Text with gaps and white text

Question

I have a kind of learning document with exercises. In this document I have texts with gaps. The gaps are currently filled by dots. I am using the following commands for this purpose:

\newcommand{\gapunit}{.....}
\newcommand{\tinygap}{\gapunit}


% ===== EGREG VARIANT =====
\makeatletter
\newcount\my@repeat@count% initialize a new counter for the loop
\newcommand{\myrepeat}[3]{% new command with 2 arguments
    \begingroup% using a group here allows nested calls
    \my@repeat@count=1% initialize at 1, so that there are argument - 1 iterations and the last iterations doesn't have a separator following it
    \@whilenum\my@repeat@count<#1\do{#2#3\advance\my@repeat@count1}#2% as long as the iteration count is smaller than the argument, advance, meaning that the counter will be increased by 1
    \endgroup%
}
\makeatother

\newcommand{\shortgap}{%
    \myrepeat{3}{\gapunit}{\ }
}
\newcommand{\mediumgap}{%
    \myrepeat{5}{\gapunit}{\ }
}
\newcommand{\longgap}{%
    \myrepeat[10]{\gapunit}{\ }
}

Now I want to add solutions to the PDF which I compile. I thought maybe I could add white text, which I write on top of the dots (want to have the dots and the white text) to add solutions. I am quite OK with the idea, that someone could highlight white text to read it. In the end either someone "cheats themselves" or really learns something.

But how can I write white text on top of the gaps?

If there is a not too complicated better way of getting solutions into my PDF, I am open to ideas as well. I just don't want to lose the gap dots and don't want to add pages of code to my template, to get something super fancy, when actually white text is totally sufficient : )

Good would be a solution, which figures out the background color used by the document somehow, so that it does not have to be set in stone to be white.

Here is the document on GitHub, in case I am too vague about the gaps thing or something: repo

Edit

I am using Andrew's solution in a slightly modified way:

\newlength\blankblanklength
\newlength\blanktextlength
\newlength\blankcenterindentation
\newcommand\BlankText[2][]{%
    % do some length calculations for text length
    \if\relax\detokenize{#1}\relax%
        \settowidth{\blanktextlength}{#2}%
    \else%
        \setlength\blanktextlength{#1}%
    \fi% calculate width of text
    \rlap{\makebox[\blanktextlength][c]{\textcolor{white}{#2}}}% write the text without moving, centered in a box
    \raisebox{-0.5ex}{\hbox to \blanktextlength{\dotfill}}% add dots to cover text
}

\newcommand{\ShortBlankText}[1]{%
    \BlankText[2cm]{#1}
}
\newcommand{\MediumBlankText}[1]{%
    \BlankText[3.2cm]{#1}
}
\newcommand{\LongBlankText}[1]{%
    \BlankText[4.4cm]{#1}
}

Answer 1

Edit II

Fixing the behaviour when used at the start of a paragraph. The macro now has two optional arguments:

 %usage: \BlankText(colour)[length]{text}

The default colour for the "blank text" is white and the default length for the dots is the length of the "blank text".

Edit

Adding an optional argument to set the length of dots.

The MWE below shows how to use \BlankText with its two optional arguments. It produces:

Here is the code:

\documentclass{article}
\usepackage{xcolor}
\usepackage{xparse}

\newlength\blanktextlength
% usage: \BlankText(colour: default=white)[length]{text}
\NewDocumentCommand\BlankText{ D(){white} o m }{%
  \IfNoValueTF{#2}{% no width specified
     \settowidth{\blanktextlength}{#3}%
     \makebox[0pt][l]{\textcolor{#1}{#3}}% write the text without moving
     }{% calculate width of text
     \setlength\blanktextlength{#2}%
     \makebox[0pt][l]{\hbox to \blanktextlength{\hfil\textcolor{#1}{#3}\hfil}}
     }%
  \raisebox{-0.5ex}{\hbox to \blanktextlength{\dotfill}}% add dots for text
}

\begin{document}

\BlankText{Start of paragraph}
Here is some white text: \BlankText{Hello}
Here is a little more: \BlankText{Can you read me through the dots?}

Same again with a declared width:
\BlankText[8cm]{Can you read me through the dots?}

\bigskip\noindent With \textcolor{blue}{blue} blank text:

\BlankText(blue){Start of paragraph}
Here is some blue text: \BlankText(blue){Hello}
Here is a little more: \BlankText(blue){Can you read me through the dots?}

Same again with a declared width:
\BlankText(blue)[8cm]{Can you read me through the dots?}

\end{document}

The previous problem with using \BlankText was caused by \llap{...} not playing well with LaTeX paragraphs. As detailed in \llap (or \rlap) at the beginning of an indented paragraph, the solution is to use \makebox[0pt][l]{...}.

Note that the \BlankText text will run off the right-hand margin the text is too long to fit on the line.

Answer 2

With this example, your dotted gap is as long as the typesetted text would be in the same font:

\documentclass{article}
\usepackage{color}
\makeatletter
\def\gap#1{%
  \setbox\@tempboxa\hbox{{\color{white}#1}}%
  \@tempdima\wd\@tempboxa
  \rlap{\unhbox\@tempboxa}%
  \hb@xt@\@tempdima{\dotfill}%
}
\makeatother   

\begin{document}

  Bla bla bla \gap{Answer 1} Bla bla bla Bla bla bla Bla bla bla Bla bla
  bla Bla bla bla Bla bla bla Bla bla bla \gap{Answer 2 is a bit longer}
  bla blab bla. 

\end{document}

You could, however, add some font size commands beneath the color-command if you need more space, like

  \setbox\@tempboxa\hbox{{\color{white}\Large#1}}%