gui2/ttext_: Add a blinking cursor to textbox widgets

This is used by all widgets that inherit from the textbox abstract
class, ttext_. The cursor is updated every 750 ms by toggling the
cursor alpha every time. When the cursor position is set by a method
(e.g. in reaction to a keyboard event), the cursor is reset to full
alpha, and the blink timer restarted.

data/gui/widget/text_box_default.cfg

@@ -66,6 +66,7 @@
 		x2 = "(cursor_offset + {X_OFFSET})"
 		y2 = "(text_y_offset + text_font_height - 2)"
 		color = "255, 255, 255, 255"
+		alpha = "(cursor_alpha)"
 		thickness = 1
 	[/line]
 #enddef

src/gui/widgets/text.cpp

@@ -18,6 +18,7 @@
 
 #include "desktop/clipboard.hpp"
 #include "gui/core/log.hpp"
+#include "gui/core/timer.hpp"
 #include "serialization/string_utils.hpp"
 #include "serialization/unicode.hpp"
 
@@ -37,6 +38,9 @@ ttext_::ttext_()
 	, text_()
 	, selection_start_(0)
 	, selection_length_(0)
+	, cursor_timer_(0)
+	, cursor_alpha_(0)
+	, cursor_blink_rate_ms_(750)
 	, text_changed_callback_()
 {
 #ifdef __unix__
@@ -53,6 +57,15 @@ ttext_::ttext_()
 			&ttext_::signal_handler_receive_keyboard_focus, this, _2));
 	connect_signal<event::LOSE_KEYBOARD_FOCUS>(
 			std::bind(&ttext_::signal_handler_lose_keyboard_focus, this, _2));
+
+	cursor_timer_ = add_timer(cursor_blink_rate_ms_, std::bind(&ttext_::cursor_timer_callback, this), true);
+}
+
+ttext_::~ttext_()
+{
+	if(cursor_timer_) {
+		remove_timer(cursor_timer_);
+	}
 }
 
 void ttext_::set_active(const bool active)
@@ -109,6 +122,8 @@ void ttext_::set_value(const std::string& text)
 
 void ttext_::set_cursor(const size_t offset, const bool select)
 {
+	reset_cursor_state();
+
 	if(select) {
 
 		if(selection_start_ == offset) {
@@ -239,6 +254,41 @@ void ttext_::set_state(const tstate state)
 	}
 }
 
+void ttext_::cursor_timer_callback()
+{
+	switch(state_) {
+		case DISABLED:
+			cursor_alpha_ = 0;
+			return;
+		case ENABLED:
+			cursor_alpha_ = 255;
+			return;
+		default:
+			cursor_alpha_ = (~cursor_alpha_) & 0xFF;
+	}
+
+	for(auto& tmp : canvas()) {
+		tmp.set_variable("cursor_alpha", variant(cursor_alpha_));
+	}
+
+	set_is_dirty(true);
+}
+
+void ttext_::reset_cursor_state()
+{
+	cursor_alpha_ = 255;
+
+	for(auto& tmp : canvas()) {
+		tmp.set_variable("cursor_alpha", variant(cursor_alpha_));
+	}
+
+	// Restart the blink timer.
+	if(cursor_timer_) {
+		remove_timer(cursor_timer_);
+	}
+	cursor_timer_ = add_timer(cursor_blink_rate_ms_, std::bind(&ttext_::cursor_timer_callback, this), true);
+}
+
 void ttext_::handle_key_left_arrow(SDL_Keymod modifier, bool& handled)
 {
 	/** @todo implement the ctrl key. */

src/gui/widgets/text.hpp

@@ -46,6 +46,8 @@ class ttext_ : public tcontrol
 public:
 	ttext_();
 
+	~ttext_();
+
 	/** See @ref tcontrol::set_active. */
 	virtual void set_active(const bool active) override;
 
@@ -262,6 +264,11 @@ private:
 
 	void set_state(const tstate state);
 
+	/** Implements blinking cursor functionality. */
+	virtual void cursor_timer_callback();
+
+	virtual void reset_cursor_state();
+
 	/**
 	 * Current state of the widget.
 	 *
@@ -285,6 +292,11 @@ private:
 	 */
 	int selection_length_;
 
+	size_t cursor_timer_;
+
+	unsigned short cursor_alpha_;
+	unsigned short cursor_blink_rate_ms_;
+
 	/****** handling of special keys first the pure virtuals *****/
 
 	/**