  We present how to control interactions between solitons, either bright or
dark, in Bose-Einstein condensates by synchronizing Feshbach resonance and
harmonic trap. Our results show that as long as the scattering length is to be
modulated in time via a changing magnetic field near the Feshbach resonance,
and the harmonic trapping frequencies are also modulated in time, exact
solutions of the one-dimensional nonlinear Schr\"{o}dinger equation can be
found in a general closed form, and interactions between two solitons are
modulated in detail in currently experimental conditions. We also propose
experimental protocols to observe the phenomena such as fusion, fission, warp,
oscillation, elastic collision in future experiments.
