🔗 Issue Links

https://stream-io.atlassian.net/browse/CIS-1854
https://stream-io.atlassian.net/browse/CIS-2059

🎯 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 _isBackgroundMappingEnabled , unless forceLazy is passed

All this implementation is guarded now behind _isBackgroundMappingEnabled. When the flag is false, the same behaviour as before will be present.

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

Pitfalls:

Because now background mapping tries to map every single updated dto as new updates come in, we end up resolving the whole tree for each object, which results in way too many fetches. This has no direct impact on the main thread per se, but the overhead we add to Core Data can be significant in some cases.

Also, in some cases, there are cyclic relationships, leading to Stack Overflow. This is now prevented from passing forceLazy: true to some models. This preserves the laziness of those properties. This is a temporary solution to those problems, but it is not meant to be the final one

☑️ 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