(Re)introduce a backend interface

This will allow us to unit test essential operations on package.

Author: leolost2605

src/Core/Backend.vala

@@ -0,0 +1,15 @@
+/*
+ * SPDX-FileCopyrightText: 2025 elementary, Inc. (https://elementary.io)
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Authored by: Leonhard Kargl <leo.kargl@proton.me>
+ */
+
+public interface AppCenterCore.Backend : Object {
+    public signal void operation_finished (Package package, Package.State operation, Error? error);
+
+    public abstract void notify_package_changed (Package package);
+    public abstract async void install_package (Package package, ChangeInformation change_info) throws Error;
+    public abstract async void remove_package (Package package, ChangeInformation change_info) throws Error;
+    public abstract async void update_package (Package package, ChangeInformation change_info) throws Error;
+}

src/Core/FlatpakBackend.vala

@@ -22,6 +22,7 @@ public class AppCenterCore.FlatpakPackage : Package {
 
     public FlatpakPackage (string uid, Flatpak.Installation installation, AppStream.Component component) {
         Object (
+            backend: FlatpakBackend.get_default (),
             uid: uid,
             installation: installation,
             component: component
@@ -49,8 +50,7 @@ public class AppCenterCore.FlatpakPackage : Package {
     }
 }
 
-public class AppCenterCore.FlatpakBackend : Object {
-    public signal void operation_finished (Package package, Package.State operation, Error? error);
+public class AppCenterCore.FlatpakBackend : Object, Backend {
     public signal void on_metadata_remote_preprocessed (string remote_title);
     public signal void package_list_changed ();
 
@@ -231,7 +231,7 @@ public class AppCenterCore.FlatpakBackend : Object {
         runtime_updates_component.summary = _("Updates to app runtimes");
         runtime_updates_component.add_icon (runtime_icon);
 
-        runtime_updates = new AppCenterCore.Package ("runtime-updates", runtime_updates_component);
+        runtime_updates = new AppCenterCore.Package (this, "runtime-updates", runtime_updates_component);
 
         additional_updates = new GLib.ListStore (typeof (Package));
         additional_updates.append (runtime_updates);

src/Core/Package.vala

@@ -96,6 +96,7 @@ public class AppCenterCore.Package : Object {
     public const string LOCAL_ID_SUFFIX = ".appcenter-local";
     public const string DEFAULT_PRICE_DOLLARS = "1";
 
+    public unowned Backend backend { get; construct; }
     public string uid { get; construct; }
 
     public AppStream.Component component { get; protected set; }
@@ -418,8 +419,8 @@ public class AppCenterCore.Package : Object {
         change_information = new ChangeInformation ();
     }
 
-    public Package (string uid, AppStream.Component component) {
-        Object (uid: uid, component: component);
+    public Package (Backend backend, string uid, AppStream.Component component) {
+        Object (backend: backend, uid: uid, component: component);
     }
 
     public void replace_component (AppStream.Component component) {
@@ -455,7 +456,7 @@ public class AppCenterCore.Package : Object {
         // Only trigger a notify if the state has changed, quite a lot of things listen to this
         if (state != new_state) {
             state = new_state;
-            FlatpakBackend.get_default ().notify_package_changed (this);
+            backend.notify_package_changed (this);
         }
     }
 
@@ -502,15 +503,14 @@ public class AppCenterCore.Package : Object {
         change_information.start ();
         state = performing;
 
-        unowned var flatpak_backend = AppCenterCore.FlatpakBackend.get_default ();
-        flatpak_backend.notify_package_changed (this);
+        backend.notify_package_changed (this);
 
         try {
             yield perform_package_operation ();
-            flatpak_backend.operation_finished (this, performing, null);
+            backend.operation_finished (this, performing, null);
         } catch (GLib.Error e) {
             warning ("Operation failed for package %s - %s", name, e.message);
-            flatpak_backend.operation_finished (this, performing, e);
+            backend.operation_finished (this, performing, e);
             throw e;
         } finally {
             change_information.complete ();
@@ -519,8 +519,6 @@ public class AppCenterCore.Package : Object {
     }
 
     private async void perform_package_operation () throws GLib.Error {
-        unowned var backend = AppCenterCore.FlatpakBackend.get_default ();
-
         switch (state) {
             case State.UPDATING:
                 yield backend.update_package (this, change_information);

src/meson.build

@@ -5,6 +5,7 @@ appcenter_files = files(
     'SuspendControl.vala',
     'Utils.vala',
     'Core' / 'AddonFilter.vala',
+    'Core' / 'Backend.vala',
     'Core/CardUtils.vala',
     'Core' / 'CategoryManager.vala',
     'Core/ChangeInformation.vala',