Cache results from respondsToSelector.

Source/core/platform/mac/NSScrollerImpDetails.mm

Index: Source/core/platform/mac/NSScrollerImpDetails.mm
diff --git a/Source/core/platform/mac/NSScrollerImpDetails.mm b/Source/core/platform/mac/NSScrollerImpDetails.mm
index a5f09c5d519c0df39a5713c72bc1acc9152a3ce1..e8325b13b3b6bd76fa982fe92e47ccf40f5301f5 100644
--- a/Source/core/platform/mac/NSScrollerImpDetails.mm
+++ b/Source/core/platform/mac/NSScrollerImpDetails.mm
@@ -29,28 +29,65 @@
 #include "RuntimeEnabledFeatures.h"
 #include "core/platform/mac/NSScrollerImpDetails.h"
 
+namespace {
+
+// Declare notification names from the 10.7 SDK.
+#if !defined(MAC_OS_X_VERSION_10_7) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
+NSString* NSPreferredScrollerStyleDidChangeNotification = @"NSPreferredScrollerStyleDidChangeNotification";
+#endif
+
+// Storing the current NSScrollerStyle as a global is appreciably faster than
+// having it be a property of ScrollerStylerObserver.
+NSScrollerStyle g_scrollerStyle = NSScrollerStyleLegacy;
+
+}  // anonymous namespace
+
+@interface ScrollerStyleObserver : NSObject
+- (id)init;
+- (void)preferredScrollerStyleDidChange:(NSNotification*)notification;
+@end
+
+@implementation ScrollerStyleObserver
+- (id)init
+{
+  self = [super init];
+  if (self) {

[eseidel, 2013/10/15 21:56:02]

I might have written if (!self)\n return nil;

Also, why 2-space indent?

[Nico, 2013/10/15 22:00:14]

I think the usual stanza in chromium is

    if ((self = [super init]))

I like the early return better too, but oh well.

[ccameron, 2013/10/15 22:19:02]

Done.

Thought I'd caught all of the 2-spaces...

+      if ([NSScroller respondsToSelector:@selector(preferredScrollerStyle)]) {
+          g_scrollerStyle = [NSScroller preferredScrollerStyle];
+          NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
+          [center addObserver:self
+                     selector:@selector(preferredScrollerStyleDidChange:)
+                         name:NSPreferredScrollerStyleDidChangeNotification
+                       object:nil];
+      }
+  }
+  return self;
+}
+
+- (void)preferredScrollerStyleDidChange:(NSNotification*)notification
+{
+    g_scrollerStyle = [NSScroller preferredScrollerStyle];

[eseidel, 2013/10/15 21:57:53]

Do we need to tell somebody that this changed?  Send some global notification of
our own, or are we content with the fact that the user will start to see the
srollbars once they refresh.

I'm curious what Apple's code looks like for the same in WebKit2.

[ccameron, 2013/10/15 22:19:02]

Something else /does/ trigger a re-layout when the properties are changed, but
I'm not sure what the mechanism is offhand (it may be in RenderThemeChromiumMac
...).

+}
+@end
+
 namespace WebCore {
 
 bool isScrollbarOverlayAPIAvailable()
 {
-    static bool apiAvailable;
-    static bool shouldInitialize = true;
-    if (shouldInitialize) {
-        shouldInitialize = false;
-        Class scrollerImpClass = NSClassFromString(@"NSScrollerImp");
-        Class scrollerImpPairClass = NSClassFromString(@"NSScrollerImpPair");
-        apiAvailable = [scrollerImpClass respondsToSelector:@selector(scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp:)]
-                       && [scrollerImpPairClass instancesRespondToSelector:@selector(scrollerStyle)];
-    }
+    static bool apiAvailable =
+        [NSClassFromString(@"NSScrollerImp") respondsToSelector:@selector(scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp:)]
+        && [NSClassFromString(@"NSScrollerImpPair") instancesRespondToSelector:@selector(scrollerStyle)];
     return apiAvailable;
 }
 
-NSScrollerStyle recommendedScrollerStyle() {
+NSScrollerStyle recommendedScrollerStyle()
+{
     if (RuntimeEnabledFeatures::overlayScrollbarsEnabled())
         return NSScrollerStyleOverlay;
-    if ([NSScroller respondsToSelector:@selector(preferredScrollerStyle)])
-        return [NSScroller preferredScrollerStyle];
-    return NSScrollerStyleLegacy;
+
+    // The ScrollerStyleObserver will update g_scrollerStyle at init and when needed.
+    static ScrollerStyleObserver* scrollerStyleObserver = [[ScrollerStyleObserver alloc] init];

[eseidel, 2013/10/15 21:56:02]

new] would have done the same as alloc] init];

[Nico, 2013/10/15 22:00:14]

Yeah, but alloc init is more common for some reason.

[ccameron, 2013/10/15 22:19:02]

Left this one as-is.

+    return g_scrollerStyle;
 }
 
 }