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/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]) {

[tapted, 2016/02/22 22:34:57]

needs extra parens around assignment used as condition

[spqchan, 2016/02/22 23:26:03]

Done.

+    delegate_ = delegate;
+    [[NSNotificationCenter defaultCenter]
+        addObserver:self
+           selector:@selector(onScrollerStyleChanged:)
+               name:NSPreferredScrollerStyleDidChangeNotification
+             object:nil];
+  }
+  return self;
+}
+
+- (void)dealloc {
+  [super dealloc];
+  [[NSNotificationCenter defaultCenter] removeObserver:self];

[tapted, 2016/02/22 22:34:57]

I'm not really a fan of this approach. I guess here it's unlikely to come up,
but ObjC objects have a different lifetime than C++ objects. If something
creates an extra reference to a `ViewsScrollbarBridge`, dealloc won't be called,
and it can still receive notifications; sending them on to a |delegate_| which
could now be a freed object.

I'd prefer an explicit `clearDelegate` method which removes the observer and
sets the delegate to nullptr; Then keep the DCHECK in dealloc.

[spqchan, 2016/02/22 23:26:03]

Added a clearDelegate method

+}
+
+- (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