feat(companion): launch and terminate apps through the automation session

companion/Sources/AppLifecycle.swift

@@ -0,0 +1,51 @@
+import XCTest
+
+// Launches and terminates apps through the automation session. Lifecycle
+// performed outside the session leaves its cached app proxies bound to dead
+// processes, after which snapshots hang and typing asserts.
+enum AppLifecycle {
+    enum LifecycleError: Error {
+        case failed(String)
+    }
+
+    static func launch(bundleIdentifier: String, foregroundIfRunning: Bool) throws {
+        try onMainCatching {
+            let application = XCUIApplication(bundleIdentifier: bundleIdentifier)
+            if foregroundIfRunning {
+                // activate starts the app when needed and foregrounds it when
+                // already running, without a forced relaunch.
+                application.activate()
+            } else {
+                application.launch()
+            }
+        }
+    }
+
+    static func terminate(bundleIdentifier: String) throws {
+        try onMainCatching {
+            let application = XCUIApplication(bundleIdentifier: bundleIdentifier)
+            if application.state == .notRunning {
+                return
+            }
+            application.terminate()
+        }
+    }
+
+    // onMainCatching runs automation work on the main thread and converts a
+    // framework assertion into a thrown error so the server survives it.
+    private static func onMainCatching(_ work: @escaping () -> Void) throws {
+        var caughtError: NSError?
+        var completed = false
+        let run = {
+            completed = CompanionRunCatching(work, &caughtError)
+        }
+        if Thread.isMainThread {
+            run()
+        } else {
+            DispatchQueue.main.sync(execute: run)
+        }
+        if !completed {
+            throw LifecycleError.failed(caughtError?.localizedDescription ?? "unknown")
+        }
+    }
+}

companion/Sources/Server.swift

@@ -103,6 +103,15 @@ final class Server {
             return ["ok": true]
         case "gestureProbe":
             return Gesture.probe()
+        case "launch":
+            let bundleIdentifier = params["bundleId"] as? String ?? currentBundleIdentifier
+            let foregroundIfRunning = params["foregroundIfRunning"] as? Bool ?? false
+            try AppLifecycle.launch(bundleIdentifier: bundleIdentifier, foregroundIfRunning: foregroundIfRunning)
+            return ["ok": true]
+        case "terminate":
+            let bundleIdentifier = params["bundleId"] as? String ?? currentBundleIdentifier
+            try AppLifecycle.terminate(bundleIdentifier: bundleIdentifier)
+            return ["ok": true]
         case "typeText":
             let text = params["text"] as? String ?? ""
             let replace = params["replace"] as? Bool ?? false