  If dark energy interacts with dark matter, there is a change in the
background evolution of the universe, since the dark matter density no longer
evolves as a^{-3}. In addition, the non-gravitational interaction affects the
growth of structure. In principle, these changes allow us to detect and
constrain an interaction in the dark sector. Here we investigate the growth
factor and the weak lensing signal for a new class of interacting dark energy
models. In these models, the interaction generalises the simple cases where one
dark fluid decays into the other. In order to calculate the effect on structure
formation, we perform a careful analysis of the perturbed interaction and its
effect on peculiar velocities. Assuming a normalization to today's values of
dark matter density and overdensity, the signal of the interaction is an
enhancement (suppression) of both the growth factor and the lensing power, when
the energy transfer in the background is from dark matter to dark energy (dark
energy to dark matter).
