  In this work, we propose a Crank-Nicolson-type scheme with variable steps for
the time fractional Allen-Cahn equation. The proposed scheme is shown to be
unconditionally stable (in a variational energy sense), and is maximum bound
preserving. Interestingly, the discrete energy stability result obtained in
this paper can recover the classical energy dissipation law when the fractional
order $\alpha \rightarrow 1.$ That is, our scheme can asymptotically preserve
the energy dissipation law in the $\alpha \rightarrow 1$ limit. This seems to
be the first work on variable time-stepping scheme that can preserve both the
energy stability and the maximum bound principle.
  Our Crank-Nicolson scheme is build upon a reformulated problem associated
with the Riemann-Liouville derivative. As a by product, we build up a
reversible transformation between the L1-type formula of the Riemann-Liouville
derivative and a new L1-type formula of the Caputo derivative, with the help of
a class of discrete orthogonal convolution kernels. This is the first time such
a \textit{discrete} transformation is established between two discrete
fractional derivatives. We finally present several numerical examples with an
adaptive time-stepping strategy to show the effectiveness of the proposed
scheme.
