Improve PRs/issues loading performance

[Fs00]

While I was investigating why loading a PRs with lots of comments was extremely slow, I opened up the Android Studio network profiler and saw this:

What was happening in thread RxNewThreadScheduler-23 was that these API calls used to load the PR conversation:

gh4a/app/src/main/java/com/gh4a/fragment/PullRequestFragment.java

Lines 315 to 320 in dfdf3e1

	return Single.zip(
	issueCommentItemSingle,
	eventItemSingle,
	reviewTimelineSingle,
	commitCommentWithoutReviewSingle,
	(comments, events, reviewItems, commentsWithoutReview) -> {

were all being executed sequentially!
To execute them in parallel, I only needed to explicitly subscribe each of those Singles on a background thread (63d4032).
After this change, this is what I saw in the profiler:

Much better. The conversation of that specific PR (PowerShell/PowerShell#9900) now takes half the time to load (from 20 to 10 secs).

I noticed that also issues had the same performance problem (albeit much less serious), so I fixed it also for them (ff17c68).

There are some other misc small changes in this PR:

• all Singles are now subscribed on Schedulers.io() rather than on newThread() (0aef6f6). This avoids spawning a thread for literally every asynchronous action in the app, which is unnecessarily costly.
  io() uses a shared thread pool that grows as needed, so we get the benefits of newThread but reusing previously allocated threads whenever possible.
• I've removed preview headers for checks API, since it became an official part of the API last year.
• I've renamed the PullRequestFragment to PullRequestConversationFragment, to improve clarity and symmetry with PullRequestFilesFragment.

[maniac103]

Great, thanks! Code looks good, I'll give it a run and merge before release tomorrow.

[smichel17]

It's nice to have another person besides @maniac103 helping with development, again :)

[maniac103]

It's nice to have another person besides @maniac103 helping with development, again :)

Indeed, exactly that was my thought as well! Unfortunately I lack time due to work and family time, so it's nice to see someone who can help maintaining the app to at least keep it working decently :-)

[Fs00]

It's nice to have another person besides @maniac103 helping with development, again :)

Indeed, exactly that was my thought as well!

Nice to hear that :)
I've been using this app since a long time and I've decided it was time to start improving it. The good thing is, it isn't neither dead like FastHub nor ugly like the official GH app 😆

Currently I have a list of things I want to address/improve. You can be sure that I won't make ground-breaking changes or add big new features 😅 (I don't even have time to).

[Fs00]

Unfortunately I lack time due to work and family time

Btw no problem for that, life happens :)
I might occasionally ask you for some help in case I run into difficulties I can't find a way through. Is there a way I can contact you without opening an issue here on the repo? @maniac103

[maniac103]

@Fs00 My email is in the commits ;-)

[Fs00]

Oh, right 😂