Introduce app metadata system, default to parsing Swift Bundler metadata

App metadata is static metadata loaded at app start-up. The Gtk backends
now make use of this metadata when computing that app's identifier. Apps
built with older Swift Bundler versions (or without Swift Bundler at
all) still benefit from these changes as they can integrate their own
metadata loading system (e.g. loading app identifier and version from
an Info.plist on macOS).

Author: stackotter

Sources/AppKitBackend/AppKitBackend.swift

@@ -3,6 +3,10 @@ import SwiftCrossUI
 
 extension App {
     public typealias Backend = AppKitBackend
+
+    public var backend: AppKitBackend {
+        AppKitBackend()
+    }
 }
 
 public final class AppKitBackend: AppBackend {

Sources/Gtk3Backend/Gtk3Backend.swift

@@ -5,6 +5,10 @@ import SwiftCrossUI
 
 extension App {
     public typealias Backend = Gtk3Backend
+
+    public var backend: Gtk3Backend {
+        Gtk3Backend(appIdentifier: Self.metadata?.identifier)
+    }
 }
 
 extension SwiftCrossUI.Color {
@@ -35,14 +39,16 @@ public final class Gtk3Backend: AppBackend {
     /// precreated window until it gets 'created' via `createWindow`.
     var windows: [Window] = []
 
-    /// Creates a backend instance using the default app identifier `com.example.SwiftCrossUIApp`.
-    convenience public init() {
-        self.init(appIdentifier: "com.example.SwiftCrossUIApp")
+    // A separate initializer to satisfy ``AppBackend``'s requirements.
+    public convenience init() {
+        self.init(appIdentifier: nil)
     }
 
-    public init(appIdentifier: String) {
+    /// Creates a backend instance. If `appIdentifier` is `nil`, the default
+    /// identifier `com.example.SwiftCrossUIApp` is used.
+    public init(appIdentifier: String?) {
         gtkApp = Application(
-            applicationId: appIdentifier,
+            applicationId: appIdentifier ?? "com.example.SwiftCrossUIApp",
             flags: G_APPLICATION_HANDLES_OPEN
         )
         gtkApp.registerSession = true

Sources/GtkBackend/GtkBackend.swift

@@ -5,6 +5,10 @@ import SwiftCrossUI
 
 extension App {
     public typealias Backend = GtkBackend
+
+    public var backend: GtkBackend {
+        GtkBackend(appIdentifier: Self.metadata?.identifier)
+    }
 }
 
 extension SwiftCrossUI.Color {
@@ -35,17 +39,20 @@ public final class GtkBackend: AppBackend {
     /// precreated window until it gets 'created' via `createWindow`.
     var windows: [Window] = []
 
-    /// Creates a backend instance using the default app identifier `com.example.SwiftCrossUIApp`.
-    convenience public init() {
-        self.init(appIdentifier: "com.example.SwiftCrossUIApp")
+    // A separate initializer to satisfy ``AppBackend``'s requirements.
+    public convenience init() {
+        self.init(appIdentifier: nil)
     }
 
-    public init(appIdentifier: String) {
+    /// Creates a backend instance. If `appIdentifier` is `nil`, the default
+    /// identifier `com.example.SwiftCrossUIApp` is used.
+    public init(appIdentifier: String?) {
         gtkApp = Application(
-            applicationId: appIdentifier,
+            applicationId: appIdentifier ?? "com.example.SwiftCrossUIApp",
             flags: G_APPLICATION_HANDLES_OPEN
         )
         gtkApp.registerSession = true
+        print(appIdentifier)
     }
 
     public func runMainLoop(_ callback: @escaping () -> Void) {

Sources/SwiftCrossUI/App.swift

@@ -9,6 +9,11 @@ public protocol App {
     /// The app's observed state.
     associatedtype State: Observable
 
+    /// Metadata loaded at app start up. By default SwiftCrossUI attempts
+    /// to load metadata inserted by Swift Bundler if present. Used by backends'
+    /// default ``App/backend`` implementations if not `nil`.
+    static var metadata: AppMetadata? { get }
+
     /// The application's backend.
     var backend: Backend { get }
 
@@ -26,9 +31,19 @@ public protocol App {
 /// this in your own code then something has gone very wrong...
 public var _forceRefresh: () -> Void = {}
 
+/// Metadata embedded by Swift Bundler if present. Loaded at app start up.
+private var swiftBundlerAppMetadata: AppMetadata?
+
 extension App {
+    /// Metadata loaded at app start up.
+    public static var metadata: AppMetadata? {
+        swiftBundlerAppMetadata
+    }
+
     /// Runs the application.
     public static func main() {
+        swiftBundlerAppMetadata = extractSwiftBundlerMetadata()
+
         let app = Self()
         let _app = _App(app)
         _forceRefresh = {
@@ -38,16 +53,56 @@ extension App {
         }
         _app.run()
     }
+
+    private static func extractSwiftBundlerMetadata() -> AppMetadata? {
+        guard let executable = Bundle.main.executableURL else {
+            print("No executable url")
+            return nil
+        }
+
+        guard let data = try? Data(contentsOf: executable) else {
+            print("warning: Executable failed to read self (to extract metadata)")
+            return nil
+        }
+
+        // Check if executable has Swift Bundler metadata magic bytes.
+        let bytes = Array(data)
+        guard bytes.suffix(8) == Array("SBUNMETA".utf8) else {
+            print("no magic bytes")
+            return nil
+        }
+
+        let lengthStart = bytes.count - 16
+        let jsonLength = parseBigEndianUInt64(startingAt: lengthStart, in: bytes)
+        let jsonStart = lengthStart - Int(jsonLength)
+        let jsonData = Data(bytes[jsonStart..<lengthStart])
+
+        do {
+            return try JSONDecoder().decode(
+                AppMetadata.self,
+                from: jsonData
+            )
+        } catch {
+            print("warning: Swift Bundler metadata present but couldn't be parsed")
+            print("  -> \(error)")
+            return nil
+        }
+    }
+
+    private static func parseBigEndianUInt64(
+        startingAt startIndex: Int,
+        in bytes: [UInt8]
+    ) -> UInt64 {
+        bytes[startIndex..<(startIndex + 8)].withUnsafeBytes { pointer in
+            let bigEndianValue = pointer.assumingMemoryBound(to: UInt64.self)
+                .baseAddress!.pointee
+            return UInt64(bigEndian: bigEndianValue)
+        }
+    }
 }
 
 extension App where State == EmptyState {
     public var state: State {
         EmptyState()
     }
 }
-
-extension App {
-    public var backend: Backend {
-        Backend()
-    }
-}

Sources/SwiftCrossUI/AppMetadata.swift

@@ -0,0 +1,17 @@
+/// Metadata loaded at app start up.
+public struct AppMetadata: Codable {
+    /// The app's reverse domain name identifier.
+    public var identifier: String
+    /// The app's version (generally a semantic version string).
+    public var version: String
+
+    public init(identifier: String, version: String) {
+        self.identifier = identifier
+        self.version = version
+    }
+
+    private enum CodingKeys: String, CodingKey {
+        case identifier = "appIdentifier"
+        case version = "appVersion"
+    }
+}

Sources/SwiftCrossUI/Backend/AppBackend.swift

@@ -43,8 +43,6 @@ public protocol AppBackend {
     associatedtype Menu
     associatedtype Alert
 
-    init()
-
     /// The default height of a table row excluding cell padding. This is a
     /// recommendation by the backend that SwiftCrossUI won't necessarily
     /// follow in all cases.