Add on-monitor-changed config callback

When a monitor is connected or disconnected, users often need to
reload external tools (e.g. sketchybar) that depend on the display
configuration. AeroSpace already detects monitor count changes in
gcMonitors() and rearranges workspaces, but provides no hook for
users to run commands in response.

Add on-monitor-changed callback that fires when the set of connected
monitors changes. It follows the same pattern as the existing
on-focus-changed and on-focused-monitor-changed callbacks: broadcast
a ServerEvent for `aerospace subscribe`, and run the configured
commands in a deferred runLightSession Task.

Config syntax:
  on-monitor-changed = 'exec-and-forget sketchybar --reload'

Also adds monitor-changed to the subscribe event types so that
`aerospace subscribe monitor-changed` works for external tooling.

Author: willzeng274

Sources/AppBundle/config/Config.swift

@@ -54,6 +54,7 @@ struct Config: ConvenienceCopyable {
     var onFocusChanged: [any Command] = []
     // var onFocusedWorkspaceChanged: [any Command] = []
     var onFocusedMonitorChanged: [any Command] = []
+    var onMonitorChanged: [any Command] = []
 
     var gaps: Gaps = .zero
     var workspaceToMonitorForceAssignment: [String: [MonitorDescription]] = [:]

Sources/AppBundle/config/parseConfig.swift

@@ -102,6 +102,7 @@ private let configParser: [String: any ParserProtocol<Config>] = [
     "on-focus-changed": Parser(\.onFocusChanged) { parseCommandOrCommands($0).toParsedConfig($1) },
     "on-mode-changed": Parser(\.onModeChanged) { parseCommandOrCommands($0).toParsedConfig($1) },
     "on-focused-monitor-changed": Parser(\.onFocusedMonitorChanged) { parseCommandOrCommands($0).toParsedConfig($1) },
+    "on-monitor-changed": Parser(\.onMonitorChanged) { parseCommandOrCommands($0).toParsedConfig($1) },
     // "on-focused-workspace-changed": Parser(\.onFocusedWorkspaceChanged, { parseCommandOrCommands($0).toParsedConfig($1) }),
 
     "enable-normalization-flatten-containers": Parser(\.enableNormalizationFlattenContainers, parseBool),

Sources/AppBundle/model/ServerEvent.swift

@@ -45,4 +45,8 @@ public struct ServerEvent: Codable, Sendable {
     public static func bindingTriggered(mode: String, binding: String) -> ServerEvent {
         ServerEvent(_event: .bindingTriggered, mode: mode, binding: binding)
     }
+
+    public static func monitorChanged(monitorCount: Int) -> ServerEvent {
+        ServerEvent(_event: .monitorChanged, monitorId: monitorCount)
+    }
 }

Sources/AppBundle/subscriptions.swift

@@ -30,7 +30,7 @@ func handleSubscribeAndWaitTillError(_ connection: NWConnection, _ args: Subscri
                         workspace: f.workspace.name,
                         monitorId_oneBased: f.workspace.workspaceMonitor.monitorId_oneBased ?? 0,
                     )
-                case .windowDetected, .bindingTriggered: continue
+                case .windowDetected, .bindingTriggered, .monitorChanged: continue
             }
             if await connection.writeAtomic(event, jsonEncoder).error != nil {
                 return

Sources/AppBundle/tree/Workspace.swift

@@ -136,6 +136,18 @@ extension Monitor {
 func gcMonitors() {
     if screenPointToVisibleWorkspace.count != monitors.count {
         rearrangeWorkspacesOnMonitors()
+        onMonitorChanged()
+    }
+}
+
+@MainActor private func onMonitorChanged() {
+    broadcastEvent(.monitorChanged(monitorCount: monitors.count))
+    if config.onMonitorChanged.isEmpty { return }
+    guard let token: RunSessionGuard = .isServerEnabled else { return }
+    Task {
+        try await runLightSession(.onMonitorChanged, token) {
+            _ = try await config.onMonitorChanged.runCmdSeq(.defaultEnv, .emptyStdin)
+        }
     }
 }
 

Sources/Common/cmdArgs/impl/SubscribeCmdArgs.swift

@@ -61,4 +61,5 @@ public enum ServerEventType: String, Codable, CaseIterable, Sendable {
     case modeChanged = "mode-changed"
     case windowDetected = "window-detected"
     case bindingTriggered = "binding-triggered"
+    case monitorChanged = "monitor-changed"
 }

Sources/Common/util/commonUtil.swift

@@ -80,6 +80,7 @@ public enum RefreshSessionEvent: Sendable, CustomStringConvertible {
     case onFocusedMonitorChanged
     case onFocusChanged
     case onModeChanged
+    case onMonitorChanged
 
     public var isStartup: Bool {
         if case .startup = self { return true } else { return false }
@@ -99,6 +100,7 @@ public enum RefreshSessionEvent: Sendable, CustomStringConvertible {
             case .onFocusedMonitorChanged: "onFocusedMonitorChanged"
             case .onFocusChanged: "onFocusChanged"
             case .onModeChanged: "onModeChanged"
+            case .onMonitorChanged: "onMonitorChanged"
         }
     }
 }