[POC] Move Model mappings to Background

[polqf]

🔗 Issue Links

https://stream-io.atlassian.net/browse/CIS-1854

🎯 Goal

The goal of this PR is to investigate the viability of moving the mapping of DTOs to Model to a background thread, reducing the pressure we put in the view/main thread.

📝 Summary

This PR introduces a new ListDatabaseObserver, matching the same API for simplicity purposes. This new object, named BackgroundListDatabaseObserver does exactly the same as its predecessor, but in a background and asynchronous way, and only notifies about results when those are already mapped.

🛠 Implementation

As of now:

• The observer listens to a particular fetch request and processes the results of any update that might be coming from the Database
• It converts the latest snapshot of DTOs into Models in a background thread, thus not leading to pressure on the main thread.
• It calls its delegate (in this case a block) whenever all the objects have been parsed
• Because all the mapped objects are assigned to a property on the same (internal) thread, that should result in no data races
• CoreDataLazy is effectively disabled under the runtime flag _isLazyMappingEnabled

In order for background mapping to work, we cannot use the diffing mechanism on the messages list (for now)
There are several deficiencies that were found during this process and should be addressed before iterating this POC even further.

All this implementation is guarded now behind the following mechanisms:

• ListDatabaseObserverWrapper, which based on a flag passed on init, internally uses either ListDatabaseObserver or BackgroundListDatabaseObserver
• _isBackgroundMappingEnabled which enables the usage of BackgroundListDatabaseObserver
• It is necessary to also disable _isLazyMappingEnabled when enabling _isBackgroundMappingEnabled

When using DemoApp-StreamDevelopers, it uses Background Mapping by default

☑️ Contributor Checklist

• I have signed the Stream CLA (required)
• Changelog is updated with client-facing changes
• New code is covered by unit tests
• This change follows zero ⚠️ policy (required)
• Comparison screenshots added for visual changes
• Affected documentation updated (docusaurus, tutorial, CMS)

🎁 Meme

[emerge-tools]

📏 Size Analysis

Total install size 9.4 MB | This change: ⬆️ +26.6 kB (+0.283%)

🗂 See size breakdown

Item	Install size
➕ StreamChat.framework/StreamChat.BackgroundListDatabaseObserver.listenForRemoveAllDataNotifications	⬆️ 4.9 kB
➕ StreamChat.framework/StreamChat.BackgroundListDatabaseObserver.mapItems(completion)	⬆️ 4.5 kB
StreamChat.framework/StreamChat.LazyCachedMapCollection.init(source,map)	⬆️ 2.3 kB
➕ StreamChat.framework/StreamChat.BackgroundListDatabaseObserver.startObserving	⬆️ 2.1 kB
StreamChat.framework/StreamChat.CoreDataLazy.projectedValue.setter	⬆️ 1.9 kB
➕ StreamChat.framework/StreamChat.BackgroundListDatabaseObserver.init(context,fetchRequest,itemCreator,fetchedResultsControllerType)	⬆️ 1.8 kB
➕ StreamChat.framework/StreamChat.BackgroundListDatabaseObserver.processItems	⬆️ 1.5 kB
StreamChat.framework/StreamChat.ListDatabaseObserver.listenForRemoveAllDataNotifications	⬆️ 1.3 kB
➕ ChatSample/ChatSample.DemoAppConfiguration.showPerformanceTracker	⬆️ 1.0 kB
StreamChat.framework/StreamChat.ListChangeAggregator.controller(didChange,at,for,newIndexPath)	900 B
➕ StreamChat.framework/StreamChat.BackgroundListDatabaseObserver.Swift Metadata	511 B

🔎 See the full size analysis (b126fe1) merging into develop (8a01f51)

Provide a baseSha and pullRequestNumber with your upload to ensure diffs are correct. Read more in the docs

[Stream-SDK-Bot]

	1 Message
📖	There seems to be app changes but CHANGELOG wasn't modified. Please include an entry if the PR includes user-facing changes. You can find it at CHANGELOG.md.

Generated by 🚫 Danger

[nuno-vieira]

LGTM overall 👍 Just added some minor comments

[nuno-vieira]

Since it looks like both ListDatabaseObserver's have the same API, instead of having the isBackground spread all over this wrapper, can't we export the API to a protocol, and set the ListDatabaseObserver implementation depending on the isBackground flag?

[polqf]

I tried the protocol approach but because of the generics it was not possible to then maintain certain APIs

[nuno-vieira]

Better to create a queue from scratch instead of using the global one

[martinmitrevski]

Yes, and might be good to specify qos in this case.

[martinmitrevski]

As we've discussed, we need to see what happens with the diffing solution, as well as the SwiftUI SDK (there are some glitches there).

[polqf]

Closed in benefit of #2237 with uses the new messages list