Rounded corners as a tangent in the right direction

Question

I want to split a path into two branches as in the following MWE.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
  \node (A) [rectangle] {A};
  \node (B) [rectangle,above right=1cm and 4cm of A] {B};
  \node (C) [rectangle,below right=1cm and 4cm of A] {C};
  \draw [->, rounded corners=1cm] (A) -| +(2cm,0cm) |- (B);
  \draw [->, rounded corners=1cm] (A) -| +(2cm,0cm) |- (C);
\end{tikzpicture}
\end{document}

This is the result:

There are many things wrong with that.

1. The left quater circle to C goes in the wrong direction
2. The circles should be a tangent to each other
3. They should meet in exactly the middle of A and BC

I think I still didn't get the thing about positioning in TikZ. Can someone help me?

Answer 1

Old Style

Positioning

Without the on grid option nodes are placed so that the distance between their border is correct. With on grid the .center anchors are used for calculation.

Code

\documentclass[tikz,border=2pt]{standalone}
\usetikzlibrary{positioning}
\tikzset{
    every node/.style={draw,fill,fill opacity=.1},
    helper node/.style={draw=none,fill=none,fill opacity=1},
    ruler/.style={line width=1.2\pgflinewidth,red,|-|,shorten >=-.5\pgflinewidth,shorten <=-.5\pgflinewidth},
    helper/.style={line width=1.2\pgflinewidth,red,-,dashed},
    } % for debugging purposes

\begin{document}
\begin{tikzpicture}
    \draw[help lines] (-1,-2) grid (5,2);
  \node (A) [rectangle] {A};
  \node (B) [rectangle,above right=1cm and 4cm of A] {B};
  \node (C) [rectangle,below right=1cm and 4cm of A] {C};

  \draw[ruler] (A.north east) -- (A.north east |- B.south west) node[left,pos=.5,helper node] {1\,cm};
  \draw[helper] (A.north east |- B.south west) -- (B.south east);
  \draw[ruler] (A.north east) -- (A.north east -| B.south west) node[above,pos=.5,helper node] {4\,cm};
  \draw[helper] (A.north east -| B.south west) -- (B.north west);
\end{tikzpicture}

\begin{tikzpicture}[on grid]
    \draw (-1,-2) grid (5,2);
  \node (A) [rectangle] {A};
  \node (B) [rectangle,above right=1cm and 4cm of A] {B};
  \node (C) [rectangle,below right=1cm and 4cm of A] {C};

  \draw[ruler] (A.center) -- (A.center |- B.center) node[left,pos=.5,helper node] {1\,cm};
  \draw[helper] (A.center |- B.center) -- (B.east);
  \draw[ruler] (A.center) -- (A.center -| B.center) node[above,pos=.5,helper node] {4\,cm};
  \draw[helper] (A.center -| B.center) -- (B.north);
\end{tikzpicture}
\end{document}

Output

Weird vertical lines

Don’t use -| for the first part of the path as the next point (2cm,0cm) is in fact already horizontal to the start point. Why is the weird line missing to C? Because it is the same as to B, apparently TikZ does a >= comparison with the y-values of the start and the target coordinate to calculate in which direction (left or right) the rounded circle have to go.

Code

  \draw [->, rounded corners=1cm] (A) -| +(2cm, 0cm   ) |- (B);
  \draw [->, rounded corners=1cm] (A) -| +(2cm,-0.01cm) |- (C);

Output

Rounded circles

A rounded-circle radius of 1cm needs at least 2cm to do a full 90-degree-left and 90-degree-right turn. So you either have to use .5cm (= half of vertical distance) in the on grid version or calculate the needed radius within the path operation.

The easiest way to do this is the let … in operation which needs the calc library.
I provide a curvy style that takes one optional argument (default: 2cm).

Code

\documentclass[tikz,border=2pt]{standalone}
\usetikzlibrary{positioning,calc}
\tikzset{
    curvy/.style={
        to path={
            let \p1=(\tikztostart),
                \p2=(\tikztotarget),
                \n1={abs(\y2-\y1)/2}
            in
            [rounded corners=\n1]
            (\tikztostart) -- + (0:#1) |- (\tikztotarget) \tikztonodes
        }
    },
    curvy/.default=2cm,
}
\begin{document}
\begin{tikzpicture}
  \node (A) [rectangle] {A};
  \node (B) [rectangle,above right=1cm and 4cm of A] {B};
  \node (C) [rectangle,below right=1cm and 4cm of A] {C};
  \draw [->] (A) to[curvy]     (B);
  \draw [->] (A) to[curvy=3cm] (C);
\end{tikzpicture}

\begin{tikzpicture}[on grid]
  \node (A) [rectangle] {A};
  \node (B) [rectangle,above right=1cm and 4cm of A] {B};
  \node (C) [rectangle,below right=1cm and 4cm of A] {C};
  \draw [->] (A) to[curvy]     (B);
  \draw [->] (A) to[curvy=1cm] (C);
\end{tikzpicture}
\end{document}

Output

New Style

You might want to check out the tree-producing facilities of TikZ.

Code

\documentclass[tikz,border=2pt]{standalone}
\usetikzlibrary{positioning,calc}
\begin{document}
\begin{tikzpicture}[
    grow'=right,% clockwise, not counterclockwise (which would be "grow")
    level 1/.style={level distance=4cm,sibling distance=2cm},
    level 2/.style={level distance=2cm,sibling distance=1.5cm},
    edge from parent path={% the let ... in operator is only used to set rounded corners 
                           % to zero if parent and child are on the same height, but due to
                           % imprecision in the math-engine the comparison value is 0.001
        let \p1=(\tikzparentnode\tikzparentanchor),
            \p2=(\tikzchildnode\tikzchildanchor),
            \n1={abs(\y2-\y1) > 0.001} 
        in
        [rounded corners=\n1*\tikzsiblingdistance/4]
             (\tikzparentnode\tikzparentanchor)
            -- + (0:\tikzleveldistance/2)
            |-   (\tikzchildnode\tikzchildanchor)
    }
]
\node {A}
    child {node {B} 
        child {node {B1}}
        child {node {B2}}
        child {node {B3}}
    }
    child {node {C}}
;
\end{tikzpicture}

Output

Answer 2

Is it like this?

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
  \node (A) [rectangle] {A};
  \node (B) [rectangle,above right=1cm and 4cm of A] {B};
  \node (C) [rectangle,below right=1cm and 4cm of A] {C};
  \draw [->, rounded corners=.7cm] (A) -- (0:2cm)  |- (B);
  \draw [->, rounded corners=.7cm] (A) -- (0:2cm) |- (C);
\end{tikzpicture}
\end{document}