If $\angle DAC=\angle B$, then $DA$ is the tangent line of the circumcircle of $\triangle ABC$. \begin{proof}Make tangent $AD'$ of the circumcircle of $\triangle ABC$ at $A$, then we have $D'AC=\angle B$, so $AD$ and $AD'$ must be the same line.\end{proof}

Make tangent $AD'$ of the circumcircle of $\triangle ABC$ at $A$, then we have $D'AC=\angle B$, so $AD$ and $AD'$ must be the same line.