Curved text (on multiple lines) within curved arrow using tikz

Question

I have a curved arrow and want to put text inside of it (having the same curve as the arrow, of course). If the text is longer than the arrow, the text should be put on multiple lines (optimally with the arrow automatically getting thicker appropriately). See figure below.

(How) can I do this?

---

Update:

I now use

  \coordinate (specRoot) at (-10,0);
  \coordinate (testTreeRoot) at (0,0);
  \draw[->, >=latex, blue!20!white, line width=15pt, bend left]
  (specRoot) to (testTreeRoot);
  \path [decorate,decoration={text along path, text={traverse part of
      the STS and heuristically choose the best test tree}}]
     [out=30] (specRoot) to [in=130] (testTreeRoot);

But how can I combine the decorated text along the path and the actual arrow?

How can I do a carriage return, i.e. decorate two lines of text along a single path?

---

Update:

And now I use the following, in the file lazyOTFoverviewSmall.tex:

\begin{tikzpicture}[->,>=stealth',shorten >=1pt,auto,node distance=2.2cm, semithick, mypostaction/.style 2 args={ 
        decoration={
            text align={
                left indent=#1},
                text along path,
                text={#2}
             },
         decorate
         }
  ]

  \begin{scope}[shift={(-10cm,0cm)}]
      \node[state] (specq0) {$q_0$};
  \end{scope}
  \begin{scope}
      \node[state] (q0) {$q_0$};
  \end{scope}

  \coordinate (specRoot) at (-10,0);
  \coordinate (testTreeRoot) at (0,0);

  \draw[-latex, blue!20!white, line width=5ex, bend left]
  (specRoot) ++(1cm,1cm)
  to (testTreeRoot) ++(-1cm,1cm);
  \path [postaction={mypostaction={1cm}{traverse part of the STS}},
postaction={mypostaction={1cm}{and heuristically choose the best test
    tree}, /pgf/decoration/raise=-3mm}]
     [out=30] (specRoot) ++(1cm,1cm) to [in=130] (testTreeRoot) ++(-1cm,1cm);
\end{tikzpicture}

and in master.tex I have:

\usepackage{tikz}
\usetikzlibrary{decorations.text}
\usetikzlibrary{backgrounds,shapes} %pictures
\usetikzlibrary{arrows,automata}

....

\begin{document}

...

\begin{figure}[hbt]
  \centering
  \scalebox{.7}{\input{gfx/MBT/lazyOTFoverviewSmall.tex}}
  \caption{Overview of Lazy On-the-fly MBT}\label{fig:lazyOTFoverview}
\end{figure}

....

Answer 1

TikZ is trying hard but text decorations are not really perfect as the text is decomposed into individual letters and spaces then boxed and rotated. So in general it needs further tweaks to make it look good. The main obstacle is the large arrowhead that changes the bending of the path to fit the arrowhead so the path needs to be adjusted manually. Here is a quick demo but I would recommend different paths for each line to be placed.

\documentclass[tikz]{standalone}
\usetikzlibrary{decorations.text}
\begin{document}
\begin{tikzpicture}[mypostaction/.style 2 args={
                         decoration={
                              text align={
                                    left indent=#1},
                                    text along path, 
                                    text={#2}
                                    },
                           decorate
                        }
                    ]
  \coordinate (specRoot) at (-10,0);
  \coordinate (testTreeRoot) at (0,0);
  \draw[-latex, blue!20!white, line width=5ex]  (specRoot) to[in=135,out=45] (testTreeRoot);

  \path [postaction={mypostaction={1cm}{traverse part of the STS}},
         postaction={mypostaction={1cm}{and heuristically choose the best test tree},
         /pgf/decoration/raise=-3mm}]
    (specRoot) to [in=120,out=45] (testTreeRoot);
\end{tikzpicture}
\end{document}

Answer 2

The answer below uses the wheelchart package, which I wrote.

The text in the arrow is given by the third variable. This text is denoted by \WCvarC and used in the key arc data. Here, \\ can be used to split the text over multiple lines.

This text is centered in the arc with the key arc data pos=0.5.

The arrow is obtained with the key slices end arrow={1}{-1}.

\documentclass[border=6pt]{standalone}
\usepackage{wheelchart}
\usetikzlibrary{decorations.text}
\begin{document}
\begin{tikzpicture}
\wheelchart[
  arc data=\WCvarC,
  arc data angle shift=2,
  arc data expand=f,
  arc data pos=0.5,
  data=,
  radius={4}{5},
  slices end arrow={1}{-1},
  slices style={\WCvarB,draw=black,very thick},
  start half,
  total angle=60
]{1/blue!20/text that is curved and\\can be on multiple lines}
\end{tikzpicture}
\end{document}

Edit: Text from an external file can be placed in the arc data by first saving this text in a macro using for example the catchfile package (see Read a file and store its contents in a variable).

\begin{filecontents}{testfile.tex}
text from an\\external file
\end{filecontents}
\documentclass[border=6pt]{standalone}
\usepackage{catchfile}
\usepackage{wheelchart}
\usetikzlibrary{decorations.text}
\begin{document}
\begin{tikzpicture}
\CatchFileDef{\mytext}{testfile}{\endlinechar=-1 }
\wheelchart[
  arc data=\mytext,
  arc data angle shift=2,
  arc data expand=f,
  arc data pos=0.5,
  data=,
  radius={3}{4},
  slices end arrow={1}{-1},
  slices style={\WCvarB,draw=black,very thick},
  start half,
  total angle=60
]{1/red!20}
\end{tikzpicture}
\end{document}

The text from the command \lipsum[1] can not be placed directly in the arc data because it is unexpandable. This can be solved by using the macros \lipsum_get_paragraph:n and \lipsum_get_sentences:nnV from the lipsum package.

\documentclass[border=6pt]{standalone}
\usepackage{lipsum}
\usepackage{wheelchart}
\usetikzlibrary{decorations.text}
\begin{document}
\begin{tikzpicture}
\ExplSyntaxOn
\edef \testa { \lipsum_get_paragraph:n { 1 } }
\edef \testb
  {
    \lipsum_get_sentences:nnV { 1 } { 1 } \testa \\
    \lipsum_get_sentences:nnV { 2 } { 2 } \testa
  }
\ExplSyntaxOff
\wheelchart[
  arc data=\testb,
  arc data angle shift=2,
  arc data expand=f,
  arc data pos=0.5,
  data=,
  radius={7}{8},
  slices end arrow={1}{-1},
  slices style={\WCvarB,draw=black,very thick},
  start half,
  total angle=90
]{1/green!20}
\end{tikzpicture}
\end{document}