`create_range` and `SlidingSyncList::next_request` return a `Result`
instead of an `Option`. It was a legacy from the old `impl Iterator` of
`SlidingSyncListRequestGenerator`. But actually, when `create_range`
fails, it must return a `Err` value, not a `None` value.
And since it's a regular error, `sync_once` can propagate the
error. Thus, it is no longer necessary to “update” the list of
`SlidingSyncList`. It's not necessary to remove some items in this list
when the `next_request` method can no return a value: it always returns
a value, except when a error happens.
First off, `set_ranges`, `set_range`, `add_range` and `reset_ranges`
take a `&[(U, U)]` instead of a `Vec<(U, U)>` when a vector was needed,
or takes a `(U, U)` instead of 2 arguments when it was needed.
Second, all those methods now return a `Result<(), Error>`. The
`Error::CannotModifyRanges` is raised if the chosen `SlidingSyncMode`
doesn't allow to modify ranges. Basically, `Selective` does allow a
user to modufy the ranges, but there is no real ranges with `Growing` or
`Paging`. Let's make it an explicit error.
Finally, `SlidingSyncListInner` has 2 methods: `set_ranges` and
`add_range`, but without any “ranges check”; `SlidingSyncList` does call
the inner methods, but does the checks.
The `SlidingSyncListInner::find_rooms_in_list` method can be greatly
improved by using the `Iterator` API.
It was already using `Iterator::skip`, great! Let's continue by using
`Iterator::take` instead of using a stop index.
And let's use `Iterator::enumerate` to avoid using a mutable index.
`SlidingSyncList::find_room_in_list` and
`SlidingSyncList::find_rooms_in_list` aren't useful (and never used) by
the public consumer of this API. Let's remove it. They are only used for
internal purposes.
Before, the `update_request_generator_state` method was emitting an
`error!` log, but not an error. But that's clearly an error!
This patch updates the method to return a `Result<(), Error>`, and adds
a new variant to `Error`.
There are problems with `SlidingSyncListRequestGenerator`:
* It's part of the module API,
* It contains a clone of `SlidingSyncList`.
To create a `SlidingSyncListRequestGenerator`, one has to
call `SlidingSyncList::request_generator`. It was done in
`SlidingSync::stream`. The problem is that it clones `SlidingSyncList`.
So theoritically it is possible to create multiple request generators
for the _same_ list, and use them to send many requests and to update
the _same_ list with multiple responses. This is utterly error-prone and
can lead to really complex bugs to discover.
Moreover, it's a lot of clones. Cloning a
`SlidingSyncListRequestGenerator` isn't cheap as it means cloning a
`SlidingSyncList`. Moreover, cloning a `SlidingSyncList` isn't cheap as
it means cloning the entire struct.
Having `SlidingSyncListRequestGenerator` inside the module API also
makes the code of `SlidingSync` more complex (why having to deal with
lists and request generators at the same time? we must be very careful
to maintain both side by side? what if a list is removed but not its
request generator? and so on).
So. This patch simplifies all that.
First off, it extracts all the fields of `SlidingSyncList` into a
`SlidingSyncListInner` struct. Then, `SlidingSyncList` has only one
field: `Arc<SlidingSyncListInner>`. Boom, it's now cheap to clone it.
Second, `SlidingSyncListRequestGenerator` is only a
struct with constructors, but there is no extra methods.
`SlidingSyncListRequestGenerator` _no longer_ contains a
`SlidingSyncList`, and doesn't no need a list at all to work. It's just
values.
Third, `SlidingSyncList` holds a `SlidingSyncListRequestGenerator`, and
only one.
Fourth, `SlidingSyncList` (and `SlidingSyncListInner`) now has methods
to handle the internal request generator. The initial `impl Iterator for
SlidingSyncListRequestGenerator` becomes a simple
`SlidingSyncList::next_request` method.
Fifth, previously, the `SlidingSyncList::handle_response`
was never called directly by `SlidingSync`. It was called by
`SlidingSyncListRequestGenerator`! How confusing! `SlidingSync` called
`SlidingSyncListRequestGenerator::handle_response` which was calling
`SlidingSyncList::handle_response`. Now, the flow is more natural:
`SlidingSync` calls `SlidingSyncList::handle_response` and that's it.
Sixth, the `SlidingSyncList::handle_response` is now composed of 2
parts: updating the list itself, and updating the request generator. It
was kind of the case before, but onto two different types.
It was unclear which types were updating `SlidingSyncList`.
For example, `SlidingSyncList::state` was updated by…
`SlidingSyncListRequestGenerator`. Now `SlidingSyncList` is responsible
to update itself, and no one else.
Finally, `SlidingSync` no longer have to deal with
`SlidingSyncListRequestGenerator`. All it has is a set of
`SlidingSyncList`, and that's it!
The tests are still passing, hurray.
Damir Jelić