Bug 588664. Cache DPI in nsCocoaWindow. r=joshmoz,a=blocking

widget/src/cocoa/nsChildView.mm

@@ -969,22 +969,11 @@ float
 nsChildView::GetDPI()
 {
   NSWindow* window = [mView window];
-  NSScreen* screen = [window screen];
-  if (!screen)
-    return 96.0f;
+  if (window && [window isKindOfClass:[BaseWindow class]]) {
+    return [(BaseWindow*)window getDPI];
+  }
 
-  CGDirectDisplayID displayID =
-    [[[screen deviceDescription] objectForKey:@"NSScreenNumber"] intValue];
-  CGFloat heightMM = CGDisplayScreenSize(displayID).height;
-  size_t heightPx = CGDisplayPixelsHigh(displayID);
-  CGFloat scaleFactor = [window userSpaceScaleFactor];
-
-  // Currently we don't do our own scaling to take account
-  // of userSpaceScaleFactor, so every "pixel" we draw is actually
-  // userSpaceScaleFactor screen pixels. So divide the screen height
-  // by userSpaceScaleFactor to get the number of "device pixels"
-  // available.
-  return (heightPx / scaleFactor) / (heightMM / MM_PER_INCH_FLOAT);
+  return 96.0;
 }
 
 LayerManager*

widget/src/cocoa/nsCocoaWindow.h

@@ -76,6 +76,11 @@ typedef struct _nsCocoaWindowList {
 
   // Shadow
   BOOL mScheduledShadowInvalidation;
+
+  // DPI cache. Getting the physical screen size (CGDisplayScreenSize)
+  // is ridiculously slow, so we cache it in the toplevel window for all
+  // descendants to use.
+  float mDPI;
 }
 
 - (void)importState:(NSDictionary*)aState;
@@ -87,6 +92,7 @@ typedef struct _nsCocoaWindowList {
 
 - (void)deferredInvalidateShadow;
 - (void)invalidateShadow;
+- (float)getDPI;
 
 @end
 

widget/src/cocoa/nsCocoaWindow.mm

@@ -1937,6 +1937,31 @@ void nsCocoaWindow::SetPopupWindowLevel()
 
 @end
 
+static float
+GetDPI(NSWindow* aWindow)
+{
+  NSScreen* screen = [aWindow screen];
+  if (!screen)
+    return 96.0f;
+
+  CGDirectDisplayID displayID =
+    [[[screen deviceDescription] objectForKey:@"NSScreenNumber"] intValue];
+  CGFloat heightMM = ::CGDisplayScreenSize(displayID).height;
+  size_t heightPx = ::CGDisplayPixelsHigh(displayID);
+  CGFloat scaleFactor = [aWindow userSpaceScaleFactor];
+  if (scaleFactor < 0.01 || heightMM < 1 || heightPx < 1) {
+    // Something extremely bogus is going on
+    return 96.0f;
+  }
+
+  // Currently we don't do our own scaling to take account
+  // of userSpaceScaleFactor, so every "pixel" we draw is actually
+  // userSpaceScaleFactor screen pixels. So divide the screen height
+  // by userSpaceScaleFactor to get the number of "device pixels"
+  // available.
+  return (heightPx / scaleFactor) / (heightMM / MM_PER_INCH_FLOAT);
+}
+
 @implementation BaseWindow
 
 - (id)initWithContentRect:(NSRect)aContentRect styleMask:(NSUInteger)aStyle backing:(NSBackingStoreType)aBufferingType defer:(BOOL)aFlag
@@ -1947,6 +1972,8 @@ void nsCocoaWindow::SetPopupWindowLevel()
   mActiveTitlebarColor = nil;
   mInactiveTitlebarColor = nil;
   mScheduledShadowInvalidation = NO;
+  mDPI = GetDPI(self);
+
   return self;
 }
 
@@ -2034,6 +2061,11 @@ static const NSString* kStateShowsToolbarButton = @"showsToolbarButton";
   mScheduledShadowInvalidation = NO;
 }
 
+- (float)getDPI
+{
+  return mDPI;
+}
+
 - (void) doCommandBySelector:(SEL)aSelector
 {
   // We override this so that it won't beep if it can't act.