Add UpdateCheck and some supporting code

Sources/AppUpdater/API/UpdateCheck.swift

@@ -0,0 +1,149 @@
+//
+//  Created by Nico Verbruggen on 30/05/2024.
+//  Copyright © 2024 Nico Verbruggen. All rights reserved.
+//
+
+import Foundation
+import Cocoa
+
+open class UpdateCheck
+{
+    let caskUrl: URL
+    let promptOnFailure: Bool
+    let selfUpdaterName: String
+    let selfUpdaterDirectory: String
+
+    var caskFile: CaskFile!
+    var newerVersion: AppVersion!
+
+    /**
+     * Create a new update check instance. Once created, you should call `perform` on this instance.
+     *
+     * - Parameter selfUpdaterName: The name of the self-updater .app file. For example, "App Self-Updater.app".
+     *   This binary should exist as a resource of the current application.
+     *
+     * - Parameter selfUpdaterDirectory: The directory that is used by the self-updater. A file `update.json`
+     *   will be placed in this directory and this should be correspond to the `baseUpdaterPath` in `SelfUpdater`.
+     *
+     * - Parameter caskUrl: The URL where the Cask file is expected to be located. Redirects will
+     *   be followed when retrieving and validating the Cask file.
+     *
+     * - Parameter promptOnFailure: Whether user interaction is required when failing to check 
+     *   or no new update is found. A user usually expects a prompt if they manually searched
+     *   for updates.
+     */
+    public init(
+        selfUpdaterName: String,
+        selfUpdaterDirectory: String,
+        caskUrl: URL,
+        promptOnFailure: Bool
+    ) {
+        self.selfUpdaterName = selfUpdaterName
+        self.selfUpdaterDirectory = selfUpdaterDirectory
+        self.caskUrl = caskUrl
+        self.promptOnFailure = promptOnFailure
+    }
+
+    /**
+     Perform the check for a new version.
+     */
+    public func perform() async {
+        guard let caskFile = await CaskFile.from(url: caskUrl) else {
+            Log.text("The contents of the CaskFile at '\(caskUrl.absoluteString)' could not be retrieved.")
+            return await presentCouldNotRetrieveUpdate()
+        }
+
+        self.caskFile = caskFile
+
+        let currentVersion = AppVersion.fromCurrentVersion()
+
+        guard let onlineVersion = AppVersion.from(caskFile.version) else {
+            Log.text("The version string from the CaskFile could not be read.")
+            return await presentCouldNotRetrieveUpdate()
+        }
+
+        self.newerVersion = onlineVersion
+
+        Log.text("The latest version read from '\(caskUrl.lastPathComponent)' is: v\(onlineVersion.computerReadable).")
+        Log.text("The current version is v\(currentVersion.computerReadable).")
+
+        if onlineVersion > currentVersion {
+            await presentNewerVersionAvailable()
+        } else if promptOnFailure {
+            await presentVersionIsUpToDate()
+        }
+    }
+
+    // MARK: - Alerts
+
+    private func presentCouldNotRetrieveUpdate() async {
+        Log.text("Could not retrieve update manifest!")
+
+        if promptOnFailure {
+            await Alert.confirm(
+                title: "Could not retrieve update information!",
+                description: "There was an issue retrieving information about possible updates. This could be a connection or server issue. Check your internet connection and try again later."
+            )
+        }
+    }
+
+    private func presentVersionIsUpToDate() async {
+        Log.text("Version is up-to-date!")
+
+        if promptOnFailure {
+            await Alert.confirm(
+                title: "The app is up-to-date!",
+                description: "The version on the server is not newer than this version, so you're all good."
+            )
+        }
+    }
+
+    private func presentNewerVersionAvailable() async {
+        Log.text("A newer version is available!")
+
+        let current = AppVersion.fromCurrentVersion()
+
+        let outcome = await Alert.choose(
+            title: "An updated version of \(Executable.name) is available.",
+            description: """
+                Version \(newerVersion.version) is available for download.
+                (This is currently version \(current.version).)
+
+                Do you want to download and install this updated version?
+            """,
+            options: [
+                "Update Now",
+                "Cancel"
+            ])
+
+        if outcome == .alertFirstButtonReturn {
+            launchSelfUpdater()
+        }
+    }
+
+    // MARK: - Functional
+
+    private func launchSelfUpdater() {
+        let updater = Bundle.main.resourceURL!.path + "/\(selfUpdaterName)"
+
+        system_quiet("mkdir -p \(selfUpdaterDirectory) 2> /dev/null")
+
+        let updaterDirectory = selfUpdaterDirectory
+            .replacingOccurrences(of: "~", with: NSHomeDirectory())
+
+        system_quiet("cp -R \"\(updater)\" \"\(updaterDirectory)/\(selfUpdaterName)\"")
+
+        try! "{ \"url\": \"\(caskFile.url)\", \"sha256\": \"\(caskFile.sha256)\" }".write(
+            to: URL(fileURLWithPath: "\(updaterDirectory)/update.json"),
+            atomically: true,
+            encoding: .utf8
+        )
+
+        NSWorkspace.shared.openApplication(
+            at: NSURL(fileURLWithPath: updater, isDirectory: true) as URL,
+            configuration: NSWorkspace.OpenConfiguration()
+        ) { _, _ in
+            Log.text("The updater has been launched successfully!")
+        }
+    }
+}