  We introduce a time-dependent generalized Floquet (TDGF) approach to
calculate attosecond transient absorption spectra of helium atoms subjected to
the combination of an attosecond extreme ultraviolet (XUV) pulse and a delayed
few-cycle infrared (IR) laser pulse. This TDGF approach provides a Floquet
understanding of the laser-induced change of resonant absorption lineshape. It
is analytically demonstrated that, the phase shift of the time-dependent dipole
moment that results in the lineshape changes consists of the \emph{adiabatic}
laser-induced phase (LIP) due to the IR-induced stark shifts of adiabatic
Floquet states and the \emph{non-adiabatic} phase correction due to the
non-adiabatic IR-induced coupling between adiabatic Floquet states. Comparisons
of the spectral lineshape calculated based on the TDGF approach with the
results obtained with the LIP model [S. Chen \emph{et al.}, Phys. Rev. A
\textbf{88}, 033409(2013)] and the rotating-wave approximation (RWA) are made
in several typical cases. It is suggested in the picture of adiabatic Floquet
states that, the LIP model works as long as the generalized adiabatic theorem
[A. Dodin \emph{et al.}, Phys. Rev. X Quantum \textbf{2}, 030302(2021)]
fulfils, and the RWA works when the higher-order IR-coupling effect in the
formation of adiabatic Floquet states is neglectable.
