\tikztonodes not picking up the nodes

Question

The following declaration

\tikzset{
      ncbar/.style={
        to path={%
        ($(\tikztostart)!#1!90:(\tikztotarget)$)
        -- ($(\tikztotarget)!#1!270:(\tikztostart)$)\tikztonodes}},
      ncbar/.default=0.5cm,
    }

does not pick up the \tikztonodes command, since the two nodes

\draw[<->] (A) to[ncbar=0.2](C) node[midway, sloped, above] {\small $30dm$}; 
\draw[<->] (C) to[ncbar=0.33](B) node[midway, sloped, above] {\small $40dm$}; 

do not appear in the picture:

Code inspired by

• How can I invert a 'clip' selection within TikZ?
• Parallel line segments connecting points in tikz
• Tikz - Custom Path Style with a Node to Label the Path

\documentclass{article}
\usepackage{amsmath}
\usepackage{pgf,tikz,pgfplots, tikz-cd}
\usetikzlibrary{arrows, intersections,decorations.markings, patterns, calc}
\usepackage{tkz-euclide}
\usepackage{calc}
\tikzset{
      ncbar/.style={
        to path={%
        ($(\tikztostart)!#1!90:(\tikztotarget)$)
        -- ($(\tikztotarget)!#1!270:(\tikztostart)$)\tikztonodes}},
      ncbar/.default=0.5cm,
    }
\begin{document}
\begin{tikzpicture}[remember picture]
% A path that follows the edges of the current page
\tikzstyle{reverseclip}=[insert path={(current page.north east) --
  (current page.south east) --
  (current page.south west) --
  (current page.north west) --
  (current page.north east)}
]
        \coordinate[] (A) at (-1,0);
        \coordinate[] (B) at (1,0);
        \coordinate[] (C) at (-0.5,0.86602);
\draw (A) -- (B) -- (C) -- cycle;
\draw (B) arc[start angle=0, end angle=180, radius=1];
\begin{pgfinterruptboundingbox} 
    \clip  (A) -- (B) -- (C) -- cycle [reverseclip]; 
\end{pgfinterruptboundingbox}
\fill[fill=green!20!white] (B) arc[start angle=0, end angle=180, radius=1];
\draw[<->] (A)+(0,-.2)--($(B)+(0,-.2)$) node[midway, below] {\small $50dm$}; 
\draw[<->] (A) toncbar=0.2 node[midway, sloped, above] {\small $30dm$}; 
\draw[<->] (C) toncbar=0.33 node[midway, sloped, above] {\small $40dm$}; 
\end{tikzpicture}
\end{document}

Answer 1

Sorry, I was not able to figured out the most part of your code. If the problem is reproduce showed image, than the MWE, which do this, can be (much simpler):

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows.meta, 
                calc,
                decorations.markings, 
                quotes}
\usepackage{siunitx}
\tikzset{
every edge/.style = {draw, Straight Barb-Straight Barb},
every edge quotes/.style = {auto, font=\footnotesize, sloped}
        }
\begin{document}
    \begin{tikzpicture}
\coordinate (A) at (-1,0);
\coordinate (B) at (1,0);
\coordinate (C) at (-0.5,0.86602);
%
\draw[fill=green]   (B) arc (0:180:1);
\draw[fill=white]   (A) -- (B) -- (C) -- cycle;
%
\draw 
    ([yshift=-2mm] A) 
        edge["\qty{50}{\deci\metre}" below]
    ([yshift=-2mm] B)
    ($(B)!7mm!-90:(C)$)
        edge["\qty{40}{\deci\metre}"]
    ($(C)!7mm!+90:(B)$)
    ($(A)!3mm!90:(C)$)
        edge["\qty{30}{\deci\metre}"]
    ($(C)!3mm!-90:(A)$)
    ;
    \end{tikzpicture}
\end{document}

Addendum:

What is wrong with your code?

• it is unnecessary complicated, so it easy to lost in it
• code for arrows with labels is wrong. Instead of used

\draw[<->] (A) to[ncbar=0.2](C) node[midway, sloped, above] {\small $30dm$}; 
\draw[<->] (C) to[ncbar=0.33](B) node[midway, sloped, above] {\small $40dm$}; 

it should be:

\draw[<->]  (A) 
        to[ncbar=0.3] node[font=\small, sloped, above] {$30dm$}
            (C);
\draw[<->]  (C) 
        to[ncbar=0.4] node[font=\small, sloped, above] {$30dm$}
            (B);

• After cleanup your document example, your document example can be:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows, 
                calc,
                decorations.markings, 
                intersections,
                }
\usepackage{tkz-euclide}
\usepackage{calc}
\tikzset{
ncbar/.style={
        to path={%
        ($(\tikztostart)!#1!90:(\tikztotarget)$)
        -- ($(\tikztotarget)!#1!270:(\tikztostart)$)\tikztonodes}},
      ncbar/.default=0.5cm,
    }
\begin{document}
\begin{tikzpicture}
        \coordinate[] (A) at (-1,0);
        \coordinate[] (B) at (1,0);
        \coordinate[] (C) at (-0.5,0.86602);
\draw[fill=green!20!white] 
        (B) arc[start angle=0, end angle=180, radius=1];
\draw[fill=white] 
        (A) -- (B) -- (C) -- cycle;
%
\draw[<->]  ($(A)+(0,-.2)$) --

        node[font=\small, below] {$50dm$}
            ($(B)+(0,-.2)$);
\draw[<->]  (A) 
        to[ncbar=0.3] node[font=\small, sloped, above] {$30dm$}
            (C);
\draw[<->]  (C) 
        to[ncbar=0.4] node[font=\small, sloped, above] {$30dm$}
            (B);
\end{tikzpicture}
\end{document}

and gives the following result:

However, the same result can be obtained with your code too if in it is considered suggested changes. Using it you need at least two compilation to get correct result.

Comparison above MWE (Minimal Working Example) and your code clearly show, that the most of code in your document example are unnecessary and therefore can be consider as clutter and should be omitted.

Answer 2

Another solution with TikZ, you can use even odd rule for more concise code.

\documentclass[tikz,border=5mm]{standalone}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
\def\r{5/2}
\pgfmathsetmacro{\t}{atan(3/4)}
\path
(-\r,0) coordinate (A)      
(\r,0)  coordinate (B)
({180-2*\t}:\r) coordinate (C)
;
\tikzset{n/.style={midway,sloped,fill=white,text=black}}
\draw[|<->|,gray] ($(B)!13mm!-90:(C)$)--($(C)!13mm!90:(B)$)
node[n]{$40\,\mathrm{dm}$};
\draw[|<->|,gray] ($(A)!9mm!90:(C)$)--($(C)!9mm!-90:(A)$)
node[n]{$30\,\mathrm{dm}$};
\draw[|<->|,gray] ([yshift=-5mm]A)--([yshift=-5mm]B)
node[n]{$50\,\mathrm{dm}$};
\draw[fill=green,even odd rule]
(B) arc(0:180:\r)--cycle
(A)--(B)--(C)--cycle;
\end{tikzpicture}
\end{document}

---

A try with Asymptote. It seems that siunitx does not work on http://asymptote.ualberta.ca/

//usepackage("siunitx");  // It seems does not work on http://asymptote.ualberta.ca/
unitsize(1cm);
real r=5/2;
real t=aTan(3/4);      // in degrees
pair A=(-r,0), B=(r,0), C=r*dir(180-2t);
path tri=A--B--C--cycle;
path cir=arc((0,0),r,0,180)--cycle;
transform s=shift(0,-.4);
draw(Label("$50 ,\mathrm{dm}$",black,Fill(white),align=(0,0)),sA--sB,gray,Arrows(TeXHead),Bars);
pair Bt=B+1.3unit(rotate(-90,B)(C-B)); //dot(Bt);
pair Ct=C+1.3unit(rotate(90,C)(B-C)); //dot(Ct);
draw(Label("$40 ,\mathrm{dm}$",black,Rotate(Bt-Ct),Fill(white),align=(0,0)),Bt--Ct,gray,Arrows(TeXHead),Bars);
pair As=A+.9unit(rotate(90,A)(C-A)); //dot(Bt);
pair Cs=C+.9unit(rotate(-90,C)(A-C)); //dot(Ct);
draw(Label("$30 ,\mathrm{dm}$",black,Rotate(Cs-As),Fill(white),align=(0,0)),Cs--As,gray,Arrows(TeXHead),Bars);
filldraw(tri^^cir,evenodd+yellow);
shipout(bbox(5mm,invisible));