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>

5 files changed, 21 insertions, 3 deletions

diff --git a/src/plugins/platforms/android/androidjnimain.cpp b/src/plugins/platforms/android/androidjnimain.cpp
index 99cb58830c..d419e42cd5 100644
--- a/src/plugins/platforms/android/androidjnimain.cpp
+++ b/src/plugins/platforms/android/androidjnimain.cpp
@@ -55,6 +55,7 @@
 #include <QtCore/private/qjnihelpers_p.h>
 #include <QtCore/private/qjni_p.h>
 #include <QtGui/private/qguiapplication_p.h>
+#include <QtGui/private/qhighdpiscaling_p.h>
 
 #include <qpa/qwindowsysteminterface.h>
 
@@ -109,6 +110,7 @@ static QAndroidPlatformIntegration *m_androidPlatformIntegration = nullptr;
 static int m_desktopWidthPixels  = 0;
 static int m_desktopHeightPixels = 0;
 static double m_scaledDensity = 0;
+static double m_density = 1.0;
 
 static volatile bool m_pauseApplication;
 
@@ -157,6 +159,11 @@ namespace QtAndroid
         return m_scaledDensity;
     }
 
+    double pixelDensity()
+    {
+        return m_density;
+    }
+
     JavaVM *javaVM()
     {
         return m_javaVM;
@@ -547,7 +554,8 @@ static void setSurface(JNIEnv *env, jobject /*thiz*/, jint id, jobject jSurface,
 static void setDisplayMetrics(JNIEnv */*env*/, jclass /*clazz*/,
                             jint widthPixels, jint heightPixels,
                             jint desktopWidthPixels, jint desktopHeightPixels,
-                            jdouble xdpi, jdouble ydpi, jdouble scaledDensity)
+                            jdouble xdpi, jdouble ydpi,
+                            jdouble scaledDensity, jdouble density)
 {
     // Android does not give us the correct screen size for immersive mode, but
     // the surface does have the right size
@@ -558,6 +566,7 @@ static void setDisplayMetrics(JNIEnv */*env*/, jclass /*clazz*/,
     m_desktopWidthPixels = desktopWidthPixels;
     m_desktopHeightPixels = desktopHeightPixels;
     m_scaledDensity = scaledDensity;
+    m_density = density;
 
     if (!m_androidPlatformIntegration) {
         QAndroidPlatformIntegration::setDefaultDisplayMetrics(desktopWidthPixels,
@@ -683,7 +692,7 @@ static JNINativeMethod methods[] = {
     {"startQtApplication", "(Ljava/lang/String;Ljava/lang/String;)V", (void *)startQtApplication},
     {"quitQtAndroidPlugin", "()V", (void *)quitQtAndroidPlugin},
     {"terminateQt", "()V", (void *)terminateQt},
-    {"setDisplayMetrics", "(IIIIDDD)V", (void *)setDisplayMetrics},
+    {"setDisplayMetrics", "(IIIIDDDD)V", (void *)setDisplayMetrics},
     {"setSurface", "(ILjava/lang/Object;II)V", (void *)setSurface},
     {"updateWindow", "()V", (void *)updateWindow},
     {"updateApplicationState", "(I)V", (void *)updateApplicationState},
diff --git a/src/plugins/platforms/android/androidjnimain.h b/src/plugins/platforms/android/androidjnimain.h
index 4d037f4b74..cdedeb38f8 100644
--- a/src/plugins/platforms/android/androidjnimain.h
+++ b/src/plugins/platforms/android/androidjnimain.h
@@ -71,6 +71,7 @@ namespace QtAndroid
     int desktopWidthPixels();
     int desktopHeightPixels();
     double scaledDensity();
+    double pixelDensity();
     JavaVM *javaVM();
     AAssetManager *assetManager();
     jclass applicationClass();
diff --git a/src/plugins/platforms/android/qandroidplatformscreen.cpp b/src/plugins/platforms/android/qandroidplatformscreen.cpp
index 7a509e4d61..209ce2f7db 100644
--- a/src/plugins/platforms/android/qandroidplatformscreen.cpp
+++ b/src/plugins/platforms/android/qandroidplatformscreen.cpp
@@ -382,6 +382,11 @@ QDpi QAndroidPlatformScreen::logicalDpi() const
     return QDpi(lDpi, lDpi);
 }
 
+qreal QAndroidPlatformScreen::pixelDensity() const
+{
+    return QtAndroid::pixelDensity();
+}
+
 Qt::ScreenOrientation QAndroidPlatformScreen::orientation() const
 {
     return QAndroidPlatformIntegration::m_orientation;
diff --git a/src/plugins/platforms/android/qandroidplatformscreen.h b/src/plugins/platforms/android/qandroidplatformscreen.h
index 403d6036f0..c01dbbc712 100644
--- a/src/plugins/platforms/android/qandroidplatformscreen.h
+++ b/src/plugins/platforms/android/qandroidplatformscreen.h
@@ -96,6 +96,7 @@ protected:
 
 private:
     QDpi logicalDpi() const;
+    qreal pixelDensity()  const;
     Qt::ScreenOrientation orientation() const;
     Qt::ScreenOrientation nativeOrientation() const;
     void surfaceChanged(JNIEnv *env, jobject surface, int w, int h);
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"));