答案：
解: $\because$ 四边形 $A B C D$ 是矩形,
$$
\begin{aligned}
& \therefore \angle D C E=90^{\circ}, A D / / B C, \\
& \therefore \angle A D F=\angle D E C . \\
& \because A F \perp D E, \therefore \angle A F D=90^{\circ}, \\
& \therefore \angle A F D=\angle D C E, \\
& \therefore \triangle A F D \backsim \triangle D C E, \\
& \therefore \frac{D F}{C E}=\frac{A D}{D E}, \\
& \text { 又 } \because C E=2, D E=2 \sqrt{10}, \\
& \therefore \frac{D F}{2}=\frac{4}{2 \sqrt{10}}, \\
& \therefore D F=\frac{2 \sqrt{10}}{5},
\end{aligned}
$$