如何在按下按钮后在Cairo gtk3.0中正确绘制一条线

Question

我正在关注zetecode（http://zetcode.com/gfx/cairo/basicdrawing/）并尝试构建我按下按钮后绘制线条的第一个示例，但它失败了，我无法弄清楚原因。单击按钮时出现segmentation fault错误。

#include <cairo.h>
#include <gtk/gtk.h>

static void do_drawing(cairo_t *);

static gboolean on_draw_event(GtkWidget *widget, cairo_t *cr, 
    gpointer user_data)
{
    do_drawing(cr);
    return FALSE;
}


static void do_drawing(cairo_t *cr)
{
    cairo_set_source_rgb(cr,0,0,0);
    cairo_set_line_width(cr,0.5);

    cairo_move_to(cr,400,400);
    cairo_line_to(cr,400,200);

    cairo_stroke(cr);


}


int main(int argc, char *argv[])
{
    GtkWidget *window;
    GtkWidget *darea;

    gtk_init(&argc, &argv);

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_default_size(GTK_WINDOW(window), 800, 480);
    gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);

    darea = gtk_drawing_area_new();
    GtkWidget *btn_draw = gtk_button_new_with_label("Draw a line");


    GtkWidget *mainwindow = gtk_grid_new();
        gtk_grid_set_row_spacing (GTK_GRID (mainwindow), 16);
        gtk_grid_set_column_spacing (GTK_GRID (mainwindow), 16);
    gtk_grid_set_row_homogeneous(GTK_GRID(mainwindow), TRUE);
    gtk_grid_set_column_homogeneous(GTK_GRID(mainwindow), TRUE);

    gtk_widget_set_margin_left(mainwindow,20);
    gtk_widget_set_margin_right(mainwindow,20);
    gtk_widget_set_margin_top(mainwindow,20);
    gtk_widget_set_margin_bottom(mainwindow,20);

    gtk_grid_attach(GTK_GRID(mainwindow),btn_draw,0,0,1,1);  
    gtk_grid_attach(GTK_GRID(mainwindow),darea,1,0,5,1);  

    gtk_container_add(GTK_CONTAINER(window),mainwindow);

    g_signal_connect(G_OBJECT(btn_draw),"clicked",G_CALLBACK(on_draw_event),NULL);

    gtk_widget_show_all(window);

        gtk_main ();

        return(0);

}

这是它的样子：

PS：

我按照@jku的建议，使用gboolean draw_a_line记录按钮状态，并使用gtk_widget_queue_draw（widget）进行重绘。我注意到的唯一问题是，当我单击按钮时，它不会立即绘制，但我必须隐藏窗口或拉伸窗口才能显示。我想我需要添加一些automatic_update（）函数，但我是GUI设计的新手，所以有人可以告诉我如何在单击按钮后立即显示该行吗？

#include <cairo.h>
#include <gtk/gtk.h>

static void do_drawing(cairo_t *);
GtkWidget *window;
GtkWidget *darea;
gboolean draw_a_line = false;

static gboolean on_draw_event(GtkWidget *widget, GdkEventExpose *event, 
    gpointer user_data)
{
    cairo_t *cr = gdk_cairo_create(gtk_widget_get_window(GTK_WIDGET(widget)));
    do_drawing(cr);

    return FALSE;
}


static void do_drawing(cairo_t *cr)
{
    cairo_set_source_rgb(cr,0,0,0);
    cairo_set_line_width(cr,0.5);
    if (draw_a_line){
        cairo_move_to(cr,400,400);
        cairo_line_to(cr,400,200);
        cairo_stroke(cr);
    }

}


static void on_clicked(GtkWidget *widget, gpointer data)
{
    draw_a_line = true;
    gtk_widget_queue_draw(widget);

}

int main(int argc, char *argv[])
{


    gtk_init(&argc, &argv);

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_default_size(GTK_WINDOW(window), 800, 480);
    gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);

    darea = gtk_drawing_area_new();
    GtkWidget *btn_draw = gtk_button_new_with_label("Draw a line");


    GtkWidget *mainwindow = gtk_grid_new();
        gtk_grid_set_row_spacing (GTK_GRID (mainwindow), 16);
        gtk_grid_set_column_spacing (GTK_GRID (mainwindow), 16);
    gtk_grid_set_row_homogeneous(GTK_GRID(mainwindow), TRUE);
    gtk_grid_set_column_homogeneous(GTK_GRID(mainwindow), TRUE);

    gtk_widget_set_margin_left(mainwindow,20);
    gtk_widget_set_margin_right(mainwindow,20);
    gtk_widget_set_margin_top(mainwindow,20);
    gtk_widget_set_margin_bottom(mainwindow,20);

    gtk_grid_attach(GTK_GRID(mainwindow),btn_draw,0,0,1,1);  
    gtk_grid_attach(GTK_GRID(mainwindow),darea,1,0,5,1);  

    gtk_container_add(GTK_CONTAINER(window),mainwindow);


    g_signal_connect(G_OBJECT(darea), "draw", G_CALLBACK(on_draw_event), NULL); 
    g_signal_connect(G_OBJECT(btn_draw),"clicked",G_CALLBACK(on_clicked),darea);

    gtk_widget_show_all(window);

        gtk_main ();

        return(0);

}

Answer 1

您已经（大概）为绘图区域编写了一个绘制处理程序，但将其连接到按钮的单击信号。这两个具有完全不同的功能签名，因此段错误并不令人惊讶。

不要直接在按钮的单击处理程序上绘制，而是更改某些应用程序状态（如gboolean should_draw_line）并使用gtk_widget_queue_draw ()将绘图区域标记为需要重绘。然后在绘图区域的绘制处理程序中根据应用程序状态决定要绘制的内容（如should_draw_line）。

在查看您链接的示例后，它似乎几乎完全按照我的建议行事......

Answer 2

这条线

gtk_widget_queue_draw(widget);

应该

gtk_widget_queue_draw((GtkWidget*)data);

按下按钮时重绘它。

第一个参数'GtkWidget * widget'是'button'（自动传递），第二个参数是'darea'小部件。发生了什么事情，当按下按钮时重新绘制按钮：D

'on_clicked'函数的代码：

static void on_clicked(GtkWidget *widget, gpointer data)
{
    draw_a_line = TRUE;
    gtk_widget_queue_draw((GtkWidget*)data);
}

顺便说一句，谢谢你发布这个。我学会了从你的代码中在GTK中画线。我也是GUI开发的新手。