OS X - Cocoa backing store and drawRect

m_backingStore pointer has a limited 'lifetime' - usually
it is set in -flushBackingStore:region:offset: method, then -drawRect:
is invoked/forced by -setNeedsDisplayInRect:, after that it's dangerous to
have a non-null pointer to a backing store (and we reset it).
But if Cocoa invokes drawRect (due to some reason) our null backing store pointer is also
not good. This patch instead is using a shared resource (QImage) from a backing store.
This patch also makes getBackingStoreCGImage() redundant - -drawRect: was the only
place we called it.

Task-number: QTBUG-42206
Change-Id: Ie7726336f05d07c52f660f6326ae5cef114201dd
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>

src/plugins/platforms/cocoa/qcocoabackingstore.h

@@ -51,12 +51,9 @@ public:
 
     QPaintDevice *paintDevice();
     void flush(QWindow *widget, const QRegion &region, const QPoint &offset);
-#ifndef QT_NO_OPENGL
-    QImage toImage() const Q_DECL_OVERRIDE;
-#endif
+    QImage toImage() const;
     void resize (const QSize &size, const QRegion &);
     bool scroll(const QRegion &area, int dx, int dy);
-    CGImageRef getBackingStoreCGImage();
     qreal getBackingStoreDevicePixelRatio();
 
 private:

src/plugins/platforms/cocoa/qcocoabackingstore.mm

@@ -96,12 +96,10 @@ void QCocoaBackingStore::flush(QWindow *win, const QRegion &region, const QPoint
     }
 }
 
-#ifndef QT_NO_OPENGL
 QImage QCocoaBackingStore::toImage() const
 {
     return m_qImage;
 }
-#endif
 
 void QCocoaBackingStore::resize(const QSize &size, const QRegion &)
 {
@@ -121,17 +119,6 @@ bool QCocoaBackingStore::scroll(const QRegion &area, int dx, int dy)
     return true;
 }
 
-CGImageRef QCocoaBackingStore::getBackingStoreCGImage()
-{
-    if (!m_cgImage)
-        m_cgImage = qt_mac_toCGImage(m_qImage);
-
-    // Warning: do not retain/release/cache the returned image from
-    // outside the backingstore since it shares data with a QImage and
-    // needs special memory considerations.
-    return m_cgImage;
-}
-
 qreal QCocoaBackingStore::getBackingStoreDevicePixelRatio()
 {
     return m_qImage.devicePixelRatio();

src/plugins/platforms/cocoa/qnsview.h

@@ -51,7 +51,8 @@ QT_END_NAMESPACE
 Q_FORWARD_DECLARE_OBJC_CLASS(QNSViewMouseMoveHelper);
 
 @interface QT_MANGLE_NAMESPACE(QNSView) : NSView <NSTextInputClient> {
-    QCocoaBackingStore* m_backingStore;
+    QImage m_backingStore;
+    qreal m_pixelRatio;
     QPoint m_backingStoreOffset;
     CGImageRef m_maskImage;
     uchar *m_maskData;

src/plugins/platforms/cocoa/qnsview.mm

@@ -143,7 +143,7 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
 {
     self = [super initWithFrame : NSMakeRect(0,0, 300,300)];
     if (self) {
-        m_backingStore = 0;
+        m_pixelRatio = 1.;
         m_maskImage = 0;
         m_shouldInvalidateWindowShadow = false;
         m_window = 0;
@@ -466,8 +466,9 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
 
 - (void) flushBackingStore:(QCocoaBackingStore *)backingStore region:(const QRegion &)region offset:(QPoint)offset
 {
-    m_backingStore = backingStore;
-    m_backingStoreOffset = offset * m_backingStore->getBackingStoreDevicePixelRatio();
+    m_backingStore = backingStore->toImage();
+    m_pixelRatio = backingStore->getBackingStoreDevicePixelRatio();
+    m_backingStoreOffset = offset * m_pixelRatio;
     foreach (QRect rect, region.rects()) {
         [self setNeedsDisplayInRect:NSMakeRect(rect.x(), rect.y(), rect.width(), rect.height())];
     }
@@ -531,7 +532,7 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
     if (m_platformWindow->m_drawContentBorderGradient)
         NSDrawWindowBackground(dirtyRect);
 
-    if (!m_backingStore)
+    if (m_backingStore.isNull())
         return;
 
     // Calculate source and target rects. The target rect is the dirtyRect:
@@ -539,11 +540,10 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
 
     // The backing store source rect will be larger on retina displays.
     // Scale dirtyRect by the device pixel ratio:
-    const qreal devicePixelRatio = m_backingStore->getBackingStoreDevicePixelRatio();
-    CGRect dirtyBackingRect = CGRectMake(dirtyRect.origin.x * devicePixelRatio,
-                                         dirtyRect.origin.y * devicePixelRatio,
-                                         dirtyRect.size.width * devicePixelRatio,
-                                         dirtyRect.size.height * devicePixelRatio);
+    CGRect dirtyBackingRect = CGRectMake(dirtyRect.origin.x * m_pixelRatio,
+                                         dirtyRect.origin.y * m_pixelRatio,
+                                         dirtyRect.size.width * m_pixelRatio,
+                                         dirtyRect.size.height * m_pixelRatio);
 
     NSGraphicsContext *nsGraphicsContext = [NSGraphicsContext currentContext];
     CGContextRef cgContext = (CGContextRef) [nsGraphicsContext graphicsPort];
@@ -569,7 +569,7 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
         dirtyBackingRect.size.width,
         dirtyBackingRect.size.height
     );
-    CGImageRef bsCGImage = m_backingStore->getBackingStoreCGImage();
+    CGImageRef bsCGImage = qt_mac_toCGImage(m_backingStore);
     CGImageRef cleanImg = CGImageCreateWithImageInRect(bsCGImage, backingStoreRect);
 
     // Optimization: Copy frame buffer content instead of blending for
@@ -586,8 +586,6 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
     CGImageRelease(subMask);
 
     [self invalidateWindowShadowIfNeeded];
-
-    m_backingStore = 0;
 }
 
 - (BOOL) isFlipped