X11 platform: follow when the OS changes the Xft.dpi resource

Under gnome, Tweaks -> Fonts ->Scaling factor allows to change
the size of all fonts handled by gnome.
With that change, FLTK apps obey to changes to Xft.dpi.

3 files changed, 38 insertions, 11 deletions

diff --git a/src/Fl_x.cxx b/src/Fl_x.cxx
index 212f14db2..0d63218cd 100644
--- a/src/Fl_x.cxx
+++ b/src/Fl_x.cxx
@@ -1267,6 +1267,28 @@ static void react_to_screen_reconfiguration() {
 }
 #endif // USE_XRANDR
 
+#if USE_XFT
+static void after_display_rescale(float *p_current_xft_dpi) {
+  FILE *pipe = popen("xrdb -query", "r");
+  if (!pipe) return;
+  char line[100];
+  while (fgets(line, sizeof(line), pipe) != NULL) {
+    if (memcmp(line, "Xft.dpi:", 8)) continue;
+    float dpi;
+    if (sscanf(line+8, "%f", &dpi) == 1) {
+      //fprintf(stderr," previous=%g dpi=%g \n", *p_current_xft_dpi, dpi);
+      if (fabs(dpi - *p_current_xft_dpi) > 0.01) {
+        *p_current_xft_dpi = dpi;
+        float f = dpi/96.;
+        for (int i = 0; i < Fl::screen_count(); i++)
+          Fl::screen_driver()->rescale_all_windows_from_screen(i, f);
+      }
+    }
+    break;
+  }
+  pclose(pipe);
+}
+#endif // USE_XFT
 
 int fl_handle(const XEvent& thisevent)
 {
@@ -1305,7 +1327,11 @@ int fl_handle(const XEvent& thisevent)
 #endif // USE_XRANDR
 
   if (xevent.type == PropertyNotify && xevent.xproperty.atom == fl_NET_WORKAREA) {
-    ((Fl_X11_Screen_Driver*)Fl::screen_driver())->init_workarea();
+    Fl_X11_Screen_Driver *d = (Fl_X11_Screen_Driver*)Fl::screen_driver();
+    d->init_workarea();
+#if USE_XFT
+    after_display_rescale(&(d->current_xft_dpi));
+#endif // USE_XFT
   }
   
   switch (xevent.type) {
diff --git a/src/drivers/X11/Fl_X11_Screen_Driver.H b/src/drivers/X11/Fl_X11_Screen_Driver.H
index 600a54584..deaaafdcf 100644
--- a/src/drivers/X11/Fl_X11_Screen_Driver.H
+++ b/src/drivers/X11/Fl_X11_Screen_Driver.H
@@ -52,6 +52,7 @@ protected:
 
 public:
 #if USE_XFT // scaling does not work without Xft
+  float current_xft_dpi; // current value of the Xft.dpi X resource
   virtual APP_SCALING_CAPABILITY rescalable() { return PER_SCREEN_APP_SCALING; }
   virtual float scale(int n) {return screens[n].scale;}
   virtual void scale(int n, float f) { screens[n].scale = f;}
diff --git a/src/drivers/X11/Fl_X11_Screen_Driver.cxx b/src/drivers/X11/Fl_X11_Screen_Driver.cxx
index 7875cc3c1..d71ef7486 100644
--- a/src/drivers/X11/Fl_X11_Screen_Driver.cxx
+++ b/src/drivers/X11/Fl_X11_Screen_Driver.cxx
@@ -116,6 +116,7 @@ Fl_Screen_Driver *Fl_Screen_Driver::newScreenDriver()
   Fl_X11_Screen_Driver *d = new Fl_X11_Screen_Driver();
 #if USE_XFT
   for (int i = 0;  i < MAX_SCREENS; i++) d->screens[i].scale = 1;
+  d->current_xft_dpi = 0.; // means the value of the Xft.dpi resource is still unknown
 #else
   secret_input_character = '*';
 #endif
@@ -1403,16 +1404,15 @@ static void* value_of_key_in_schema(const char **known, const char *schema, cons
 // set the desktop's default scaling value
 void Fl_X11_Screen_Driver::desktop_scale_factor()
 {
-  float factor = 1;
-  int dpi;
-  // Try getting the Xft.dpi resource value
-  char *s = XGetDefault(fl_display, "Xft", "dpi");
-  if (s && sscanf(s, "%d", &dpi) == 1) {
-    factor = dpi / 96.;
-    // checks to prevent potential crash (factor <= 0) or very large factors
-    if (factor < 0.25) factor = 0.25;
-    else if (factor > 10.0) factor = 10.0;
-    for (int i = 0; i < screen_count(); i++)  scale(i, factor);
+  if (this->current_xft_dpi == 0.) { // Try getting the Xft.dpi resource value
+    char *s = XGetDefault(fl_display, "Xft", "dpi");
+    if (s && sscanf(s, "%f", &(this->current_xft_dpi)) == 1) {
+      float factor = this->current_xft_dpi / 96.;
+      // checks to prevent potential crash (factor <= 0) or very large factors
+      if (factor < 0.25) factor = 0.25;
+      else if (factor > 10.0) factor = 10.0;
+      for (int i = 0; i < screen_count(); i++)  scale(i, factor);
+    }
   }
 }