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# Extension points in detail
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This page is about *behaviour*: what each extension point is really good for, how the host drives it,
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and the caveats that only show up once you run it. Signatures are in
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[api-reference.md](api-reference.md); copy-paste recipes are in [examples.md](examples.md).
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Registration order does not matter — everything registered inside `Initialize` is collected and then
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applied once the plugin finishes initialising.
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---
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## 1. Overlays — drawing on the desktop
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`context.Extensions.AddOverlay(new MyOverlay())` (permission `DesktopOverlay`).
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**Where it draws.** LiveWall owns one transparent, layered, always-behind window per display inside
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the shell's desktop layer (the same WorkerW the wallpaper lives in). Above the wallpaper and *below*
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the icons, it hosts a `Canvas` — `DesktopIconWindow.OverlayLayer` — and each plugin's view is
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stretched across it, in registration order (so a later plugin paints over an earlier one).
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**Lifecycle**
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| Host action | Your callbacks |
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|---|---|
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| Plugin enabled / rescan | `CreateView(context)` once per display |
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| Wallpaper or playback changed | `Update(state)` |
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| Overlays hidden (tray *Hide overlay*), plugin disabled, wallpaper paused | `OnVisibilityChanged(false)` |
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| Overlays shown again | `OnVisibilityChanged(true)` |
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| Plugin disabled | your view is removed and collected |
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**Rules that matter**
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* `CreateView` must return a *fresh* element each time it is called (it is called once per display,
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and again if the overlay host is rebuilt). Do not return a `Window`, and do not add your element to
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another parent.
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* Position with `HorizontalAlignment`/`VerticalAlignment`/`Margin`, or use a `Canvas` and
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`Canvas.SetLeft/Top`. Do not assume a specific size: the root is stretched to the display, in DIPs.
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* **No hit testing.** The layer is `IsHitTestVisible = false`, so mouse events never reach you and
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never block the desktop. If you need clicks, use a renderer with `InteractiveMode`.
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* Transparent pixels cost nothing visually; opaque full-screen rectangles do. Prefer text, `Path`,
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`Ellipse`, gradients with alpha.
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* Avoid `BitmapEffect`/heavy `Effect` on large elements and avoid animating layout properties; prefer
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`RenderTransform` animations.
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* Timers: `DispatcherTimer` with `DispatcherPriority.Background`, interval ≥ 1 s for clocks, ~16 ms for
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animation, and stop them in `OnVisibilityChanged(false)`. Remember `Update` is not a timer.
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* Don't paint over the icon area with anything opaque — the whole point of the layer is that icons
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stay usable.
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**Multi-monitor.** One `CreateView` per display, each with its own `OverlayContext`; use
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`context.Primary` if you only want the main screen (return an empty `Canvas` for the others).
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---
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## 2. Wallpaper filters — rewriting requests
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`context.Extensions.AddFilter(new MyFilter())` (permission `Filters`).
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The engine builds a `WallpaperRequest` (`Kind`, `Source`, `Title`, `Metadata`) and asks every filter,
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highest `Priority` first, to transform it. What you return is applied — including the built-in
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renderers' input, playlist advance, and requests made by *other* plugins via
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`host.SetWallpaper`.
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Good uses:
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* resolve a shorthand (`yt:meatloaf` → a real URL),
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* expand environment variables (`%USERPROFILE%\pics\a.mp4`),
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* pick a random file from a folder when the source is `random:folder`,
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* add metadata for your own renderer (`request.Metadata["scene"] = "rain"`),
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* veto: return the current wallpaper (`host.Current`) to refuse the change,
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* normalise casing/slashes so two plugins agree on a path.
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Bad uses:
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* doing I/O on the UI thread (filters run synchronously in the wallpaper path — a 3-second network
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call looks like a frozen app),
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* returning `null` (treated as "no change", but do it explicitly by returning the argument),
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* mutating the request object you were given *and* returning a different one.
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Filter exceptions are caught per filter per pass; the pass continues with the previous request.
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---
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## 3. Renderers — playing what nobody else can
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`context.Extensions.AddRenderer(new MyRenderer())` (permission `Renderer`).
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`CanRender(request)` is asked for every request, in descending `Priority` (ties run in plugin load
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order). The first `true` wins and the built-in players are skipped entirely for that source, so you can
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also *replace* video playback with your own pipeline.
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```csharp
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public bool CanRender(WallpaperRequest r)
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=> r.Kind.Equals("Custom", StringComparison.OrdinalIgnoreCase) && r.Source.StartsWith("myprefix:");
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```
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Then `Create()` → `Load(request)` → (`Play`/`Pause`/`ApplyPlayback` as the user acts) → `Unload()` →
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`Dispose()`.
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**Rules**
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* `View` must be a single element you own; LiveWall puts it in the wallpaper window. Keep the
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background opaque (or it shows the desktop colour behind).
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* `Load` may throw — that is how you report "this file is not what I expected"; the message goes to
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the status line and the log.
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* `ApplyPlayback(muted, volume /*0–100*/, speed /*0.25–4*/)` can arrive before or after `Load`, and
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repeatedly; make it idempotent.
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* Raise `Failed(message)` for problems after load (device lost, decode error) and `Ended` when a
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non-looping source finished, so LiveWall can advance the playlist.
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* Reported `WallpaperState.RendererName` shows in diagnostics — return a stable `Name`.
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* Do not start a second renderer yourself; LiveWall creates one and keeps it for the source's
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lifetime.
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**Interaction.** With `Displays.Interaction = Interactive` the wallpaper window accepts clicks, so a
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renderer can host real UI (a game, a web view, an editor). In the default `ClickThrough` mode the
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window is transparent to the mouse and the desktop icons above it stay clickable.
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---
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## 4. Settings pages
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`context.Extensions.AddSettingsPage(new MyPage())` (permission `SettingsUi`).
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The page shows up in **Plugins** on the plugin's row via **Settings**. One page → one window; several
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pages → one window with tabs. `CreateView` runs when the window opens, so read your storage there and
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build fresh controls each time (do not cache the element).
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LiveWall's resource dictionary is available through `Application.Current.Resources`, so plugin pages
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can look native:
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```csharp
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var label = new TextBlock
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{
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Text = "Colour",
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Foreground = (Brush)Application.Current.Resources["TextDim"]
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};
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```
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Keep it declarative: you get a `ScrollViewer` around your content, so a long `StackPanel` is fine.
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---
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## 5. Commands and the tray menu
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`context.Extensions.AddCommand(new PluginCommand("id", "Title", ctx => …))`.
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* Every command is listed in **Plugins ▸ Plugin commands** with a **Run** button.
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* With the `TrayMenu` permission it also appears in the tray under **Plugins ▸**; the submenu is
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rebuilt whenever plugins load/unload.
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* `CanExecute` is evaluated when the menu opens and in the row rendering — keep it cheap and
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side-effect free.
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* The `ctx` gives you `ctx.Plugin` (your context) and `ctx.Host` (actions).
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* Exceptions inside a command are caught, logged, and reported in the status line.
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Use commands for anything the user should be able to trigger: "Next quote", "Toggle rain", "Open my
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plugin's data folder", "Reload my config".
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---
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## 6. Hotkeys
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`context.Extensions.AddHotkey(new PluginHotkey("id", "Ctrl+Alt+K", () => …))` (permission `Hotkeys`).
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Gestures use LiveWall's own parser: one or more modifiers (`Ctrl`, `Alt`, `Shift`, `Win`), then one
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key — `F1`–`F24`, `A`–`Z`, `0`–`9`, `D0`–`D9`, `Space`, `Pause`, `PrintScreen`, `Insert`, `Delete`,
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`Home`, `End`, `PageUp`, `PageDown`, arrows, `NumPad0`–`NumPad9`, `NumPad*`. LiveWall's own shortcuts
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are registered first; a collision logs a warning and the plugin hotkey is skipped. Hotkeys are
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unregistered when the plugin unloads.
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The action runs on the UI thread — nothing long inside it.
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---
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## 7. Storage
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`context.Storage` is a JSON object per plugin in `plugin-data\<id>\storage.json`.
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```csharp
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var colour = context.Storage.Get("colour", "#FFFFFF");
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context.Storage.Set("colour", "#00FF88"); // debounced write
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File.WriteAllText(context.Storage.PathFor("cache.json"), json); // your own files too
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```
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Use `PathFor` for anything larger than a few values — everything in the key/value store is rewritten
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as one small document. `Flush()` forces a write, and LiveWall flushes automatically on shutdown.
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---
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## 8. Events and host actions
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See [api-reference.md](api-reference.md#events-eventscs) for the catalogue. Two habits:
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```csharp
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// keep the subscription, dispose it in Shutdown
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_subs.Add(context.Events.Subscribe<WallpaperChangedEvent>(OnWallpaperChanged));
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public void Shutdown() { foreach (var s in _subs) s.Dispose(); _subs.Clear(); }
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```
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and never block the UI thread from a handler — they run on it.
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`IHostActions` is the write side: `SetWallpaper`, `Play`, `Pause`, `TogglePause`, `NextWallpaper`,
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`ReloadWallpaper`, `Reattach`, `OpenSettings`, `SetStatus`, `ShowNotification`, `SetOverlayVisible`.
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Each is a no-op (with a log line) if you did not declare the matching permission.
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`SetWallpaper` goes back through the filter chain, so two plugins that both set wallpapers cannot
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deadlock each other — the last write wins and every filter gets a say.
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---
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## Putting it together: what a "big" plugin looks like
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The sample in `samples/ClockOverlayPlugin/` uses all of the above at once and is the best reference:
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* overlay + settings page + command + filter + hotkey + storage + three event subscriptions,
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* the settings page writes storage and the overlay picks the change up immediately,
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* the filter pins the wallpaper while its option is on,
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* the command reports the current wallpaper through `ctx.Host.Current`.
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