Android: Add opt-in high-dpi support

Opt-in by setting android.app.auto_screen_scale_factor
to true in AndroidManifest.xml. This will enable
devicePixelRatio scaling in QtGui using a scale
factor provided by Android DisplayMetrics.

Note that the Android style is not currently supported:
it already accounts for different display densities
which results in incorrect visual sizes when enabling
devicePixelRatio scaling.

Implementation: Bring DisplayMetrics::density through
to setDisplayMetrics in androidjnimain.cpp, similar
to what is done for "scaledDensity". Override
QPlatformScreen::pixelDensity(), which forwards the
scale factor to QtGui.

[The difference between "density" and "scaledDensity"
is that the former is a physical display density factor
which corresponds closely to devicePixelRatio in Qt,
while the latter also includes the Android global font
scale factor.]

Scale the global font pixel size in qandroidplatformtheme.cpp
to keep the visual font size constant.

Based on an initial patch from Daiwei Li <daiweili@suitabletech.com>

Task-number: QTBUG-46615
Change-Id: Ia51f99bf6dda485a57413949246c7b32cb47e8dd
Reviewed-by: BogDan Vatra <bogdan@kdab.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/plugins/platforms/android/qandroidplatformtheme.cpp b/src/plugins/platforms/android/qandroidplatformtheme.cpp
index 3e2ae7c939..71a3f910d2 100644
--- a/src/plugins/platforms/android/qandroidplatformtheme.cpp
+++ b/src/plugins/platforms/android/qandroidplatformtheme.cpp
@@ -31,6 +31,7 @@
 **
 ****************************************************************************/
 
+#include "androidjnimain.h"
 #include "androidjnimenu.h"
 #include "qandroidplatformtheme.h"
 #include "qandroidplatformmenubar.h"
@@ -216,6 +217,7 @@ QJsonObject AndroidStyle::loadStyleData()
 
 static std::shared_ptr<AndroidStyle> loadAndroidStyle(QPalette *defaultPalette)
 {
+    double pixelDensity = qEnvironmentVariableIsSet("QT_AUTO_SCREEN_SCALE_FACTOR") ? QtAndroid::pixelDensity() : 1.0;
     std::shared_ptr<AndroidStyle> style(new AndroidStyle);
     style->m_styleData = AndroidStyle::loadStyleData();
     if (style->m_styleData.isEmpty())
@@ -245,7 +247,7 @@ static std::shared_ptr<AndroidStyle> loadAndroidStyle(QPalette *defaultPalette)
             // Font size (in pixels)
             attributeIterator = item.find(QLatin1String("TextAppearance_textSize"));
             if (attributeIterator != item.constEnd())
-                font.setPixelSize(int(attributeIterator.value().toDouble()));
+                font.setPixelSize(int(attributeIterator.value().toDouble() / pixelDensity));
 
             // Font style
             attributeIterator = item.find(QLatin1String("TextAppearance_textStyle"));