Refactor code, create abstract class ExternalUpdater

(Hopefully) this makes implementing updaters using external libraries easier on other platforms. To implement an updater on a new platform, create a new class that implements the pure virtual methods from `ExternalUpdater` and add code in the `UpdateChecker` initializer to initialize the new class.

launcher/updater/MacSparkleUpdater.h

@@ -0,0 +1,126 @@
+// SPDX-License-Identifier: GPL-3.0-only
+/*
+ *  PolyMC - Minecraft Launcher
+ *  Copyright (C) 2022 Kenneth Chew <kenneth.c0@protonmail.com>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, version 3.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef LAUNCHER_MACSPARKLEUPDATER_H
+#define LAUNCHER_MACSPARKLEUPDATER_H
+
+#include <QObject>
+#include <QSet>
+#include "ExternalUpdater.h"
+
+/*!
+ * An implementation for the updater on macOS that uses the Sparkle framework.
+ */
+class MacSparkleUpdater : public ExternalUpdater
+{
+    Q_OBJECT
+
+public:
+    /*!
+     * Start the Sparkle updater, which automatically checks for updates if necessary.
+     */
+    MacSparkleUpdater();
+    ~MacSparkleUpdater() override;
+
+    /*!
+     * Check for updates manually, showing the user a progress bar and an alert if no updates are found.
+     */
+    void checkForUpdates() override;
+
+    /*!
+     * Indicates whether or not to check for updates automatically.
+     */
+    bool getAutomaticallyChecksForUpdates() override;
+
+    /*!
+     * Indicates the current automatic update check interval in seconds.
+     */
+    double getUpdateCheckInterval() override;
+
+    /*!
+     * Indicates the set of Sparkle channels the updater is allowed to find new updates from.
+     */
+    QSet<QString> getAllowedChannels();
+
+    /*!
+     * Indicates whether or not beta updates should be checked for in addition to regular releases.
+     */
+    bool getBetaAllowed() override;
+
+    /*!
+     * Set whether or not to check for updates automatically.
+     *
+     * As per Sparkle documentation, "By default, Sparkle asks users on second launch for permission if they want
+     * automatic update checks enabled and sets this property based on their response. If SUEnableAutomaticChecks is
+     * set in the Info.plist, this permission request is not performed however.
+     *
+     * Setting this property will persist in the host bundle’s user defaults. Only set this property if you need
+     * dynamic behavior (e.g. user preferences).
+     *
+     * The update schedule cycle will be reset in a short delay after the property’s new value is set. This is to allow
+     * reverting this property without kicking off a schedule change immediately."
+     */
+    void setAutomaticallyChecksForUpdates(bool check) override;
+
+    /*!
+     * Set the current automatic update check interval in seconds.
+     *
+     * As per Sparkle documentation, "Setting this property will persist in the host bundle’s user defaults. For this
+     * reason, only set this property if you need dynamic behavior (eg user preferences). Otherwise prefer to set
+     * SUScheduledCheckInterval directly in your Info.plist.
+     *
+     * The update schedule cycle will be reset in a short delay after the property’s new value is set. This is to allow
+     * reverting this property without kicking off a schedule change immediately."
+     */
+    void setUpdateCheckInterval(double seconds) override;
+
+    /*!
+     * Clears all allowed Sparkle channels, returning to the default updater channel behavior.
+     */
+    void clearAllowedChannels();
+
+    /*!
+     * Set a single Sparkle channel the updater is allowed to find new updates from.
+     *
+     * Items in the default channel can always be found, regardless of this setting. If an empty string is passed,
+     * return to the default behavior.
+     */
+    void setAllowedChannel(const QString& channel);
+
+    /*!
+     * Set a set of Sparkle channels the updater is allowed to find new updates from.
+     *
+     * Items in the default channel can always be found, regardless of this setting. If an empty set is passed,
+     * return to the default behavior.
+     */
+    void setAllowedChannels(const QSet<QString>& channels);
+
+    /*!
+     * Set whether or not beta updates should be checked for in addition to regular releases.
+     */
+    void setBetaAllowed(bool allowed) override;
+
+private:
+    class Private;
+
+    Private *priv;
+
+    void loadChannelsFromSettings();
+};
+
+#endif //LAUNCHER_MACSPARKLEUPDATER_H