iOS: Flesh out device-pixel-ratio handling in QIOSWindow

We don't need to cache the device-pixel-ratio, as we can ask the UIView
directly. We do need to set it though, as the default behavior of
matching the screen scale does not apply for EAGL-backed views,
but the ratio needs to match the current screen the view is on.

Change-Id: I29e4a4fa4f4b767d86265ec899fb43a355b5c3a3
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>

src/plugins/platforms/ios/qioswindow.h

@@ -87,7 +87,6 @@ private:
 
     QRect m_normalGeometry;
     int m_windowLevel;
-    qreal m_devicePixelRatio;
 
     void raiseOrLower(bool raise);
     void updateWindowLevel();

src/plugins/platforms/ios/qioswindow.mm

@@ -127,6 +127,16 @@
     return self;
 }
 
+- (void)willMoveToWindow:(UIWindow *)newWindow
+{
+    // UIKIt will normally set the scale factor of a view to match the corresponding
+    // screen scale factor, but views backed by CAEAGLLayers need to do this manually.
+    self.contentScaleFactor = newWindow && newWindow.screen ?
+        newWindow.screen.scale : [[UIScreen mainScreen] scale];
+
+    // FIXME: Allow the scale factor to be customized through QSurfaceFormat.
+}
+
 - (void)layoutSubviews
 {
     // This method is the de facto way to know that view has been resized,
@@ -331,19 +341,9 @@ QIOSWindow::QIOSWindow(QWindow *window)
     , m_view([[QUIView alloc] initWithQIOSWindow:this])
     , m_normalGeometry(QPlatformWindow::geometry())
     , m_windowLevel(0)
-    , m_devicePixelRatio(1.0)
 {
     setParent(parent());
     setWindowState(window->windowState());
-
-    // Retina support: get screen scale factor and set it in the content view.
-    // This will make framebufferObject() create a 2x frame buffer on retina
-    // displays. Also set m_devicePixelRatio which is used for scaling the
-    // paint device.
-    if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)] == YES) {
-        m_devicePixelRatio = [[UIScreen mainScreen] scale];
-        [m_view setContentScaleFactor: m_devicePixelRatio];
-    }
 }
 
 QIOSWindow::~QIOSWindow()
@@ -522,7 +522,7 @@ void QIOSWindow::handleContentOrientationChange(Qt::ScreenOrientation orientatio
 
 qreal QIOSWindow::devicePixelRatio() const
 {
-    return m_devicePixelRatio;
+    return m_view.contentScaleFactor;
 }
 
 QT_END_NAMESPACE