Deprecate and Remove Non-Functional HotReload Infrastructure

## Summary

The MAUI codebase contains a complex, **non-functional HotReload infrastructure** that was intended for .NET hot reload support via metadata updates. This infrastructure is:

- **Not registered** - No `[assembly: MetadataUpdateHandler(...)]` attribute exists in MAUI Core or Controls
- **Not called** - No metadata update entry points are invoked
- **Unused** - No callers to `Register/UnRegister/RegisterReplacedView/TriggerReload`
- **Non-functional** - Empty handler stubs, no actual reload happens

However, one piece is genuinely needed: **`IReplaceableView.ReplacedView`** is used by MVU in `ElementExtensions.ToHandler/ToPlatform()` for collapsing the view tree.

Additionally, **BlazorWebView has a completely separate MetadataUpdateHandler** for CSS/JS hot reload that should NOT be removed.

---

## Current State Assessment

### HotReload Infrastructure
Located in `src/Core/src/HotReload/`:

| Component | Status | Evidence |
|-----------|--------|----------|
| `IReloadHandler` interface | Unused | Only implementations are platform containers; never called |
| `IHotReloadableView` interface | Unused | Implemented by View/ContentPage but `Reload()` is empty |
| `MauiHotReloadHelper` | Partially used | `RegisterHandlers()` called during app init; rest is dead code |
| Metadata update entry points | Dead code | `UpdateApplication()/ClearCache()` - no MetadataUpdateHandler registered |
| View registration system | Dead code | `Register/UnRegister` dictionary - never populated or used |
| `[OnHotReload]` attribute | Dead code | Stubs in handler base classes, never invoked |
| `IReplaceableView.ReplacedView` | **NEEDED** | Used by MVU for view tree collapsing in `ElementExtensions` |

### Evidence
- **No MetadataUpdateHandler**: Search for `[assembly: MetadataUpdateHandler` returns only BlazorWebView result
- **No registration calls**: No callers to `Register/UnRegister/RegisterReplacedView/TriggerReload`
- **Empty implementations**: Handler `OnHotReload()` methods are empty stubs
- **All public APIs**: All types are in `PublicAPI.Shipped.txt` across 8 platforms

### What Actually Works
- **Container wiring**: Containers still set `ReloadHandler` on views (harmless no-op)
- **`IReplaceableView.ReplacedView`**: Used by MVU in `ElementExtensions.ToHandler()` for view collapsing
  - Lines 62-63: `if (view is IReplaceableView ir) view = ir.ReplacedView;`

---

## Proposed Solution

### Phase 1: Deprecation (This Change)
Add `[Obsolete]` attributes to mark infrastructure as deprecated while maintaining compatibility.

**Deprecate these methods/types:**
- `IReloadHandler` interface
- `IHotReloadableView` interface  
- `OnHotReloadAttribute`
- `HotReloadExtensions` static class
- `MauiHotReloadHelper` methods (except `RegisterHandlers()`)
- Metadata update entry points: `UpdateApplication()`, `ClearCache()`

**Keep (but no-op):**
- `IReplaceableView` - required for MVU
- `MauiHotReloadHelper.RegisterHandlers()` - called during app startup
- Container hot reload wiring - harmless if disabled in future
- View/ContentPage implementations - harmless if never called

**Remove:**
- `[OnHotReload]` attributes from handler base classes (empty stubs)

**Files affected (Phase 1):**
- 6 HotReload source files (add `[Obsolete]`)
- 2 handler files (remove `[OnHotReload]`)
- 8 PublicAPI.Shipped.txt files (update entries)
- Container files (add comments explaining no-op status)

### Phase 2: Removal (Future Major Version)
Remove the infrastructure entirely. This would be a breaking change suitable for next major version.

**Remove:**
- Entire `src/Core/src/HotReload/` folder
- `IHotReloadableView` implementation from `View` and `ContentPage`
- Hot reload wiring from platform containers

**Simplify:**
- `IReplaceableView` - keep only `ReplacedView { get; }` property
- Minimal implementation in View/ContentPage (just return self by default)

---

## Files to Modify (Phase 1)

### Core HotReload
- `src/Core/src/HotReload/IReloadHandler.cs` - Add [Obsolete]
- `src/Core/src/HotReload/IHotReloadableView.cs` - Add [Obsolete]
- `src/Core/src/HotReload/OnHotReloadAttribute.cs` - Add [Obsolete]
- `src/Core/src/HotReload/HotReloadExtensions.cs` - Add [Obsolete]
- `src/Core/src/HotReload/HotReloadHelper.cs` - Add [Obsolete] to methods

### Handlers
- `src/Core/src/Handlers/View/ViewHandlerOfT.cs` - Remove `[OnHotReload]` attribute
- `src/Core/src/Handlers/Element/ElementHandlerOfT.cs` - Remove `[OnHotReload]` attribute

### Implementations  
- `src/Controls/src/Core/View/View.cs` - Keep implementation, add comment
- `src/Controls/src/Core/ContentPage/ContentPage.cs` - Keep implementation, add comment

### Platforms
- `src/Core/src/Platform/Android/ContainerView.cs` - Add comment
- `src/Core/src/Platform/iOS/ContainerViewController.cs` - Add comment
- `src/Core/src/Platform/Tizen/ContainerView.cs` - Add comment

### PublicAPI (8 files)
- `src/Core/src/PublicAPI/net-android/PublicAPI.Shipped.txt`
- `src/Core/src/PublicAPI/net-ios/PublicAPI.Shipped.txt`
- `src/Core/src/PublicAPI/net-maccatalyst/PublicAPI.Shipped.txt`
- `src/Core/src/PublicAPI/net-windows/PublicAPI.Shipped.txt`
- `src/Core/src/PublicAPI/netstandard/PublicAPI.Shipped.txt`
- `src/Core/src/PublicAPI/netstandard2.0/PublicAPI.Shipped.txt`
- `src/Core/src/PublicAPI/net-tizen/PublicAPI.Shipped.txt`
- `src/Core/src/PublicAPI/net/PublicAPI.Shipped.txt`

---

## User Migration Path

### Phase 1 (Deprecation)
- Users not using HotReload: No change needed
- Users with custom `[OnHotReload]` handlers: Suppress warnings with `#pragma disable CS0618` or `<NoWarn>618</NoWarn>`

### Phase 2 (Removal)  
- Must remove custom HotReload implementations
- Recommend using VS/Rider hot reload or custom tooling instead
- Provide migration guide in release notes

---

## Discussion Points

1. **Metadata Update Registration**: Should we ever register a MetadataUpdateHandler for View hot reload, or is this infrastructure permanently deprecated?

2. **BlazorWebView Exception**: Confirmed that `StaticContentHotReloadManager` in BlazorWebView is separate and active (CSS/JS hot reload). This should NOT be removed.

3. **Phase 1 Timing**: Can this deprecation be merged in current release, or should we wait?

4. **Phase 2 Timeline**: Target for removal (e.g., next major version)?

5. **Migration Documentation**: Should we provide guidance for users wanting custom hot reload?

---

## Obsolete Message

Suggested message for all deprecated types:

```csharp
[Obsolete(
    "The HotReload infrastructure is deprecated and will be removed in a future version. " +
    "Metadata-driven hot reload is not currently supported in MAUI. " +
    "For hot reload support, use Visual Studio/Rider hot reload or implement custom solutions. " +
    "The IReplaceableView interface will be retained for internal MVU support.",
    error: false)]
```

---

## Testing

**Phase 1 validation:**
- Build succeeds with [Obsolete] attributes
- No compilation errors in core tests
- `IReplaceableView.ReplacedView` still works in MVU (ElementExtensions)
- Container wiring still executes (no-op)
- Controls sample launches without issues
- All UI tests pass
- BlazorWebView hot reload still works (separate system)

**Phase 2 validation (future):**
- Code compiles after removing HotReload folder
- All public API tests pass
- Control tests pass
- Sample apps launch successfully


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Deprecate and Remove Non-Functional HotReload Infrastructure #34714

Summary

Current State Assessment

HotReload Infrastructure

Evidence

What Actually Works

Proposed Solution

Phase 1: Deprecation (This Change)

Phase 2: Removal (Future Major Version)

Files to Modify (Phase 1)

Core HotReload

Handlers

Implementations

Platforms

PublicAPI (8 files)

User Migration Path

Phase 1 (Deprecation)

Phase 2 (Removal)

Discussion Points

Obsolete Message

Testing

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Component	Status	Evidence
`IReloadHandler` interface	Unused	Only implementations are platform containers; never called
`IHotReloadableView` interface	Unused	Implemented by View/ContentPage but `Reload()` is empty
`MauiHotReloadHelper`	Partially used	`RegisterHandlers()` called during app init; rest is dead code
Metadata update entry points	Dead code	`UpdateApplication()/ClearCache()` - no MetadataUpdateHandler registered
View registration system	Dead code	`Register/UnRegister` dictionary - never populated or used
`[OnHotReload]` attribute	Dead code	Stubs in handler base classes, never invoked
`IReplaceableView.ReplacedView`	NEEDED	Used by MVU for view tree collapsing in `ElementExtensions`

Deprecate and Remove Non-Functional HotReload Infrastructure #34714

Description

Summary

Current State Assessment

HotReload Infrastructure

Evidence

What Actually Works

Proposed Solution

Phase 1: Deprecation (This Change)

Phase 2: Removal (Future Major Version)

Files to Modify (Phase 1)

Core HotReload

Handlers

Implementations

Platforms

PublicAPI (8 files)

User Migration Path

Phase 1 (Deprecation)

Phase 2 (Removal)

Discussion Points

Obsolete Message

Testing

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions