exsen-beamer.sty

% This style file is loaded automatically when `example-sentences` is 
% used with `beamer`. 
%
% Note that this file assumes that beamer is already loaded, it does not 
% check for this itself.
%
% There will be very few instances where it makes sense to load this 
% file directly.
%
\ProvidesPackage{exsen-beamer}[%
    2021/06/22 Linguistic example sentences v.0.8.0 
    / beamer compatibility layer%
]
%
% BEAMER HELPER COMMANDS
%
% Beamer unfortunately does not provide ways to execute code at the 
% beginning of each frame or overlay. We define such commands here, as 
% they are needed for the counter-management below.
%
% \ExSen@beamer@BeforeBeginFrame executes its argument at (just before) 
% the beginning of each frame environment.
\newcommand{%
    \ExSen@beamer@BeforeBeginFrame%
}[1]{%
    \BeforeBeginEnvironment{frame}{#1}%
}
%
% \ExSen@beamer@AtBeginSlide executes its argument at the beginning of 
% each "slide" - a single frame can correspond to multiple "slides" if 
% overlays are used. In this case, the argument of the command will be 
% executed at the beginning of each overlay.
%
% (Implementation note: We prepend the argument to 
% \beamer@checkframetitle,  which is a macro without arguments that 
% happens to be called just before the beginning of each slide. This is 
% decidedly hacky and relies on beamer internals, but it seems to be the
% only way to make this work at present.)
\newcommand{%
  \ExSen@beamer@AtBeginSlide%
}[1]{%
  \preto{\beamer@checkframetitle}{#1}%
}

% DEPENDENCIES
\RequirePackage{etoolbox}
%
% (RE)DEFINE THE EXAMPLES environment
%
% By default, beamer defines an `examples`-environment, which we alias 
% as `beamerexamples` and then overwrite.
%
% However, beamer can be prevented from creating this environment (via
% the `notheorems` option), so we must test whether the environments 
% exist.
% 
\ifdef{\examples}{
    % Alias beamer's examples environment
    \let\beamerexamples\examples
    \let\endbeamerexamples\endexamples
    % Overwrite beamer's examples environment
    \renewlist{examples}{enumerate}{3}
}{
    \newlist{examples}{enumerate}{3}
}
\@ifclassloaded{beamer}{
    % Counter/overlay handling
    %
    % When overlays are used, a frame can be typeset multiple times. This 
    % means that the example counter needs to be reset at the beginnging of
    % each overlay (to the value it had at the beginnging of the frame).
    %
    % To complicate matters, enumitem actually does not save the value in 
    % a global counter, but defines the command `\enit@resume@series@examples` which 
    % is used to initialize the local counter of the next examples-environment.
    % This means we cannot rely on beamer's facilities to handle global counters
    % with overlays (\resetcounteronoverlay).
    %
    % Instead, we squirrel away the current value of the resume command at the
    % beginning of each frame:
    \ExSen@beamer@BeforeBeginFrame{\let\ExSen@beamer@resumecommand\enit@resume@series@examples}%
    %
    % Then, we reset the resume command at the beginning of each slide (=overlay):
    \ExSen@beamer@AtBeginSlide{\let\enit@resume@series@examples\ExSen@beamer@resumecommand}%
}{}