Fall back to default font if Libertinus is unavailable

ChangeLog

@@ -1,3 +1,11 @@
+2018-10-02  Ista Zahn  <istazahn@gmail.com>
+
+        * inst/iqssDemo/beamerfontthemeiqss.sty: Fall back to default font
+	if Libertinus is unavailable
+
+	* inst/rmarkdown/templates/iqss/skeleton/beamerfontthemeiqss.sty:
+	Ditto
+
 2018-09-27  Dirk Eddelbuettel  <edd@debian.org>
 
 	* DESCRIPTION (Version, Date): Release 0.0.2

README.md

@@ -191,7 +191,7 @@ These themes use additional (free) fonts you may need to install:
 
 - [Metropolis](https://github.com/matze/mtheme) wants [Fira Sans](https://github.com/mozilla/Fira)
   but can proceed with alternate fonts;
-- [IQSS Beamer Theme](https://github.com/IQSS/iqss-beamer-theme) really requires
+- [IQSS Beamer Theme](https://github.com/IQSS/iqss-beamer-theme) wants
   [Libertinus](https://github.com/libertinus-fonts/libertinus), see the
   [IQSS Beamer Theme](https://github.com/IQSS/iqss-beamer-theme)  page for details.
 - [Presento Theme](https://github.com/RatulSaha/presento) wants 

inst/iqssDemo/beamerfontthemeiqss.sty

@@ -1,7 +1,48 @@
 \usefonttheme{professionalfonts}
+
 \RequirePackage{fontspec}
 
+%% checkfont and iffontsavaiable copied from the metropolis theme
+%% by Matthias Vo­gelge­sang.
+%    Checks if a font is installed; if not, |fontsnotfound| is increased.
+\newcounter{fontsnotfound}
+\newcommand{\checkfont}[1]{%
+  \suppressfontnotfounderror=1%
+  \font\x = "#1" at 10pt
+  \selectfont
+  \ifx\x\nullfont%
+  \stepcounter{fontsnotfound}%
+  \fi%
+  \suppressfontnotfounderror=0%
+}
+%   Resets the |fontsnotfound| counter and calls |\checkfont| for each font in
+%   the comma separated list in the first argument.
+\newcommand{\iffontsavailable}[3]{%
+  \setcounter{fontsnotfound}{0}%
+  \expandafter\forcsvlist\expandafter%
+  \checkfont\expandafter{#1}%
+  \ifnum\value{fontsnotfound}=0%
+  #2%
+  \else%
+  #3%
+  \fi%
+}
+
+\iffontsavailable{Libertinus Serif,%
+  Libertinus Sans,%
+  LibertinusMath,%
+  Libertinus Mono}%
+{%
   \setmainfont{Libertinus Serif}
   \setsansfont{Libertinus Sans}
   \setmathfont{Libertinus Math}
   \setmonofont{Libertinus Mono}
+}{%
+  \PackageWarning{beamerthememeiqss}{%
+    Could not find Libertinus fonts.
+    Please install them from
+    https://github.com/libertinus-fonts/libertinus/releases
+    following the instructions at
+    https://github.com/IQSS/iqss-beamer-theme/blob/master/README.md% 
+  }
+}

inst/rmarkdown/templates/iqss/skeleton/beamerfontthemeiqss.sty

@@ -1,7 +1,48 @@
 \usefonttheme{professionalfonts}
+
 \RequirePackage{fontspec}
 
+%% checkfont and iffontsavaiable copied from the metropolis theme
+%% by Matthias Vo­gelge­sang.
+%    Checks if a font is installed; if not, |fontsnotfound| is increased.
+\newcounter{fontsnotfound}
+\newcommand{\checkfont}[1]{%
+  \suppressfontnotfounderror=1%
+  \font\x = "#1" at 10pt
+  \selectfont
+  \ifx\x\nullfont%
+  \stepcounter{fontsnotfound}%
+  \fi%
+  \suppressfontnotfounderror=0%
+}
+%   Resets the |fontsnotfound| counter and calls |\checkfont| for each font in
+%   the comma separated list in the first argument.
+\newcommand{\iffontsavailable}[3]{%
+  \setcounter{fontsnotfound}{0}%
+  \expandafter\forcsvlist\expandafter%
+  \checkfont\expandafter{#1}%
+  \ifnum\value{fontsnotfound}=0%
+  #2%
+  \else%
+  #3%
+  \fi%
+}
+
+\iffontsavailable{Libertinus Serif,%
+  Libertinus Sans,%
+  LibertinusMath,%
+  Libertinus Mono}%
+{%
   \setmainfont{Libertinus Serif}
   \setsansfont{Libertinus Sans}
   \setmathfont{Libertinus Math}
   \setmonofont{Libertinus Mono}
+}{%
+  \PackageWarning{beamerthememeiqss}{%
+    Could not find Libertinus fonts.
+    Please install them from
+    https://github.com/libertinus-fonts/libertinus/releases
+    following the instructions at
+    https://github.com/IQSS/iqss-beamer-theme/blob/master/README.md% 
+  }
+}