOL animated = NO;
  [oldSelectedViewController beginAppearanceTransition:NO animated:animated];
  [selectedViewController beginAppearanceTransition:YES animated:animated];

  [self transitionViewsWithoutAnimationFromViewController:oldSelectedViewController
                                         toViewController:selectedViewController];

  [oldSelectedViewController endAppearanceTransition];
  [selectedViewController endAppearanceTransition];

  if (selectedViewController) {
    self.tabBar.selectedItem = selectedViewController.tabBarItem;
  }
  [self setNeedsStatusBarAppearanceUpdate];

  UIAccessibilityPostNotification(UIAccessibilityScreenChangedNotification, nil);
}

- (void)setSelectedViewController:(nullable UIViewController *)selectedViewController {
  UIViewController *oldSelectedViewController = _selectedViewController;
  _selectedViewController = selectedViewController;
  [self updateOldSelectedViewController:oldSelectedViewController to:selectedViewController];
}

#pragma mark - private

// Encapsulate the actual view handling.
- (void)transitionViewsWithoutAnimationFromViewController:(UIViewController *)from
                                         toViewController:(UIViewController *)to {
  [self.view setNeedsLayout];
  [self.view layoutIfNeeded];
  from.view.hidden = YES;
  to.view.hidden = NO;
}

// Either this has just come into existence or its array of viewControllers has changed.
// Update the TabBar from the array of viewControllers.
- (void)updateTabBarItems {
  NSMutableArray<UITabBarItem *> *items = [NSMutableArray array];
  BOOL hasTitles = NO;
  BOOL hasImages = NO;
  for (UIViewController *child in _viewControllers) {
    UITabBarItem *tabBarItem = child.tabBarItem;
    [items addObject:tabBarItem];
    if (tabBarItem.title.length) {
      hasTitles = YES;
    }
    if (tabBarItem.image) {
      hasImages = YES;
    }
  }
  // This class preserves the invariant that if the selected controller is not nil, it is contained
  // in the array of viewControllers.
  if (![_viewControllers containsObject:_selectedViewController]) {
    self.selectedViewController = nil;
  }
  self.tabBar.items = items;
  // The default height of the tab bar depends on the underlying UITabBarItems of
  // the viewControllers.
  if (hasImages && hasTitles) {
    self.tabBar.itemAppearance = MDCTabBarItemAppearanceTitledImages;
  } else if (hasImages) {
    self.tabBar.itemAppearance = MDCTabBarItemAppearanceImages;
  } else {
    self.tabBar.itemAppearance = MDCTabBarItemAppearanceTitles;
  }
  [self.view bringSubviewToFront:_tabBarShadow];
  [self.view bringSubviewToFront:self.tabBar];
}

- (nullable UIViewController *)controllerWithView:(nullable UIView *)view {
  for (UIViewController *child in _viewControllers) {
    if (child.view == view) {
      return child;
    }
  }
  return nil;
}

- (void)updateLayout {
  CGRect bounds = self.view.bounds;
  CGFloat tabBarHeight = [[_tabBar class] defaultHeightForBarPosition:UIBarPositionBottom
                                                       itemAppearance:_tabBar.itemAppearance];
  if (@available(iOS 11.0, *)) {
    tabBarHeight += self.view.safeAreaInsets.bottom;
  }

  CGRect currentViewFrame = bounds;
  CGRect tabBarFrame = CGRectMake(bounds.origin.x, bounds.origin.y + bounds.size.height,
                                  bounds.size.width, tabBarHeight);
  if (!_tabBarWantsToBeHidden) {
    CGRectDivide(bounds, &tabBarFrame, &currentViewFrame, tabBarHeight, CGRectMaxYEdge);
  }
  _tabBar.frame = tabBarFrame;
  _tabBarShadow.frame = tabBarFrame;
  _selectedViewController.view.frame = currentViewFrame;
}

#pragma mark -  MDCTabBarDelegate

- (BOOL)tabBar:(UITabBar *)tabBar shouldSelectItem:(UITabBarItem *)item {
  if ([_delegate respondsToSelector:@selector(tabBarController:shouldSelectViewController:)]) {
    NSUInteger index = [tabBar.items indexOfObject:item];
    if (index < _viewControllers.count) {
      UIViewController *newSelected = _viewControllers[index];
      return [_delegate tabBarController:self shouldSelectViewController:newSelected];
    }
  }
  return YES;
}

- (void)tabBar:(UITabBar *)tabBar didSelectItem:(UITabBarItem *)item {
  NSUInteger index = [tabBar.items indexOfObject:item];
  if (index < _viewControllers.count) {
    UIViewController *newSelected = _viewControllers[index];
    if (newSelected != self.selectedViewController) {
      self.selectedViewController = newSelected;
    }
    if ([_delegate respondsToSelector:@selector(tabBarController:didSelectViewController:)]) {
      [_delegate tabBarController:self didSelectViewController:newSelected];
    }
  }
}

- (UIBarPosition)positionForBar:(id<UIBarPositioning>)bar {
  if (_tabBar == bar) {
    return UIBarPositionBottom;
  } else {
    return UIBarPositionAny;
  }
}

#pragma mark - UIViewController status bar

- (nullable UIViewController *)childViewControllerForStatusBarStyle {
  return _selectedViewController;
}

- (nullable UIViewController *)childViewControllerForStatusBarHidden {
  return _selectedViewController;
}

@end