iOS: Clarify implementation of QIOSWindow::isExposed()

We treat windows as exposed even if the application state is still
inactive (e.g. during startup), otherwise there's a visible moment
of black screen between the launch screen fading out and the app
rendering its first pixel.

Change-Id: I48368459a8a46fa1c5b2853ea88adfe1ac61b6f7
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/plugins/platforms/ios/qioswindow.mm

@@ -206,6 +206,25 @@ void QIOSWindow::applyGeometry(const QRect &rect)
 
 bool QIOSWindow::isExposed() const
 {
+    // Note: At startup of an iOS app it will enter UIApplicationStateInactive
+    // while showing the launch screen, and once the application returns from
+    // applicationDidFinishLaunching it will hide the launch screen and enter
+    // UIApplicationStateActive. Technically, a window is not exposed until
+    // it's actually visible on screen, and Apple also documents that "Apps
+    // that use OpenGL ES for drawing must not use didFinishLaunching to
+    // prepare their drawing environment. Instead, defer any OpenGL ES
+    // drawing calls to applicationDidBecomeActive". Unfortunately, if we
+    // wait until the applicationState reaches ApplicationActive to signal
+    // that the window is exposed, we get a lag between hiding the launch
+    // screen and blitting the first pixels of the application, as Qt
+    // spends some time drawing those pixels in response to the expose.
+    // In practice there doesn't seem to be any issues starting GL setup
+    // and drawing from within applicationDidFinishLaunching, and this is
+    // also the recommended approach for other 3rd party GL toolkits on iOS,
+    // so we 'cheat', and report that a window is exposed even if the app
+    // is in UIApplicationStateInactive, so that the startup transition
+    // between the launch screen and the application content is smooth.
+
     return qApp->applicationState() > Qt::ApplicationHidden
         && window()->isVisible() && !window()->geometry().isEmpty();
 }