设$f(x)$以$2$为周期,当$x∈[-1,1)$时 $f(x)= \begin{cases} -1-x,-1 \le x < 0, \ 2x,0 \le x < \frac {1}{2}, \\ 1-x, \frac {1}{2} \le x < 1, \end{cases}$ 为其傅里叶级数的和函数，则 $S(- \frac {7}{2})=( \quad \quad )$.
$\text{A.}$ 0
$\text{B.}$ $\frac {3}{4} $
$\text{C.}$ $\frac {1}{2} $
$\text{D.}$ 1