With continue chain, how to achieve absolute node positioning?

Question

My problem is that I want to draw several horizontal "layers" of nodes (implemented as chains), with connections between them. When I try to position a node absolutely, it is additionally offset in the chain's direction:

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{chains}
\begin{document}
\begin{tikzpicture}[
  node distance=1,
  block/.style={
    on chain,
    inner sep=0pt,outer sep=0pt,
    draw,minimum height=10,minimum width=2}
]

\path[start chain=layer1]
  node[block] {}
  node[block,join] {}
  node[block,join] {}
;

\path[start chain=layer2]
  node[block,join=with layer1-3,below=of layer1-end] {}
  node[block,join] {}
  node[block,join] {}
;

%\path[start chain=layer1right] % WORKS
\path[continue chain=layer1]    % DOES NOT WORK, BUT WHY?
  node[block,join=with layer2-end,anchor=center] at (layer2-end |- layer1-end) {}
  node[block,join] {}
  node[block,join] {}
;

\end{tikzpicture}
\end{document}

I also tried variants like

node[block,join=with layer2-end placed {at=(layer2-end |- layer1-end)},anchor=center] {}

and I don't understand why I get the additional offset (I did find a workaround, see below). According to the pgfmanual, I think this should've worked:

Independently of which form is used, the ⟨text⟩ inside ⟨direction⟩ that follows going or placed (separated by a compulsory space) can have two different effects:

(a) If it contains an equal sign, then this ⟨text⟩ is used as the placement rule, that is, it is simply executed.

(b) If it does not contain an equal sign, then ⟨text⟩=of \tikzchainprevious is used as the placement rule.

Note that in the first case, inside the ⟨text⟩ you have access to \tikzchainprevious and \tikzchaincount for doing your positioning calculations.

My workaround (see commented out code) is to start a new chain (which has the additional advantage that I could remove the anchor=center). I cannot use above=of ..., because in my real code, the blocks have different heights, and the nodes on layer1 will not be aligned anymore.

Answer 1

If I understand question correctly, than you looking for something like this:

Code:

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{chains,positioning}

    \begin{document}
\begin{tikzpicture}[
  node distance=1,
  block/.style={
    on chain,
    inner sep=0pt,outer sep=0pt,
    draw,minimum height=10,minimum width=2}
                    ]
\path[start chain=layer1]
  node[block] {}
  node[block,join] {}
  node[block,join] {};
\path[start chain=layer2]
  node[block,join=with layer1-end,below=of layer1-end] {}
  node[block,join] {}
  node[block,join] {};
\path[continue chain=layer1]   
  node[block,join=with layer2-end,above=of layer2-end] {}% <--
  node[block,join] {}
  node[block,join] {};
\end{tikzpicture}
    \end{document}

As you see, in continuation I use the same syntax as you for draw chain layer2. Options continue chain=... means that the chain will continue, where you indicate, in this case above of end node in chain2.

Main benefits to use of chains is relative positioning of nodes: next node in chain is always placed relatively to node before it.

Addendum: From comments can be concluded, that you looking for nodes on the grid:

New MWE:

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{chains,positioning}

\begin{document}
\begin{tikzpicture}[
    on grid,
  node distance=2cm and 0.5 cm,
  block/.style={
    on chain,
    inner sep=0pt,outer sep=0pt,
    draw,minimum height=#1,minimum width=2}
                    ]
\path[start chain=layer1]
  node[block=1cm] {}
  node[block=2cm,join] {}
  node[block=1cm,join] {};
\path[start chain=layer2]
  node[block=1cm,join=with layer1-end,below=of layer1-end] {}
  node[block=1cm,join] {}
  node[block=1cm,join] {};
\path[continue chain=layer1]
  node[block=2cm,join=with layer2-end,above=of layer2-end] {}% <--
  node[block=1cm,join] {}
  node[block=1cm,join] {};
\end{tikzpicture}
\end{document}

Answer 2

I would:

• set up layer2 as a branch;
• add the precisely positioned node outside the chain;
• use \chainin to add that node into the chain before continuing with the remaining nodes.

For example:

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{chains,scopes}
\begin{document}
\begin{tikzpicture}
  [
    node distance=1,
    basic block/.style={%
      inner sep=0pt,
      outer sep=0pt,
      draw,
      minimum height=10,
      minimum width=2,
    },
    block/.style={%
      basic block,
      on chain,
    },
    start chain=layer1,
  ]
  \path
  node [block] {}
  node [block,join,minimum height=20] {}
  node [block,join,minimum height=5] {}
  ;
  {[start branch=layer2 going below]
    \node [block,join,minimum height=20] {};
  }
  {[continue branch=layer2 going right]
    \path
    node [block,join] {}
    node [block,join,minimum height=30] {}
    ;
  }
  \node (a) [basic block] at (layer1/layer2-end |- layer1-3) {};
  [continue chain=layer1]
  \chainin (a) [join=with layer1/layer2-end];
  \path
  node [block,join,minimum height=20] {}
  node [block,join] {}
  ;
\end{tikzpicture}
\end{document}

Note that the chains implementation is a little ... eccentric ... at times. As I understand it, it is considered deprecated and, hence, very unlikely to get significant improvements.

[I think this is a shame because the idea has a pleasing simplicity and is more friendly to beginners than the functionality provided by the graphs syntax, which replaces it. The graphs syntax is, however, more flexible, powerful and concise.]