Upstream Chrome on iOS source code [5/11].

ios/chrome/browser/ui/ntp/centering_scrollview.mm

+// Copyright 2012 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/chrome/browser/ui/ntp/centering_scrollview.h"
+
+#include "base/logging.h"
+#include "ios/chrome/browser/ui/ui_util.h"
+
+namespace {
+// Thickness of scrollbar images inserted by UIScrollView (circa iOS 5.0).
+const CGFloat kScrollBarThickness = 7.0;
+}  // namespace
+
+@interface CenteringScrollView ()
+// Returns the one and only subview. There may be other subviews
+// (e.g. UIImageView for the scrollbars) created by UIScrollView.
+- (UIView*)findContentView;
+@end
+
+@implementation CenteringScrollView
+
+- (void)layoutSubviews {
+  [super layoutSubviews];
+
+  // Find the movable content view inside this scroll view.
+  UIView* contentView = [self findContentView];
+  // UIScrollView does not send |layoutSubviews| message to its subviews.
+  // Since layoutSubviews may change the frame of |contentView|, this must be
+  // called before the centering computation below.
+  [contentView layoutSubviews];
+
+  // Place content view in the center of frame.
+  CGRect frame = self.frame;
+  CGRect contentRect = contentView.frame;
+  contentRect.origin = AlignPointToPixel(CGPointMake(
+      std::max((frame.size.width - contentRect.size.width) / 2.0, 0.0),
+      std::max((frame.size.height - contentRect.size.height) / 2.0, 0.0)));
+  contentView.frame = contentRect;
+
+  if (contentRect.size.width > frame.size.width ||
+      contentRect.size.height > frame.size.height) {
+    // If size of content view is larger than frame size, adjust content size
+    // so it becomes scrollable.
+    self.contentSize =
+        CGSizeMake(std::max(contentRect.size.width, frame.size.width),
+                   std::max(contentRect.size.height, frame.size.height));
+  } else {
+    // Reset the content size as the view need not be scollable.
+    self.contentSize = CGSizeMake(0.0, 0.0);
+  }
+}
+
+#pragma mark -
+#pragma mark Private
+
+- (UIView*)findContentView {
+  UIView* soloView = nil;
+  NSUInteger viewCount = 0;
+  for (UIView* view in [self subviews]) {
+    // Ignore any UIImageView that has a smaller dimension less than the
+    // thickness of a scroll bar. This detection heuristics is based on the
+    // scrollbar sizing of iOS UIScrollView circa iOS 5.0. If this changes,
+    // the DCHECK following this for-loop will likely fail.
+    CGFloat thickness = std::min(view.frame.size.width, view.frame.size.height);
+    if ([view isKindOfClass:[UIImageView class]] &&
+        thickness <= kScrollBarThickness)
+      continue;
+    soloView = view;
+    ++viewCount;
+  }
+  DCHECK(viewCount == 1);
+  return soloView;
+}
+
+@end