[Tabs] Make -itemAtIndexPath: safe (#7555)

This PR makes the -itemAtIndexPath: method on MDCItemBar safe against out of bounds exceptions.

Related to #6275.

components/Tabs/src/private/MDCItemBar.m

@@ -357,7 +357,9 @@ - (BOOL)collectionView:(UICollectionView *)collectionView
     id<MDCItemBarDelegate> delegate = self.delegate;
     if ([delegate respondsToSelector:@selector(itemBar:shouldSelectItem:)]) {
       UITabBarItem *item = [self itemAtIndexPath:indexPath];
-      return [delegate itemBar:self shouldSelectItem:item];
+      if (item) {
+        return [delegate itemBar:self shouldSelectItem:item];
+      }
     }
   }
   return YES;
@@ -368,6 +370,9 @@ - (void)collectionView:(UICollectionView *)collectionView
   if (_collectionView == collectionView) {
     // Update selected item.
     UITabBarItem *item = [self itemAtIndexPath:indexPath];
+    if (!item) {
+      return;
+    }
     _selectedItem = item;
 
     // Notify delegate of new selection.
@@ -400,10 +405,12 @@ - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
 
   MDCItemBarCell *itemCell = [collectionView dequeueReusableCellWithReuseIdentifier:kItemReuseID
                                                                        forIndexPath:indexPath];
-  UITabBarItem *item = [self itemAtIndexPath:indexPath];
-
   [self configureCell:itemCell];
-  [itemCell updateWithItem:item atIndex:indexPath.item count:_items.count];
+
+  UITabBarItem *item = [self itemAtIndexPath:indexPath];
+  if (item) {
+    [itemCell updateWithItem:item atIndex:indexPath.item count:_items.count];
+  }
 
   return itemCell;
 }
@@ -416,6 +423,9 @@ - (CGSize)collectionView:(UICollectionView *)collectionView
   NSParameterAssert(_collectionView == collectionView);
 
   UITabBarItem *item = [self itemAtIndexPath:indexPath];
+  if (!item) {
+    return CGSizeZero;
+  }
 
   const CGFloat itemHeight = CGRectGetHeight(self.bounds);
   CGSize size = CGSizeMake(CGFLOAT_MAX, itemHeight);
@@ -513,7 +523,11 @@ - (void)selectItemAtIndex:(NSUInteger)index animated:(BOOL)animated {
 }
 
 - (UITabBarItem *)itemAtIndexPath:(NSIndexPath *)indexPath {
-  return _items[indexPath.item];
+  if (indexPath && indexPath.section == 0 && indexPath.item >= 0 &&
+      (NSInteger)_items.count > indexPath.item) {
+    return _items[indexPath.item];
+  }
+  return nil;
 }
 
 - (NSInteger)indexForItem:(nullable UITabBarItem *)item {
@@ -628,6 +642,9 @@ - (void)updateSelectionIndicatorToIndex:(NSInteger)index {
 
   // Construct a context object describing the selected tab.
   UITabBarItem *item = [self itemAtIndexPath:indexPath];
+  if (!item) {
+    return;
+  }
   MDCTabBarPrivateIndicatorContext *context =
       [[MDCTabBarPrivateIndicatorContext alloc] initWithItem:item
                                                       bounds:selectionIndicatorBounds

components/Tabs/tests/unit/MDCItemBarTests.m

@@ -0,0 +1,51 @@
+// 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 <XCTest/XCTest.h>
+
+#import "MDCItemBar.h"
+
+@interface MDCItemBar (Testing)
+- (UITabBarItem *)itemAtIndexPath:(NSIndexPath *)indexPath;
+@end
+
+@interface MDCItemBarTests : XCTestCase
+@end
+
+@implementation MDCItemBarTests
+
+- (void)testItemAtIndexPath {
+  // Given
+  MDCItemBar *itemBar = [[MDCItemBar alloc] init];
+  UITabBarItem *item1 = [[UITabBarItem alloc] initWithTitle:@"first tab" image:nil tag:0];
+  UITabBarItem *item2 = [[UITabBarItem alloc] initWithTitle:@"second tab" image:nil tag:0];
+
+  // When
+  itemBar.items = @[ item1, item2 ];
+
+  // Then
+  NSIndexPath *indexPathForFirstItem = [NSIndexPath indexPathForItem:0 inSection:0];
+  XCTAssertNotNil([itemBar itemAtIndexPath:indexPathForFirstItem]);
+  NSIndexPath *indexPathForSecondItem = [NSIndexPath indexPathForItem:1 inSection:0];
+  XCTAssertNotNil([itemBar itemAtIndexPath:indexPathForSecondItem]);
+  NSIndexPath *indexPathForThirdItem = [NSIndexPath indexPathForItem:2 inSection:0];
+  XCTAssertNil([itemBar itemAtIndexPath:indexPathForThirdItem]);
+  NSIndexPath *indexPathWithNonZeroSection = [NSIndexPath indexPathForItem:0 inSection:1];
+  XCTAssertNil([itemBar itemAtIndexPath:indexPathWithNonZeroSection]);
+  NSIndexPath *indexPathWithItemEqualToNegativeOne = [NSIndexPath indexPathForItem:-1 inSection:0];
+  XCTAssertNil([itemBar itemAtIndexPath:indexPathWithItemEqualToNegativeOne]);
+}
+
+@end