gui2/tfile_dialog: Add support for a filename extension hint

This allows setting a hint in save mode so that the name portion of the
filename will be selected whenever the user selects a file with a
matching extension on the fileview, and so that whenever the textbox is
cleared by code (e.g. when browsing into directories) it is pre-filled
with the extension with the cursor at the start.

With this, the dialog is now feature-complete compared to the GUI1
version.

src/gui/dialogs/file_dialog.cpp

@@ -36,6 +36,7 @@
 #include "gui/widgets/window.hpp"
 #include "gettext.hpp"
 #include "log.hpp"
+#include "serialization/unicode.hpp"
 
 #include <functional>
 
@@ -106,6 +107,7 @@ tfile_dialog::tfile_dialog()
 	: title_(_("Find File"))
 	, msg_()
 	, ok_label_()
+	, extension_()
 	, current_entry_()
 	, current_dir_()
 	, read_only_(false)
@@ -383,6 +385,36 @@ tfile_dialog::SELECTION_TYPE tfile_dialog::register_new_selection(const std::str
 	return SELECTION_PARENT_NOT_FOUND;
 }
 
+void tfile_dialog::set_input_text(ttext_box& t, const std::string& value)
+{
+	if(value.empty()) {
+		clear_input_text(t);
+		return;
+	}
+
+	t.set_value(value);
+
+	const size_t vallen = t.get_length();
+	const size_t extlen = utf8::size(extension_);
+
+	if(save_mode_ && extlen && vallen > extlen) {
+		// Highlight everything but the extension if it matches
+		if(value.substr(vallen - extlen) == extension_) {
+			t.set_selection(0, vallen - extlen);
+		}
+	}
+}
+
+void tfile_dialog::clear_input_text(ttext_box& t)
+{
+	if(save_mode_ && !extension_.empty()) {
+		t.set_value(extension_);
+		t.set_selection(0, 0);
+	} else {
+		t.clear();
+	}
+}
+
 void tfile_dialog::refresh_fileview(twindow& window)
 {
 	cursor::setter cur{cursor::WAIT};
@@ -423,7 +455,7 @@ void tfile_dialog::refresh_fileview(twindow& window)
 	}
 
 	find_widget<tcontrol>(&window, "current_dir", false).set_label(current_dir_);
-	find_widget<ttext_box>(&window, "filename", false).set_value(current_entry_);
+	set_input_text(find_widget<ttext_box>(&window, "filename", false), current_entry_);
 }
 
 void tfile_dialog::push_fileview_row(tlistbox& filelist, const std::string& name, const std::string& icon, bool check_selection)
@@ -464,9 +496,9 @@ void tfile_dialog::on_row_selected(twindow& window)
 
 	// Clear the textbox when selecting ..
 	if(current_entry_ != PARENT_DIR) {
-		file_textbox.set_value(current_entry_);
+		set_input_text(file_textbox, current_entry_);
 	} else {
-		file_textbox.clear();
+		clear_input_text(file_textbox);
 	}
 }
 

src/gui/dialogs/file_dialog.hpp

@@ -105,6 +105,24 @@ class tfile_dialog : public tdialog
 	 */
 	tfile_dialog& set_filename(const std::string& value);
 
+	/**
+	 * Sets the default file extension for file names in save mode.
+	 *
+	 * When this is set to a non-empty string and save mode is active, selecting
+	 * file entries will cause their name portions to be highlighted in the name
+	 * text box if their extensions match the provided template, and any time the
+	 * text box is cleared it will position the cursor before the extension as a
+	 * hint for the user.
+	 *
+	 * The value provided to this method should be preceded by a dot if
+	 * applicable (e.g. ".cfg").
+	 */
+	tfile_dialog& set_extension(const std::string& value)
+	{
+		extension_ = value;
+		return *this;
+	}
+
 	/**
 	 * Whether user interface elements for manipulating existing objects are provided.
 	 */
@@ -163,6 +181,7 @@ class tfile_dialog : public tdialog
 	std::string msg_;
 	std::string ok_label_;
 
+	std::string extension_;
 	std::string current_entry_;
 	std::string current_dir_;
 
@@ -237,6 +256,9 @@ class tfile_dialog : public tdialog
 	 */
 	SELECTION_TYPE register_new_selection(const std::string& name);
 
+	void set_input_text(class ttext_box& t, const std::string& value);
+	void clear_input_text(class ttext_box& t);
+
 	/**
 	 * Updates the dialog contents to match the internal state.
 	 */