MacViews: Overlay Scrollbars with Show/Hide Animations

ui/views/cocoa/views_scrollbar_bridge.mm

+// 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/sdk_forward_declarations.h"
+
+@implementation ViewsScrollbarBridge
+
+- (id)initWithDelegate:(ViewsScrollbarBridgeDelegate*)delegate {
+  DCHECK(delegate);
+  if ((self = [super init])) {
+    delegate_ = delegate;
+    [[NSNotificationCenter defaultCenter]
+        addObserver:self

[tapted, 2016/02/08 00:31:15]

usually `dealloc` should remove this, but since this is owned by a C++ instance,
which has a tighter lifetime, there should be a method to clear the delegate,
which also removes this observer.  There are many ways to structure this... my
favourite would be to scrap the `initWithDelegate` method, and just have a 

- (void)setDelegate:(ViewsScrollbarBridgeDelegate*)delegate {
  DCHECK_NE(delegate, delegate_);
  delegate_ = delegate;
  if (!delegate_) {
    [[NSNotificationCenter ..] removeObserver:self];
    return;
  }

  [[NSNotificationCenter ..] addObserver:self];
}


then optional would be a dealloc override which just does

{
  DCHECK(!delegate_);
  [super dealloc];
}

[spqchan, 2016/02/09 21:21:25]

Done.

+           selector:@selector(onScrollerStyleChanged:)
+               name:NSPreferredScrollerStyleDidChangeNotification
+             object:nil];
+  }
+  return self;
+}
+
+- (void)onScrollerStyleChanged:(NSNotification*)notification {
+  delegate_->OnScrollerStyleChanged();
+}
+
+- (NSScrollerStyle)getPreferredScrollerStyle {
+  if (![NSScroller respondsToSelector:@selector(preferredScrollerStyle)])

[tapted, 2016/02/08 00:31:15]

this shouldn't be needed much longer, since it is 10.6 only... optional:
`DCHECK(IsOSSnowLeopard) inside the `if`. That would remind us that we can
remove the check when we remove the `IsOSSnowLeopard` declaration once it
becomes a tautology. But probably we will just git-grep for
preferredScrollerStyle since lots of things do this.

[spqchan, 2016/02/09 21:21:25]

Done.

+    return NSScrollerStyleLegacy;
+  return [NSScroller preferredScrollerStyle];
+}
+
+@end