1. Given $f(x)=x^{2}-2 x$, set $A=\{x \mid f(f(x))=0\}$, then the sum of all elements in set $A$ is $\qquad$ .
$$
f(f(x))=f\left(x^{2}-2 x\right)=\left(x^{2}-2 x\right)^{2}-2\left(x^{2}-2 x\right)=\left(x^{2}-2 x\right)\left(x^{2}-2 x-2\right) \text {, }
$$

So $x_{1}+x_{2}+x_{3}+x_{4}=2+2=4$.