How with shade and standalone

Question

I want draw a figure like fig 1, but my Latex give fig 2. I think the problem is in:

\shade[top ‎color=blue!2‎0‎‎] (‎-‎‎‎1,‎-1.2‎‎‎‎‎‎‎‎) rectangle (‎3‎‎‎,‎3‎);‎‎

‎‎

Here is the Latex code:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing,patterns}
\usetikzlibrary{arrows.meta}
\begin{document}
\begin{tikzpicture}
\draw[decoration={aspect=0.3, segment length=1.5mm, amplitude=2.4mm,coil},decorate] (1,5) -- (1,3.3);
\fill [pattern = north east lines] (-0.5,5) rectangle (2.5,5.2);
\draw[thick] (-0.5,5) -- (2.5,5);
%\filldraw[fill=blue!20, draw=blue!60] (-1,-1.2) rectangle (3,3);
\draw[line width=0.5pt] (1,3.3) -- (1,2.5);
\draw[line width=1pt] (-1,3) -- (3,3);

\draw[->] (1,0.2) -- (1,-0.8);
\node[draw=none,right=.1cm] at (1.3,4.25) (a) {$K$};
\node[draw=none,right=.1cm] at (0,-0.5) (a) {$f(t)$};
\draw[line width=6pt] (1,2.5) -- (1,-0.2);
\shade[top color=blue!20] (-1,-1.2) rectangle (3,3);
\end{tikzpicture}
\end{document}

The output is not correct!

Answer 1

When drawing a TikZ picture, you should always start with things in the background, and then add objects which are in front of that. Take the following example:

\draw[red,fill] (0,0) circle (0.1);
\draw[blue,fill] (-1,-1) rectangle (1,1);

The result of this will be only the rectangle, as the circle is behind the rectangle.

The same happened to your figure: By placing the \shade command at the bottom of your code, i.e. as the last command, this shaded rectangle is shown in the foreground, while you actually want it to be in the background. As a solution, simply move the command to the start of your tikzpicture:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing,patterns}
\usetikzlibrary{arrows.meta}
\begin{document}
\begin{tikzpicture}

% First draw the background
\shade[top color=blue!20] (-1,-1.2) rectangle (3,3);

\draw[decoration={aspect=0.3, segment length=1.5mm, amplitude=2.4mm,coil},decorate] (1,5) -- (1,3.3);
\fill [pattern = north east lines] (-0.5,5) rectangle (2.5,5.2);
\draw[thick] (-0.5,5) -- (2.5,5);
%\filldraw[fill=blue!20, draw=blue!60] (-1,-1.2) rectangle (3,3);
\draw[line width=0.5pt] (1,3.3) -- (1,2.5);
\draw[line width=1pt] (-1,3) -- (3,3);

\draw[->] (1,0.2) -- (1,-0.8);
\node[draw=none,right=.1cm] at (1.3,4.25) (a) {$K$};
\node[draw=none,right=.1cm] at (0,-0.5) (a) {$f(t)$};
\draw[line width=6pt] (1,2.5) -- (1,-0.2);
\end{tikzpicture}
\end{document}

and you'll get exactly the desired result: