将case内部的等号与case外部的等号对齐

Question

我想将盒子内的等号与外面的等号对齐。怎么做？我已阅读这些问题的答案1 2 3，但我还是很困惑。有人能帮忙吗？谢谢！

\documentclass{article}
\usepackage{amsmath}
\begin{document}  
\begin{align*}
    v&=s\\
    \begin{cases}
    v'_x&=1\\
    v'_y&=2\\
    \end{cases}
\end{align*}
\end{document}

Answer 1

该包的可能性blkarray：

    \documentclass{article}
    \usepackage{blkarray}
    \usepackage{amsmath}

    \begin{document}

    \[ \begin{blockarray}{r@{}l}
        v&{}=s\\
        \begin{block}{\{r@{}l}
        v'_x &{}=1\\
        v'_y&{}=2\\
        \end{block}
    \end{blockarray} \]

    \end{document} 

Answer 2

只需调整代码即可https://tex.stackexchange.com/a/621802/4427

\documentclass{article}
\usepackage{amsmath}
\usepackage{nicematrix}

\begin{document}

\[
\setlength{\arraycolsep}{0pt}
\begin{NiceArray}{r >{{}}c<{{}} l}
    v&=&s\\[1ex]
    v'_x&=&1\\[1ex]
    v'_y&=&2
\CodeAfter\SubMatrix\{{2-1}{3-3}.
\end{NiceArray}
\]

\end{document}

Answer 3

一种方法是绘制以下方程组：

\documentclass[border=5mm, preview]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing,
                matrix}

\begin{document}
\[
\begin{tikzpicture}
\matrix (a) [matrix of math nodes, 
             nodes = {inner ysep=2pt},
             column sep=-4pt
             ]
{
v       & = s   \\
v'_x    & = 1   \\
v'_y    & = 2   \\
};
\draw[decorate,decoration={brace, mirror}] 
    (a-2-1.north -| a-3-1.west) -- (a-3-1.south west);
\end{tikzpicture}
\]
\end{document}