Prepend is ignored if page size is less than prefetch index

[dinaraparanid]

Fixes #7

[xsahil03x]

Hey, thank you for the PR but this will also trigger notification for all the items after the prefetch item. A better solution would be to count the items of last or first page based on reverse and then match it with
min(fetchIndex, length).

[dinaraparanid]

Hello, thank you for reply! It seems that checking exact item index with == will still not work even for min(fetchIndex, length). There is an example that will break it:

1. Suppose id of message is the key of page (e.g. start page from message with id = ?)
2. Initial key = 20 (message id), page size = 45, prefetchIndex = 22
3. We cannot use == with min(fetchIndex, len) since we have not reached item with index 22 (because we are loaded page from item=20, but it has initial index = 0, meaning that we have to scroll down to item 20+22=42 to trigger prepend operation)

Solution seems to be straightforward: in PageFetcher we disallow to trigger multiple _doLoad calls (we cannot do it on Ui, because of WidgetsBinding.instance.addPostFrameCallback { pager.load(...) }), which will protect pager from multiple requests of items that have passed fetchIndex

[xsahil03x]

Hey @dinaraparanid , can you try this PR #9 and check if this fixes your issue? Thanks

[dinaraparanid]

Hello! PR #9 fixes the main issue, but there is still an issue with preloading, if fetch index is far from visible zone.

1. Suppose that page key is the id of message [each message has id = text = ordering number, e.g. Message(id=1, text=1)]
2. We load the page with key = 20 (id of first visible message), pageSize = 45, prefetchIndex = 22.
3. It is expected that pager will trigger previous page (with key = 1)
4. We have loaded full page (from 20 to 65), but the prefetchIndex is far from visible zone, we have to scroll to item 20+22 = 42 to trigger prepend. In that case prefetchIndex < itemsCount, meaning that fix with if (prefetchIndex > itemCount) { ... } will be ignored

[xsahil03x]

@dinaraparanid I think i understood your issue. THis will only happen for the first page right? Maybe we can add a additional check for this.

if (pages.length == 1) i.e it's the first page, we set the prefetchIndex = 0

What do you think?

[dinaraparanid]

Yeah, it will fix the issue, thanks!

[xsahil03x]

Yeah, it will fix the issue, thanks!

Hey, I just released a new v0.2.0 with the fix. Thanks