MacViews: Overlay Scrollbars with Show/Hide Animations

ui/views/cocoa/views_scrollbar_bridge.mm

Index: ui/views/cocoa/views_scrollbar_bridge.mm
diff --git a/ui/views/cocoa/views_scrollbar_bridge.mm b/ui/views/cocoa/views_scrollbar_bridge.mm
new file mode 100644
index 0000000000000000000000000000000000000000..94527be67fccace0851f92380bc67c720ead371e
--- /dev/null
+++ b/ui/views/cocoa/views_scrollbar_bridge.mm
@@ -0,0 +1,54 @@
+// Copyright 2016 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 "ui/views/cocoa/views_scrollbar_bridge.h"
+
+#include "base/mac/mac_util.h"
+#import "base/mac/sdk_forward_declarations.h"
+
+@interface ViewsScrollbarBridge ()
+
+// Called when we receive a NSPreferredScrollerStyleDidChangeNotification.
+- (void)onScrollerStyleChanged:(NSNotification*)notification;
+
+@end
+
+@implementation ViewsScrollbarBridge
+
+- (id)initWithDelegate:(ViewsScrollbarBridgeDelegate*)delegate {
+  if ((self = [super init])) {
+    delegate_ = delegate;
+    [[NSNotificationCenter defaultCenter]
+        addObserver:self
+           selector:@selector(onScrollerStyleChanged:)
+               name:NSPreferredScrollerStyleDidChangeNotification
+             object:nil];
+  }
+  return self;
+}
+
+- (void)dealloc {
+  DCHECK(!delegate_);
+  [super dealloc];
+}
+
+- (void)clearDelegate {
+  delegate_ = nullptr;
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+}
+
+- (void)onScrollerStyleChanged:(NSNotification*)notification {
+  if (delegate_)
+    delegate_->OnScrollerStyleChanged();
+}
+
++ (NSScrollerStyle)getPreferredScrollerStyle {
+  if (![NSScroller respondsToSelector:@selector(preferredScrollerStyle)]) {
+    DCHECK(base::mac::IsOSSnowLeopard());
+    return NSScrollerStyleLegacy;
+  }
+  return [NSScroller preferredScrollerStyle];
+}
+
+@end