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<h1>Using GtkApplication<a class="headerlink" href="#using-gtkapplication" title="Permalink to this headline">#</a></h1>
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<p><code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> is the base class of a GTK Application.</p>
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<p>The philosophy of <code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> is that applications are interested in
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being told what needs to happen, when it needs to happen, in response to actions
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from the user. The exact mechanism by which the operating system starts
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applications is uninteresting.</p>
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<p>To this end, <code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> exposes a set of signals (or virtual functions)
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that an application should respond to:</p>
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<ul class="simple">
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<li><p><code class="docutils literal notranslate"><span class="pre">startup</span></code>: sets up the application when it first starts</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">shutdown</span></code>: performs shutdown tasks</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">activate</span></code>: shows the default first window of the application (like a new
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document). This corresponds to the application being launched by the desktop
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environment.</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">open</span></code>: opens files and shows them in a new window. This corresponds to
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someone trying to open a document (or documents) using the application from
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the file browser, or similar.</p></li>
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</ul>
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<p>When your application starts, the <code class="docutils literal notranslate"><span class="pre">startup</span></code> signal will be fired. This gives
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you a chance to perform initialisation tasks that are not directly related to
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showing a new window. After this, depending on how the application is started,
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either <code class="docutils literal notranslate"><span class="pre">activate</span></code> or <code class="docutils literal notranslate"><span class="pre">open</span></code> will be called next.</p>
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<p><code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> defaults to applications being single-instance. If the user
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attempts to start a second instance of a single-instance application then
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<code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> will signal the first instance and you will receive additional
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activate or open signals. In this case, the second instance will exit
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immediately, without calling <code class="docutils literal notranslate"><span class="pre">startup</span></code> or <code class="docutils literal notranslate"><span class="pre">shutdown</span></code>.</p>
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<p>All startup initialisation should be done in startup. This avoids wasting work
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in the second-instance case where the program just exits immediately.</p>
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<p>The application will continue to run for as long as it needs to. This is usually
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for as long as there are any open windows. You can additionally force the
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application to stay alive using <code class="docutils literal notranslate"><span class="pre">g_application_hold()</span></code>.</p>
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<p>On shutdown, you receive a <code class="docutils literal notranslate"><span class="pre">shutdown</span></code> signal where you can do any necessary
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cleanup tasks (such as saving files to disk).</p>
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<p>You main entry point for your application should only create a
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<code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> instance, set up the signal handlers, and then call
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<code class="docutils literal notranslate"><span class="pre">g_application_run()</span></code>.</p>
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<section id="primary-vs-local-instance">
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<h2>Primary vs. local instance<a class="headerlink" href="#primary-vs-local-instance" title="Permalink to this headline">#</a></h2>
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<p>The “primary instance” of an application is the first instance of it that was
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run. A “remote instance” is another instance that is started that is not the
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primary instance. The term “local instance” is used to refer to the current
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process which may or may not be the primary instance.</p>
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<p><code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> only ever emits signals in the primary instance. Calls to
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<code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> API can be made in primary or remote instances (and are made
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from the vantage of being the “local instance”). In the case that the local
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instance is the primary instance, function calls on <code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> will
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result in signals being emitted locally, in the primary instance. In the case
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that the local instance is a remote instance, function calls result in messages
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being sent to the primary instance and signals being emitted there.</p>
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<p>For example, calling <code class="docutils literal notranslate"><span class="pre">g_application_activate()</span></code> on the primary instance will
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emit the <code class="docutils literal notranslate"><span class="pre">activate</span></code> signal. Calling it on a remote instance will result in a
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message being sent to the primary instance and it will emit <code class="docutils literal notranslate"><span class="pre">activate</span></code>.</p>
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<p>You rarely need to know if the local instance is primary or remote. In almost
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all cases, you should just call the <code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> method you are interested
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in and either have it be forwarded or handled locally, as appropriate.</p>
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</section>
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<section id="actions">
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<h2>Actions<a class="headerlink" href="#actions" title="Permalink to this headline">#</a></h2>
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<p>An application can register a set of actions that it supports in addition to the
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default activate and open. Actions are added to the application with the
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<code class="docutils literal notranslate"><span class="pre">GActionMap</span></code> interface and invoked or queried with the <code class="docutils literal notranslate"><span class="pre">GActionGroup</span></code>
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interface.</p>
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<p>As with <code class="docutils literal notranslate"><span class="pre">activate</span></code> and <code class="docutils literal notranslate"><span class="pre">open</span></code>, calling <code class="docutils literal notranslate"><span class="pre">g_action_group_activate_action()</span></code>
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on the primary instance will activate the named action in the current process.
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Calling <code class="docutils literal notranslate"><span class="pre">g_action_group_activate_action()</span></code> on a remote instance will send a
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message to the primary instance, causing the action to be activated there.</p>
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</section>
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<section id="dealing-with-the-command-line">
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<h2>Dealing with the command line<a class="headerlink" href="#dealing-with-the-command-line" title="Permalink to this headline">#</a></h2>
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<p>Normally, <code class="docutils literal notranslate"><span class="pre">GtkApplication</span></code> will assume that arguments passed on the command
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line are files to be opened. If no arguments are passed, then it assumes that an
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application is being launched to show its main window or an empty document. In
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the case that files were given, you will receive these files (in the form of
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<code class="docutils literal notranslate"><span class="pre">GFile</span></code>) from the open signal. Otherwise you will receive an activate signal.
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It is recommended that new applications make use of this default handling of
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command line arguments.</p>
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<p>If you want to deal with command line arguments in more advanced ways, there are
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several (complementary) mechanisms by which you can do this.</p>
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<p>First, the <code class="docutils literal notranslate"><span class="pre">handle-local-options</span></code> signal will be emitted and the signal
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handler gets a dictionary with the parsed options. To make use of this, you need
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to register your options with <code class="docutils literal notranslate"><span class="pre">g_application_add_main_option_entries()</span></code>. The
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signal handler can return a non-negative value to end the process with this exit
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code, or a negative value to continue with the regular handling of commandline
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options. A popular use of for this signal is to implement a <code class="docutils literal notranslate"><span class="pre">--version</span></code>
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argument that works without communicating with a remote instance.</p>
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<p>If <code class="docutils literal notranslate"><span class="pre">handle-local-options</span></code> is not flexible enough for your needs, you can
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override the <code class="docutils literal notranslate"><span class="pre">local_command_line</span></code> virtual function to take over the handling
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of command line arguments in the local instance entirely. If you do so, you will
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be responsible for registering the application, and for handling a <code class="docutils literal notranslate"><span class="pre">--help</span></code>
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argument (the default <code class="docutils literal notranslate"><span class="pre">local_command_line</span></code> function does this for you).</p>
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<p>It is also possible to invoke actions from <code class="docutils literal notranslate"><span class="pre">handle-local-options</span></code> or
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<code class="docutils literal notranslate"><span class="pre">local_command_line</span></code> in response to command line arguments. For example, a
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mail client may choose to map the <code class="docutils literal notranslate"><span class="pre">--compose</span></code> command line argument to an
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invocation of its <code class="docutils literal notranslate"><span class="pre">compose</span></code> action. This is done by calling
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<code class="docutils literal notranslate"><span class="pre">g_action_group_activate_action()</span></code> from the <code class="docutils literal notranslate"><span class="pre">local_command_line</span></code>
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implementation. In the case that the command line being processed is in the
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primary instance then the <code class="docutils literal notranslate"><span class="pre">compose</span></code> action is invoked locally. In the case
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that it is a remote instance, the action invocation is forwarded to the primary
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instance.</p>
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<div class="admonition note">
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<p class="admonition-title">Note</p>
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<p>It is possible to use action activations instead of <code class="docutils literal notranslate"><span class="pre">activate</span></code> or <code class="docutils literal notranslate"><span class="pre">open</span></code>.
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It is perfectly reasonable that an application could start without an
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<code class="docutils literal notranslate"><span class="pre">activate</span></code> signal ever being emitted. The <code class="docutils literal notranslate"><span class="pre">activate</span></code> signal is only
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supposed to be the default “started with no options” signal. Actions are
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meant to be used for anything else.</p>
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</div>
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<p>Some applications may wish to perform even more advanced handling of command
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lines, including controlling the life cycle of the remote instance and its exit
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status once it quits as well as forwarding the entire contents of the command
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line arguments, the environment and even forwarding standard input, output, and
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error streams. This can be accomplished using
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<code class="docutils literal notranslate"><span class="pre">G_APPLICATION_HANDLES_COMMAND_LINE</span></code> and the <code class="docutils literal notranslate"><span class="pre">command-line</span></code> signal.</p>
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</section>
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</section>
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<ul>
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<li><a class="reference internal" href="#">Using GtkApplication</a><ul>
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<li><a class="reference internal" href="#primary-vs-local-instance">Primary vs. local instance</a></li>
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<li><a class="reference internal" href="#actions">Actions</a></li>
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<li><a class="reference internal" href="#dealing-with-the-command-line">Dealing with the command line</a></li>
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