signal_connect 函数中的指针工作不正确

Question

我想使用g_signal_connect() 函数来更改特定struct/class 中的数据。所以，在我看来，最好的方法是使用指向struct 的指针。问题是指针的信息似乎一直在变化。

我花了很多时间来弄清楚为什么会发生这种情况，但我不知道。 我可以编译和运行代码而没有任何错误，但输出总是不同。

以后我想用几个event_box来连接一个struct数组或者一个class数组（event_box[0]连接到data[0],...）。

我希望有人能理解我的意思，我会很高兴得到任何帮助。

#include<gtk/gtk.h>

struct d
{
bool status;
int ID;
};

void end_program(GtkWidget *wid, gpointer ptr)
{
gtk_main_quit();
}

void box_click(GtkWidget *wid, gpointer user_data)
{
    struct d *data = (struct d*)user_data;
    printf("status  = %i\n", data->status);
    printf("ID      = %i\n", data->ID);
}

int main (int argc, char *argv[])
{
    struct d data;
    data.status = 0;
    data.ID = 1;

    gtk_init(&argc, &argv);

    GtkWidget *win = gtk_window_new (GTK_WINDOW_TOPLEVEL);

    GtkWidget *event_box  = gtk_event_box_new();
    g_signal_connect(G_OBJECT(event_box), "button_press_event", G_CALLBACK(box_click), &data); 

    gtk_container_add(GTK_CONTAINER(win), event_box);
    gtk_widget_show_all(win);
    g_signal_connect(win, "delete_event", G_CALLBACK(end_program),NULL);

    gtk_main();

    return 0;
}

如果我点击该框几次输出：

status  = 4
ID      = 32193184
status  = 5
ID      = 32193184
status  = 4
ID      = 32193184
status  = 6
ID      = 32193184
status  = 4
ID      = 32193184

Answer 1

我希望有人能理解我的意思，我会为任何事情感到高兴 帮助。

嗯，是的.. 您为 button-press-event 使用了错误的函数原型。 button-press-event 的原型是：

The “button-press-event” signal

gboolean
user_function (GtkWidget *widget,
               GdkEvent  *event,
               gpointer   user_data)

（注意：正确的信号是 "button-press-event" 而不是 "button_press_event"，尽管有一个 #define 允许第二种形式工作）

见GtkWidget (Gtk+3 Manual)。所以你的函数应该是这样的：

gboolean box_click(GtkWidget *wid, GdkEvent *event, gpointer user_data)
{
    struct d *data = user_data;            /* no need for cast, gpointer is void* */
    g_print("status  = %d\n", data->status);
    g_print("ID      = %d\n", data->ID);

    return TRUE;    /* to prevent further handling, FALSE otherwise */

    (void)wid;      /* cast to void to avoid unused var warning */
    (void)event;
}

额外的尼特

使用g_print 代替printf，使用gboolean 代替bool。虽然传递 address of 对于小型结构来说很好，但对于大型结构，您应该使用 g_slice_new 进行分配。

Answer 2

我将稍微扩展一下@David C. Rankin 所说的这一部分：

(void)wid;      /* cast to void to avoid unused var warning */
(void)event;

因为有很多人不知道的重要事情。

强制转换的需要一直都是错误的，这是因为有另一个函数被调用：

g_signal_connect_swapped()

通过将第一个参数与最后一个参数交换来避免需要如此多的转换。

举个例子：

#include <gtk/gtk.h>

GtkWidget *createWindow ( const gint width, const gint height );
void user_function      ( GtkWidget *object, gpointer   user_data );

int main ( void )
{
    GtkWidget *window;
    gtk_init ( NULL, NULL );
    /// ***
    window = createWindow( 300, 300 );
    /// ***
    g_signal_connect ( window, "destroy",       G_CALLBACK( user_function ),  NULL );
    /// ***
    gtk_widget_show_all ( window );
    gtk_main();
}

GtkWidget *createWindow ( const gint width, const gint height )
{
    GtkWidget *window = gtk_window_new  ( GTK_WINDOW_TOPLEVEL );
    gtk_widget_set_size_request ( window, width, height );
    gtk_container_set_border_width ( GTK_CONTAINER ( window ), 50 );
    return window;
}

void user_function ( GtkWidget *object, gpointer   user_data )
{
    (void)object;
    (void)user_data;
    g_print( "Goodbye\n" );
    gtk_main_quit();
}

这里你可以看到回调函数签名是这样的：

void user_function ( GtkWidget *object, gpointer user_data )

现在在这种情况下需要两个强制转换，但我们可以通过调用 g_signal_connect_swapped 来删除其中一个（例如 gpointer 数据），如下所示：

g_signal_connect_swapped ( window, "destroy", G_CALLBACK( user_function ),  NULL 

甚至更好：

g_signal_connect_swapped ( window, "destroy", G_CALLBACK( user_function ),  (gpointer)"Goodbye" );

现在我们的程序如下所示：

#include <gtk/gtk.h>

GtkWidget *createWindow ( const gint width, const gint height );
void user_function      ( gpointer data );

int main ( void )
{
    GtkWidget *window;
    gtk_init ( NULL, NULL );
    /// ***
    window = createWindow( 300, 300 );
    /// ***
    g_signal_connect_swapped ( window, "destroy", G_CALLBACK( user_function ),  (gpointer)"Goodbye" );
    /// ***
    gtk_widget_show_all ( window );
    gtk_main();
}

GtkWidget *createWindow ( const gint width, const gint height )
{
    GtkWidget *window = gtk_window_new  ( GTK_WINDOW_TOPLEVEL );
    gtk_widget_set_size_request ( window, width, height );
    gtk_container_set_border_width ( GTK_CONTAINER ( window ), 50 );
    return window;
}

void user_function ( gpointer data )
{
    g_print( "%s\n", (char*)data );
    gtk_main_quit();
}

而且不再有 CAST。

这是一个更好的例子：

#include <gtk/gtk.h>

gboolean scroll_callback    ( GtkWidget *widget, GdkEvent  *event );
gboolean show_mouse_pressed ( GtkWidget *widget, GdkEventButton *event );

int main ( void )
{
    GtkWidget *window;
    GtkWidget *button;
    gtk_init( NULL, NULL );
    /// ***
    window = gtk_window_new( GTK_WINDOW_TOPLEVEL );
    gtk_window_set_default_size( GTK_WINDOW( window ), 300, 250 );
    g_signal_connect( window, "destroy", gtk_main_quit, NULL );
    gtk_container_set_border_width( GTK_CONTAINER( window ), 50 );
    /// ***
    button = gtk_button_new_with_mnemonic( "_Click me" );

    gtk_widget_add_events( button, GDK_BUTTON_PRESS );
    gtk_widget_add_events( button, GDK_SCROLL_MASK );

    ///g_signal_connect( button, "clicked", gtk_main_quit, NULL );
    gtk_container_add( GTK_CONTAINER( window ), button );
    /// ***
    g_signal_connect_swapped( button, "button_press_event", G_CALLBACK( show_mouse_pressed ), window );
    g_signal_connect_swapped( button, "scroll_event", G_CALLBACK( scroll_callback ), window );
    /// ***
    gtk_widget_show_all( window );
    gtk_main();
}

gboolean scroll_callback ( GtkWidget *widget, GdkEvent  *event )
{
    if ( event->type == GDK_SCROLL ) /// Scroll was Catched ?
    {
        if ( event->scroll.direction == GDK_SCROLL_DOWN ) /// It is down?
        {
            g_print( "Scroll-Down Detected\n" );
            gtk_window_set_title( GTK_WINDOW( widget ), "Scroll-Down Detected" );
        }

        if ( event->scroll.direction == GDK_SCROLL_UP ) /// It is up?
        {
            g_print( "Scroll-UP Detected\n" );
            gtk_window_set_title( GTK_WINDOW( widget ), "Scroll-UP Detected" );
        }
        return FALSE;
    }
    return TRUE;
}

gboolean show_mouse_pressed ( GtkWidget *widget, GdkEventButton *event )
{
    assert ( widget != NULL );
    if ( gtk_widget_get_has_window ( widget ) )
    {
        if ( event->type == GDK_BUTTON_PRESS )
        {
            g_print( "The mouse clicked the button\n" );
            gtk_window_set_title( GTK_WINDOW( widget ),  "The mouse was Clicked" );
        }
        return TRUE;
    }
    return FALSE;
}