如图，抛物线\(C_{1}：y^{2}=2px(p > 0)\)，圆\(C_{2}：(x- \dfrac {p}{2})^{2}+y^{2}=1\)，圆\(C _{2}\)与\(y\)轴相切，过\(C _{1}\)的焦点\(F\)的直线从上至下依此交\(C _{1}\)，\(C _{2}\)于\(A\)，\(B\)，\(C\)，\(D\)，且\(|AB|=|BD|\)，\(O\)为坐标原点，则\( \overrightarrow {DA}\)在\( \overrightarrow {OF}\)方向上的投影为\((\:\:\:\:)\)