按下按钮后如何在 Cairo gtk3.0 中正确绘制一条线

Question

我正在关注 zetecode (http://zetcode.com/gfx/cairo/basicdrawing/) 并尝试构建我的第一个在按下按钮后画线的示例，但它失败了，我只是想不通原因。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:

我听从了@jku 的建议，使用 gboolean draw_a_line 记录按钮状态，gtk_widget_queue_draw(widget) 用于重绘。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 window 或拉伸 window 才能显示出来。我想我需要添加某种 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我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

您已经为（大概）绘图区域编写了绘图处理程序，但将其连接到按钮的单击信号。这两个具有完全不同的函数签名，因此段错误不足为奇。

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。然后在绘图区的绘图处理程序中根据应用程序状态决定绘制什么（如should_draw_line）。

查看您链接到的示例后，它似乎几乎完全按照我的建议进行...

Answer 2

行

gtk_widget_queue_draw(widget);

应该是

gtk_widget_queue_draw((GtkWidget*)data);

按下按钮时重新绘制。

第一个参数'GtkWidget *widget'是'button'（自动传递），第二个参数是'darea'小部件。发生的事情是，当按下时按钮被重绘 :D

'on_clicked'函数代码：

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

顺便说一句，感谢您发帖。我从你的代码中学会了在 GTK 中画线。我也是GUI开发的新手。

按下按钮后如何在 Cairo gtk3.0 中正确绘制一条线

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