Overview

There was a request in #6124 to decompose some complex code in AppStore._loadStatus to make it more readable, but I also wanted to take it further and see if I can get this code under test.

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Closes #6124

Description

The goal with this PR was to be able to write some meaningful tests around complex parts of AppStore, while avoiding the overhead of getting AppStore setup and in the correct state for testing.

The initial request to extract these into functions was a start, but I also wanted to achieve two things:

• make these functions stateless, so they could be tested in isolation of AppStore
• wrap the existing behaviour in tests, so we can understand the behaviour and catch regressions later

I settled on just covering the important code paths for both updateChangedFiles and updateConflictState and was able to hit 100% coverage with a bit of work:

The pattern that I ended up with does feel a bit like reducers in Redux, but with some caveats beyond the generic function updateSomeState<T>(prevState: T, ...args): T:

• each function only cares about updating a subset of T, so making each function merge in previous state to then return T feels like overkill right now
• we already have infrastructure inside RepositoryStateCache to handle merging state changes together
• I want the tests to only focus on what's changed - so making these functions return the subset of values that need updating helps with test clarity

Things of note so far:

• I moved the functions up to app/src/lib/stores/updates so these are close to AppStore, but avoid the churn of potential conflicts in AppStore itself
• I created an interfaces for IStatsStore to stub and test the actions that we need to fire inside updateConflictState, and we can refine that as we need to test more things
• setting up tests - both of these functions need IChangesState and IStatusResult to work, and those contain a lot of noise. I fell into a pattern using createState and createStatus factory functions to construct state needed for tests by taking a base object and merging it with just the fields that were relevant to the test. These helpers now live in app/test/unit/stores/updates/changes-state-helper.ts and they were a great help to keep the tests focus on the specific scenario
• DiffSelectionType enum is now a string enum because the strings help with readability in the tests ('All' !== 'None' is much easier to understand than 0 !== 1)

TODO:

• tidy up history after generate HTML bundle of coverage stats by default #6129 is merged now that I've settled on a reasonable pattern and code organization
• write some docs around the new testing infrastructure for app store functions
• gather feedback
• address this TODO inside app/src/lib/stores/updates/changes-state.ts

// TODO: I want to use generics here so I don't need to hard-code the keys

Release notes

Notes: no-notes