tcolorbox - Define a macro for the title that can be empty

Question

I fail to use a macro that give a possibly empty title for a tcolorbox frame.

Here is the code used.

\documentclass{article}
\RequirePackage{tcolorbox}

\begin{document}

\def\thistitle{One title}
\begin{tcolorbox}[title = \thistitle]
    Bla, bla, bla....
\end{tcolorbox}

\def\thistitle{}
\begin{tcolorbox}[title = \thistitle]
    Bla, bla, bla....
\end{tcolorbox}

\begin{tcolorbox}[title = {}]
    Bla, bla, bla....
\end{tcolorbox}
        
\end{document}

Answer 1

You should read the section about pgfkeys in the TikZ & PGF manual. .expand once as well as .expanded can solve the problem here, since you want to obtain the same as title={} when the expansion of \thistitle is empty (.expand once is in my opinion a better choice than .expanded here, since it doesn't expand more than necessary).

\documentclass{article}
\usepackage{tcolorbox}

\begin{document}

\newcommand{\thistitle}{One title}
\begin{tcolorbox}[title/.expand once = \thistitle]
    Bla, bla, bla....
\end{tcolorbox}

\renewcommand{\thistitle}{}
\begin{tcolorbox}[title/.expand once = \thistitle]
    Bla, bla, bla....
\end{tcolorbox}

\begin{tcolorbox}[title = {}]
    Bla, bla, bla....
\end{tcolorbox}

\end{document}

Here is another way using etoolbox and a custom mytitle key. It works differently from the previous technique in that it never feeds any expansion of \thistitle to title. The title key is either called with \thistitle, or not called at all.

\documentclass{article}
\usepackage{tcolorbox}
\usepackage{etoolbox}

\tcbset{mytitle/.code={%
  \expandafter\ifstrempty\expandafter{\thistitle}
    {}
    {\pgfkeysalso{title=\thistitle}}%
}}

\begin{document}

\newcommand{\thistitle}{One title}
\begin{tcolorbox}[mytitle = \thistitle]
    Bla, bla, bla....
\end{tcolorbox}

\renewcommand{\thistitle}{}
\begin{tcolorbox}[mytitle = \thistitle]
    Bla, bla, bla....
\end{tcolorbox}

\begin{tcolorbox}[mytitle = {}]
    Bla, bla, bla....
\end{tcolorbox}

\end{document}

Same output as above.