Initialized repo.

src/gtk/help-overlay.ui

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+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <object class="GtkShortcutsWindow" id="help_overlay">
+    <property name="modal">True</property>
+    <child>
+      <object class="GtkShortcutsSection">
+        <property name="section-name">shortcuts</property>
+        <property name="max-height">10</property>
+        <child>
+          <object class="GtkShortcutsGroup">
+            <property name="title" translatable="yes" context="shortcut window">General</property>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="title" translatable="yes" context="shortcut window">Open</property>
+                <property name="action-name">win.open</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="title" translatable="yes" context="shortcut window">Save as</property>
+                <property name="action-name">win.save-as</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="title" translatable="yes" context="shortcut window">Show Shortcuts</property>
+                <property name="action-name">win.show-help-overlay</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="title" translatable="yes" context="shortcut window">Quit</property>
+                <property name="action-name">app.quit</property>
+              </object>
+            </child>
+          </object>
+        </child>
+      </object>
+    </child>
+  </object>
+</interface>

src/main.py

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+# main.py
+#
+# Copyright 2024 Benjamin Burkhardt
+#
+# 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, either version 3 of the License, or
+# (at your option) any later version.
+#
+# 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 <http://www.gnu.org/licenses/>.
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+import sys
+import gi
+
+gi.require_version('Gtk', '4.0')
+gi.require_version('Adw', '1')
+
+from gi.repository import Gtk, Gio, Adw, GLib
+from .window import TextViewerWindow
+
+
+class TextViewerApplication(Adw.Application):
+    """The main application singleton class."""
+
+    def __init__(self):
+        super().__init__(application_id='com.example.textviewer',
+                         flags=Gio.ApplicationFlags.DEFAULT_FLAGS)
+        self.create_action('quit', lambda *_: self.quit(), ['<primary>q'])
+        self.create_action('about', self.on_about_action)
+
+        # keyboard SHORTCUTS for window actions
+        self.set_accels_for_action('win.open', ['<Ctrl>o'])  # add a keyboard shortcut for the open action
+        self.set_accels_for_action('win.save-as', ['<Ctrl>s'])  # add a keyboard shortcut for the save-as action
+
+
+        # the SETTINGS storage
+        self.settings = Gio.Settings(schema_id="com.example.textviewer")
+
+        # set the STYLE from SETTINGS
+        force_dark_mode = self.settings.get_boolean("force-dark-mode")
+        style_manager = Adw.StyleManager.get_default()
+        if force_dark_mode:
+            style_manager.set_color_scheme(Adw.ColorScheme.FORCE_DARK)
+        else:
+            style_manager.set_color_scheme(Adw.ColorScheme.DEFAULT)
+
+
+        # the dark mode ACTION
+        dark_mode_action = Gio.SimpleAction(name="force_dark_mode",
+                                            state=GLib.Variant.new_boolean(force_dark_mode))
+        dark_mode_action.connect("activate", self.toggle_dark_mode)
+        dark_mode_action.connect("change-state", self.change_color_scheme)
+        self.add_action(dark_mode_action)
+
+
+    def do_activate(self):
+        """Called when the application is activated.
+
+        We raise the application's main window, creating it if
+        necessary.
+        """
+        win = self.props.active_window
+        if not win:
+            win = TextViewerWindow(application=self)
+        win.present()
+
+    def toggle_dark_mode(self, action, _):
+        state = action.get_state()
+        old_state = state.get_boolean()
+        new_state = not old_state
+        action.change_state(GLib.Variant.new_boolean(new_state))
+
+    def change_color_scheme(self, action, new_state):
+        force_dark_mode = new_state.get_boolean()
+        style_manager = Adw.StyleManager.get_default()
+        if force_dark_mode:
+            style_manager.set_color_scheme(Adw.ColorScheme.FORCE_DARK)
+        else:
+            style_manager.set_color_scheme(Adw.ColorScheme.DEFAULT)
+        action.set_state(new_state)
+        self.settings.set_boolean("force-dark-mode", force_dark_mode)
+
+
+    def on_about_action(self, widget, _):
+        """Callback for the app.about action."""
+        about = Adw.AboutWindow(transient_for=self.props.active_window,
+                                application_name='text-viewer',
+                                application_icon='com.example.textviewer',
+                                developer_name='Benjamin Burkhardt',
+                                version='0.1.0',
+                                developers=['Benjamin Burkhardt'],
+                                copyright='© 2024 Benjamin Burkhardt')
+        about.present()
+
+    def create_action(self, name, callback, shortcuts=None):
+        """Add an application action.
+
+        Args:
+            name: the name of the action
+            callback: the function to be called when the action is
+              activated
+            shortcuts: an optional list of accelerators
+        """
+        action = Gio.SimpleAction.new(name, None)
+        action.connect("activate", callback)
+        self.add_action(action)
+        if shortcuts:
+            self.set_accels_for_action(f"app.{name}", shortcuts)
+
+
+def main(version):
+    """The application's entry point."""
+    app = TextViewerApplication()
+    return app.run(sys.argv)

src/meson.build

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+pkgdatadir = get_option('prefix') / get_option('datadir') / meson.project_name()
+moduledir = pkgdatadir / 'text_viewer'
+gnome = import('gnome')
+
+gnome.compile_resources('text-viewer',
+  'text-viewer.gresource.xml',
+  gresource_bundle: true,
+  install: true,
+  install_dir: pkgdatadir,
+)
+
+python = import('python')
+
+conf = configuration_data()
+conf.set('PYTHON', python.find_installation('python3').path())
+conf.set('VERSION', meson.project_version())
+conf.set('localedir', get_option('prefix') / get_option('localedir'))
+conf.set('pkgdatadir', pkgdatadir)
+
+configure_file(
+  input: 'text-viewer.in',
+  output: 'text-viewer',
+  configuration: conf,
+  install: true,
+  install_dir: get_option('bindir'),
+  install_mode: 'r-xr--r--'
+)
+
+text_viewer_sources = [
+  '__init__.py',
+  'main.py',
+  'window.py',
+]
+
+install_data(text_viewer_sources, install_dir: moduledir)

src/text-viewer.gresource.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<gresources>
+  <gresource prefix="/com/example/textviewer">
+    <file preprocess="xml-stripblanks">window.ui</file>
+    <file preprocess="xml-stripblanks">gtk/help-overlay.ui</file>
+  </gresource>
+</gresources>

src/text-viewer.in

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+#!@PYTHON@
+
+# text-viewer.in
+#
+# Copyright 2024 Benjamin Burkhardt
+#
+# 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, either version 3 of the License, or
+# (at your option) any later version.
+#
+# 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 <http://www.gnu.org/licenses/>.
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+import os
+import sys
+import signal
+import locale
+import gettext
+
+VERSION = '@VERSION@'
+pkgdatadir = '@pkgdatadir@'
+localedir = '@localedir@'
+
+sys.path.insert(1, pkgdatadir)
+signal.signal(signal.SIGINT, signal.SIG_DFL)
+locale.bindtextdomain('text-viewer', localedir)
+locale.textdomain('text-viewer')
+gettext.install('text-viewer', localedir)
+
+if __name__ == '__main__':
+    import gi
+
+    from gi.repository import Gio
+    resource = Gio.Resource.load(os.path.join(pkgdatadir, 'text-viewer.gresource'))
+    resource._register()
+
+    from text_viewer import main
+    sys.exit(main.main(VERSION))

src/window.py

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+# window.py
+#
+# Copyright 2024 Benjamin Burkhardt
+#
+# 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, either version 3 of the License, or
+# (at your option) any later version.
+#
+# 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 <http://www.gnu.org/licenses/>.
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+from gi.repository import Adw, Gtk, Gio, GLib  # Adw is Adwaita and GTK GTK :)
+                                               # Gio is needed for actions etc.
+                                               # and GLib for saving the file!
+
+@Gtk.Template(resource_path='/com/example/textviewer/window.ui')
+class TextViewerWindow(Adw.ApplicationWindow):
+    __gtype_name__ = 'TextViewerWindow'
+
+    main_text_view = Gtk.Template.Child()
+    open_button = Gtk.Template.Child()
+    cursor_pos_text = Gtk.Template.Child()
+    toast_overlay = Gtk.Template.Child()
+
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+
+        open_action = Gio.SimpleAction(name="open")
+        open_action.connect("activate", self.open_file_dialog_action)
+        self.add_action(open_action)
+
+        save_action = Gio.SimpleAction(name="save-as")
+        save_action.connect("activate", self.save_file_dialog_action)
+        self.add_action(save_action)
+
+
+        buffer = self.main_text_view.get_buffer()
+        buffer.connect("notify::cursor-position", self.update_cursor_position)
+
+        self.settings = Gio.Settings(schema_id="com.example.textviewer")
+        self.settings.bind("window-width", self, "default-width",
+                           Gio.SettingsBindFlags.DEFAULT)
+        self.settings.bind("window-height", self, "default-height",
+                           Gio.SettingsBindFlags.DEFAULT)
+        self.settings.bind("window-maximized", self, "maximized",
+                           Gio.SettingsBindFlags.DEFAULT)
+        # Gio.Settings.bind method takes
+        # 1. the key defined in the settings scheme to bind to
+        # 2. the object which owns the property to bind the setting key to
+        # 3. property: the property, e.g. default-width as defined in the XML!
+        # 4. flags: BindFlags.DEFAULT means it's a kind of bidirectional rw bound
+
+
+
+    def open_file_dialog_action(self, action, _):
+        native = Gtk.FileDialog()
+        native.open(self, None, self.on_open_response)
+
+    def on_open_response(self, dialog, result):
+        try:
+            file = dialog.open_finish(result)
+        except:  # if the user dismisses the dialog, a GError/sth else is raised
+            return
+        # If the user selected a file...
+        if file is not None:
+            # ... open it
+            self.open_file(file)
+
+    def open_file(self, file):
+        file.load_contents_async(None, self.open_file_complete)
+
+    def open_file_complete(self, file, result):
+        info = file.query_info("standard::display-name", Gio.FileQueryInfoFlags.NONE)
+        if info:
+            display_name = info.get_attribute_string("standard::display-name")
+        else:
+            display_name = file.get_basename()
+
+        contents = file.load_contents_finish(result)
+        if not contents[0]:
+            path = file.peek_path()
+            print(f"Unable to open {path}: {contents[1]}")
+            self.toast_overlay.add_toast(Adw.Toast(title=f"Unable to open “{display_name}”"))
+
+        try:
+            text = contents[1].decode('utf-8')
+        except UnicodeError as err:
+            path = file.peek_path()
+            print(f"Unable to load the contents of {path}: the file is not encoded with UTF-8")
+            self.toast_overlay.add_toast(Adw.Toast(title=f"Invalid encoding for {display_name}"))
+            return
+
+
+        self.set_title(display_name)
+
+        buffer = self.main_text_view.get_buffer()
+        buffer.set_text(text)
+        start = buffer.get_start_iter()
+        buffer.place_cursor(start)
+
+    def update_cursor_position(self, buffer, _):
+        # Retrieve the value of the "cursor-position" property
+        cursor_pos = buffer.props.cursor_position
+        # Construct the text iterator for the position of the cursor
+        iter = buffer.get_iter_at_offset(cursor_pos)
+        line = iter.get_line() + 1
+        column = iter.get_line_offset() + 1
+        # Set the new contents of the label
+        self.cursor_pos_text.set_text(f"Ln {line}, Col {column}")
+
+    def save_file_dialog_action(self, action, _):
+        native = Gtk.FileDialog()
+        native.save(self, None, self.on_save_response)
+
+    def on_save_response(self, dialog, result):
+        try:
+            file = dialog.save_finish(result)
+        except:  # if the user dismisses the dialog, a GError/sth else is raised
+            return
+        if file is not None:
+            self.save_file(file)
+
+    def save_file(self, file):
+        buffer = self.main_text_view.get_buffer()
+
+        # Retrieve the iterator at the start of the buffer
+        start = buffer.get_start_iter()
+        # Retrieve the iterator at the end of the buffer
+        end = buffer.get_end_iter()
+        # Retrieve all the visible text between the two bounds
+        text = buffer.get_text(start, end, False)
+
+        # If there is nothing to save, return early
+        if not text:
+            return
+
+        bytes = GLib.Bytes.new(text.encode('utf-8'))
+
+        # Start the asynchronous operation to save the data into the file
+        file.replace_contents_bytes_async(bytes,
+                                          None,
+                                          False,
+                                          Gio.FileCreateFlags.NONE,
+                                          None,
+                                          self.save_file_complete)
+
+    def save_file_complete(self, file, result):
+        res = file.replace_contents_finish(result)
+        info = file.query_info("standard::display-name",
+                               Gio.FileQueryInfoFlags.NONE)
+        if info:
+            display_name = info.get_attribute_string("standard::display-name")
+        else:
+            display_name = file.get_basename()
+        if not res:
+            print(f"Unable to save {display_name}")
+            self.toast_overlay.add_toast(Adw.Toast(title=f"Unable to save {display_name}"))
+        else:
+            self.toast_overlay.add_toast(Adw.Toast(title=f"Saved to {display_name}"))
+
+        self.set_title(display_name)
+        

src/window.ui

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+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <requires lib="gtk" version="4.0"/>
+  <requires lib="Adw" version="1.0"/>
+  <template class="TextViewerWindow" parent="AdwApplicationWindow">
+    <property name="default-width">600</property>
+    <property name="default-height">300</property>
+    <property name="title">Text Viewer</property>
+    <style>
+      <class name="devel"/>
+    </style>
+    <property name="content">
+      <object class="AdwToolbarView">
+        <child type="top">
+          <object class="AdwHeaderBar" id="header_bar">
+            <child type="start">
+              <object class="GtkMenuButton">
+                <property name="primary">True</property>
+                <property name="icon-name">open-menu-symbolic</property>
+                <property name="tooltip-text" translatable="yes">Menu</property>
+                <property name="menu-model">primary_menu</property>
+              </object>
+            </child>
+            <child type="end">
+              <object class="GtkButton" id="open_button">
+                <property name="label">Open</property>
+                <property name="action-name">win.open</property>
+              </object>
+            </child>
+            <child type="end">
+              <object class="GtkLabel" id="cursor_pos_text">
+                <property name="label">Ln 0, Col 0</property>
+                <style>
+                  <class name="dim-label"/>
+                  <class name="numeric"/>
+                </style>
+              </object>
+            </child>
+          </object>
+        </child>
+        <property name="content">
+          <object class="AdwToastOverlay" id="toast_overlay">
+            <property name="child">
+              <object class="GtkScrolledWindow">
+                <property name="hexpand">true</property>
+                <property name="vexpand">true</property>
+                <property name="margin-top">6</property>
+                <property name="margin-bottom">6</property>
+                <property name="margin-start">6</property>
+                <property name="margin-end">6</property>
+                <property name="child">
+                  <object class="GtkTextView" id="main_text_view">
+                    <property name="monospace">true</property>
+                  </object>
+                </property>
+              </object>
+            </property>
+          </object>
+        </property>
+      </object>
+    </property>
+  </template>
+  <menu id="primary_menu">
+    <section>
+      <item>
+        <attribute name="label" translatable="yes">_Save as...</attribute>
+        <attribute name="action">win.save-as</attribute>
+      </item>
+      <item>
+        <attribute name="label" translatable="yes">_Force dark mode</attribute>
+        <attribute name="action">app.force_dark_mode</attribute>
+      </item>
+      <item>
+        <attribute name="label" translatable="yes">_Keyboard Shortcuts</attribute>
+        <attribute name="action">win.show-help-overlay</attribute>
+      </item>
+      <item>
+        <attribute name="label" translatable="yes">_About Text-viewer</attribute>
+        <attribute name="action">app.about</attribute>
+      </item>
+    </section>
+  </menu>
+</interface>