macOS: Use QMacNotificationObserver over manual notification handling

This also fixes a bug where we were implicitly capturing this inside
the block, which meant that we would crash if the theme was recreated.
The capture is now tied to the lifetime of QCocoaTheme.

Change-Id: I37df8e6c0b33bf41e76d66be3cf29576041a7546
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/plugins/platforms/cocoa/qcocoatheme.h

@@ -45,6 +45,8 @@
 
 Q_FORWARD_DECLARE_OBJC_CLASS(QT_MANGLE_NAMESPACE(QCocoaThemeAppAppearanceObserver));
 
+#include <QtCore/private/qcore_mac_p.h>
+
 QT_BEGIN_NAMESPACE
 
 class QPalette;
@@ -82,6 +84,7 @@ public:
 
 private:
     mutable QPalette *m_systemPalette;
+    QMacNotificationObserver m_systemColorObserver;
     mutable QHash<QPlatformTheme::Palette, QPalette*> m_palettes;
     mutable QHash<QPlatformTheme::Font, QFont*> m_fonts;
     QT_MANGLE_NAMESPACE(QCocoaThemeAppAppearanceObserver) *m_appearanceObserver;

src/plugins/platforms/cocoa/qcocoatheme.mm

@@ -133,10 +133,10 @@ QCocoaTheme::QCocoaTheme()
         m_appearanceObserver = [[QCocoaThemeAppAppearanceObserver alloc] initWithTheme:this];
 #endif
 
-    [[NSNotificationCenter defaultCenter] addObserverForName:NSSystemColorsDidChangeNotification
-        object:nil queue:nil usingBlock:^(NSNotification *) {
+    m_systemColorObserver = QMacNotificationObserver(nil,
+        NSSystemColorsDidChangeNotification, [this] {
             handleSystemThemeChange();
-        }];
+    });
 }
 
 QCocoaTheme::~QCocoaTheme()

src/plugins/styles/mac/qmacstyle_mac.mm

@@ -161,18 +161,6 @@ QT_NAMESPACE_ALIAS_OBJC_CLASS(NotificationReceiver);
     return self;
 }
 
-- (void)scrollBarStyleDidChange:(NSNotification *)notification
-{
-    Q_UNUSED(notification);
-
-    // purge destroyed scroll bars:
-    QMacStylePrivate::scrollBars.removeAll(QPointer<QObject>());
-
-    QEvent event(QEvent::StyleChange);
-    for (const auto &o : QMacStylePrivate::scrollBars)
-        QCoreApplication::sendEvent(o, &event);
-}
-
 - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
         change:(NSDictionary<NSKeyValueChangeKey, id> *)change context:(void *)context
 {
@@ -2095,11 +2083,18 @@ QMacStyle::QMacStyle()
     Q_D(QMacStyle);
     QMacAutoReleasePool pool;
 
+    static QMacNotificationObserver scrollbarStyleObserver(nil,
+        NSPreferredScrollerStyleDidChangeNotification, []() {
+            // Purge destroyed scroll bars
+            QMacStylePrivate::scrollBars.removeAll(QPointer<QObject>());
+
+            QEvent event(QEvent::StyleChange);
+            for (const auto &o : QMacStylePrivate::scrollBars)
+                QCoreApplication::sendEvent(o, &event);
+    });
+
     d->receiver = [[NotificationReceiver alloc] initWithPrivateStyle:d];
-    [[NSNotificationCenter defaultCenter] addObserver:d->receiver
-                                             selector:@selector(scrollBarStyleDidChange:)
-                                                 name:NSPreferredScrollerStyleDidChangeNotification
-                                               object:nil];
+
 #if QT_MACOS_PLATFORM_SDK_EQUAL_OR_ABOVE(__MAC_10_14)
     if (QOperatingSystemVersion::current() >= QOperatingSystemVersion::MacOSMojave) {
         [NSApplication.sharedApplication addObserver:d->receiver forKeyPath:@"effectiveAppearance"
@@ -2113,7 +2108,6 @@ QMacStyle::~QMacStyle()
     Q_D(QMacStyle);
     QMacAutoReleasePool pool;
 
-    [[NSNotificationCenter defaultCenter] removeObserver:d->receiver];
 #if QT_MACOS_PLATFORM_SDK_EQUAL_OR_ABOVE(__MAC_10_14)
     if (QOperatingSystemVersion::current() >= QOperatingSystemVersion::MacOSMojave)
         [NSApplication.sharedApplication removeObserver:d->receiver forKeyPath:@"effectiveAppearance"];