  Ultra-peripheral collisions (UPC) are events characterised by large impact
parameters between the two projectiles, larger than the sum of their radii. In
UPCs, the protons and ions accelerated by the LHC do not interact via the
strong interaction and can be regarded as sources of quasireal photons.
Photoproduction of vector mesons (e.g. J/$\psi$ and $\psi$') in UPC is quite
interesting since it is sensitive to the low-$x$ gluon density. Owing to the
statistics available from Run 2 data, the ALICE Collaboration has been able to
carry out differential measurements, such as the measurement of the
rapidity-differential cross section of coherent J/$\psi$ production in Pb--Pb
UPCs and the $t$-dependence of coherent and incoherent J/$\psi$, which are
sensitive to the average of the spatial distribution of the gluons and its
variance, respectively. In addition, the ALICE Collaboration has measured the
energy dependence of the photonuclear cross section with a few different
strategies, such as by measuring the rapidity-differential cross section of
coherent J/$\psi$ production in different impact parameter ranges i.e. in
ultra-peripheral and peripheral collisions, and in neutron emission classes.
The latter two provide promising tools for the resolution of the ambiguity in
Bjorken-$x$ which arises in symmetric A--A UPCs.
