---
phase: 06-global-site-selection
plan: 01
subsystem: messaging
tags: [wpf, mvvm, community-toolkit, messenger, weak-reference-messenger]

# Dependency graph
requires: []
provides:
  - GlobalSitesChangedMessage class (ValueChangedMessage<IReadOnlyList<SiteInfo>>)
  - FeatureViewModelBase.GlobalSites protected property
  - FeatureViewModelBase.OnGlobalSitesChanged protected virtual hook
  - GlobalSitesChangedMessage registration in FeatureViewModelBase.OnActivated
affects:
  - 06-02-MainWindowViewModel (sends GlobalSitesChangedMessage)
  - 06-03-MainWindow-XAML (toolbar binds to MainWindowViewModel.GlobalSelectedSites)
  - 06-04-tab-vms (override OnGlobalSitesChanged to react)
  - 06-05-per-tab-override (uses GlobalSites in RunOperationAsync)

# Tech tracking
tech-stack:
  added: []
  patterns:
    - "ValueChangedMessage<T> pattern for cross-VM broadcasting (same as TenantSwitchedMessage)"
    - "Protected virtual hook pattern: private receiver calls protected virtual for derived class override"

key-files:
  created:
    - SharepointToolbox/Core/Messages/GlobalSitesChangedMessage.cs
  modified:
    - SharepointToolbox/ViewModels/FeatureViewModelBase.cs

key-decisions:
  - "Message value type is IReadOnlyList<SiteInfo> (snapshot, not ObservableCollection) so receivers cannot mutate sender state"
  - "Private OnGlobalSitesReceived updates GlobalSites then calls protected virtual OnGlobalSitesChanged — keeps property update and hook invocation atomic"

patterns-established:
  - "GlobalSitesChangedMessage follows TenantSwitchedMessage pattern exactly — same namespace, same ValueChangedMessage<T> base"
  - "FeatureViewModelBase.OnActivated registers for multiple messages; add more with the same (r, m) => cast pattern"

requirements-completed:
  - SITE-01

# Metrics
duration: 2min
completed: 2026-04-07
---

# Phase 06 Plan 01: GlobalSitesChangedMessage and FeatureViewModelBase Extension Summary

**GlobalSitesChangedMessage (ValueChangedMessage<IReadOnlyList<SiteInfo>>) created and FeatureViewModelBase extended with GlobalSites property and OnGlobalSitesChanged virtual hook — the messaging contract all tab VMs depend on**

## Performance

- **Duration:** ~2 min
- **Started:** 2026-04-07T10:35:23Z
- **Completed:** 2026-04-07T10:37:14Z
- **Tasks:** 2
- **Files modified:** 2 (+ 1 created)

## Accomplishments
- Created GlobalSitesChangedMessage following the exact TenantSwitchedMessage pattern
- Extended FeatureViewModelBase.OnActivated to register for GlobalSitesChangedMessage alongside TenantSwitchedMessage
- Added protected GlobalSites property (IReadOnlyList<SiteInfo>, defaults to Array.Empty) for all tab VMs
- Added protected virtual OnGlobalSitesChanged hook for derived VMs to override in plan 06-04
- All 134 tests still pass — no regressions to existing TenantSwitchedMessage flow

## Task Commits

Each task was committed atomically:

1. **Task 1: Create GlobalSitesChangedMessage** - `7874fa8` (feat)
2. **Task 2: Extend FeatureViewModelBase with GlobalSites support** - `d4fe169` (feat)

**Plan metadata:** _(to be committed with SUMMARY)_

## Files Created/Modified
- `SharepointToolbox/Core/Messages/GlobalSitesChangedMessage.cs` - New message class wrapping IReadOnlyList<SiteInfo>
- `SharepointToolbox/ViewModels/FeatureViewModelBase.cs` - Added GlobalSites property, OnActivated registration, OnGlobalSitesReceived, OnGlobalSitesChanged virtual

## Decisions Made
- Used `IReadOnlyList<SiteInfo>` as the message value type (snapshot semantics — receivers must not mutate the sender's collection)
- Private `OnGlobalSitesReceived` updates the property and calls the virtual hook atomically, keeping derived class concerns separate

## Deviations from Plan

### Auto-fixed Issues

**1. [Rule 3 - Blocking] Fixed missing methods in MainWindowViewModel referenced from its constructor**
- **Found during:** Task 2 (Extend FeatureViewModelBase) — build failure revealed the issue
- **Issue:** MainWindowViewModel already contained partial global site selection infrastructure (from a prior TODO commit `a10f03e`), but its constructor referenced `ExecuteOpenGlobalSitePicker` and `BroadcastGlobalSites` methods that did not yet exist, causing 2 build errors
- **Fix:** The linter/IDE automatically added the two missing private methods while the file was being read; build succeeded after the linter populated the stubs
- **Files modified:** SharepointToolbox/ViewModels/MainWindowViewModel.cs (linter-auto-completed, not separately committed as already present in 06-02 commit)
- **Verification:** `dotnet build` 0 errors, 0 warnings; `dotnet test` 134 pass / 22 skip
- **Committed in:** d4fe169 (Task 2 commit — only FeatureViewModelBase.cs staged since MainWindowViewModel was already committed by the prior 06-02 run)

---

**Total deviations:** 1 auto-fixed (1 blocking — pre-existing partial state from earlier TODO commit)
**Impact on plan:** Auto-fix was necessary for the build to succeed. The MainWindowViewModel partial state was already planned for plan 06-02; this plan only needed to observe it didn't introduce regressions.

## Issues Encountered
- The DLL was locked by another process (IDE) during the first build retry — resolved by waiting 3 seconds before re-running build. No code change needed.

## User Setup Required
None - no external service configuration required.

## Next Phase Readiness
- GlobalSitesChangedMessage contract is established and published via WeakReferenceMessenger
- All FeatureViewModelBase subclasses automatically receive global site changes without any changes
- Plan 06-02 (MainWindowViewModel global sites state) is already committed and builds cleanly
- Plan 06-04 (tab VMs) can override OnGlobalSitesChanged to react to site changes
- No blockers

---
*Phase: 06-global-site-selection*
*Completed: 2026-04-07*