[tab_grid] New collection view layout.

ios/clean/chrome/browser/ui/tab_grid/tab_grid_collection_view_layout.mm

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import "ios/clean/chrome/browser/ui/tab_grid/tab_grid_collection_view_layout.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+namespace {
+const CGFloat kMinTabWidth = 200.0f;
+const CGFloat kMaxTabWidth = 250.0f;
+const CGFloat kInterTabSpacing = 20.0f;
+const UIEdgeInsets kSectionInset = {20.0f, 20.0f, 20.0f, 20.0f};
+}
+
+@implementation TabGridCollectionViewLayout
+
+#pragma mark - UICollectionViewLayout
+
+// This is called whenever the layout is invalidated, including during rotation.
+- (void)prepareLayout {
+  [super prepareLayout];
+  [self updateLayoutWithBounds:[[self collectionView] bounds].size];
+}
+
+#pragma mark - Private
+
+// This sets the appropriate itemSize given the bounds of the collection view.
+- (void)updateLayoutWithBounds:(CGSize)boundsSize {
+  int columns = static_cast<int>(floor(boundsSize.width - kInterTabSpacing) /
+                                 (kMinTabWidth + kInterTabSpacing));
+  CGFloat tabWidth =
+      (boundsSize.width - kInterTabSpacing * (columns + 1)) / columns;
+  while (columns < 2 || tabWidth > kMaxTabWidth) {
+    columns++;
+    tabWidth = (boundsSize.width - kInterTabSpacing * (columns + 1)) / columns;
+  }
+  self.itemSize = CGSizeMake(tabWidth, tabWidth);
+  self.sectionInset = kSectionInset;
+  self.minimumLineSpacing = kInterTabSpacing;
+  self.minimumInteritemSpacing = kInterTabSpacing;
+}
+
+@end