ex_gui_frameless_drag.c

A press on a widget in a movable frameless window is not a window drag (headless).

A press on a widget in a movable frameless window is not a window drag (headless). A frameless window without N_GUI_WIN_FIXED_POSITION is dragged by its body. The press handler used to start that drag after handing the press to the widget under the pointer, so a press on a slider started both: the window then followed the pointer exactly, the slider's position inside its own window never changed, and the slider did not move at all. The same held for a scrollbar and for anything else that is dragged.

The rule now is that a press a widget took is not a window drag. A press on the empty body still is, and so is a press on a widget that does nothing with a press - here an image - so a window made mostly of pictures can still be moved.

Author

Castagnier Mickael

Version

1.0

Date

22/09/2026

/*
 * Nilorea Library
 * Copyright (C) 2005-2026 Castagnier Mickael
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
 * implied. See the License for the specific language governing
 * permissions and limitations under the License.
 *
 * SPDX-License-Identifier: Apache-2.0
 */

 
#define ALLEGRO_UNSTABLE 1
 
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#include "nilorea/n_common.h"
#include "nilorea/n_gui.h"
#include "nilorea/n_log.h"

static int failures = 0;

static int slider_changes = 0;

static void expect_true(const char* label, int cond) {
    if (!cond) {
        n_log(LOG_ERR, "%s: FAILED", label);
        failures++;
    } else {
        n_log(LOG_INFO, "%s: OK", label);
    }
}

static void on_slider(int id, double value, void* data) {
    (void)id;
    (void)value;
    (void)data;
    slider_changes++;
}

static ALLEGRO_EVENT mouse_event(int type, int x, int y) {
    ALLEGRO_EVENT ev;
    memset(&ev, 0, sizeof(ev));
    ev.type = (unsigned int)type;
    ev.mouse.x = x;
    ev.mouse.y = y;
    ev.mouse.button = 1;
    return ev;
}

static void drag_right(N_GUI_CTX* gui, int x, int y, int step) {
    int i = 0;
 
    n_gui_process_event(gui, mouse_event(ALLEGRO_EVENT_MOUSE_BUTTON_DOWN, x, y));
    for (i = 1; i <= 3; i++) {
        n_gui_process_event(gui, mouse_event(ALLEGRO_EVENT_MOUSE_AXES, x + i * step, y));
    }
    n_gui_process_event(gui, mouse_event(ALLEGRO_EVENT_MOUSE_BUTTON_UP, x + 3 * step, y));
}
 
int main(void) {
    ALLEGRO_FONT* font;
    ALLEGRO_BITMAP* pic;
    N_GUI_CTX* gui;
    N_GUI_WINDOW* w;
    int win, slider;
    double before;
 
    set_log_level(LOG_ERR);
    if (!al_init() || !al_init_font_addon()) {
        n_log(LOG_ERR, "allegro init failed");
        return 1;
    }
    font = al_create_builtin_font(); /* no display needed */
    if (!font) {
        n_log(LOG_ERR, "al_create_builtin_font failed");
        return 1;
    }
    /* a memory bitmap, since there is no display to make a video one on */
    al_set_new_bitmap_flags(ALLEGRO_MEMORY_BITMAP);
    pic = al_create_bitmap(16, 16);
    if (!pic) {
        n_log(LOG_ERR, "al_create_bitmap failed");
        al_destroy_font(font);
        return 1;
    }
    gui = n_gui_new_ctx(font);
    if (!gui) {
        n_log(LOG_ERR, "n_gui_new_ctx failed");
        al_destroy_bitmap(pic);
        al_destroy_font(font);
        return 1;
    }
    n_gui_set_display_size(gui, 800.0f, 600.0f);
    win = n_gui_add_window(gui, "W", 0.0f, 0.0f, 400.0f, 300.0f);
    /* frameless and NOT fixed: the body is a drag handle for the window */
    n_gui_window_set_flags(gui, win, N_GUI_WIN_FRAMELESS);
    slider = n_gui_add_slider(gui, win, 0.0f, 0.0f, 200.0f, 24.0f, 0.0, 1.0, 0.0,
                              N_GUI_SLIDER_VALUE, on_slider, NULL);
    n_gui_add_image(gui, win, 0.0f, 100.0f, 50.0f, 50.0f, pic, N_GUI_IMAGE_FIT);
    n_gui_open_window(gui, win);
    w = n_gui_get_window(gui, win);
    if (!w) {
        n_log(LOG_ERR, "n_gui_get_window failed");
        n_gui_destroy_ctx(&gui);
        al_destroy_bitmap(pic);
        al_destroy_font(font);
        return 1;
    }
 
    /* A drag on the slider moves the slider and leaves the window where it
       is. Before the fix the window moved by exactly the pointer's travel and
       the slider stayed at its first value. */
    n_gui_slider_set_value(gui, slider, 0.0);
    before = n_gui_slider_get_value(gui, slider);
    slider_changes = 0;
    drag_right(gui, 40, 12, 20);
    expect_true("a drag on a slider leaves the window where it was",
                w->x == 0.0f && w->y == 0.0f);
    expect_true("and moves the slider", n_gui_slider_get_value(gui, slider) > before);
    /* one for the press and one per motion event */
    expect_true("and reports every step of it", slider_changes >= 4);
 
    /* The empty body is still a handle for the window. */
    drag_right(gui, 300, 250, 10);
    expect_true("a drag on the empty body moves the window", w->x == 30.0f && w->y == 0.0f);
 
    /* So is an image, which does nothing with a press. The window is now 30
       to the right, so the image is too. */
    drag_right(gui, 30 + 20, 120, 10);
    expect_true("a drag on an image moves the window", w->x == 60.0f);
 
    n_gui_destroy_ctx(&gui);
    al_destroy_bitmap(pic);
    al_destroy_font(font);
    if (failures) {
        n_log(LOG_ERR, "ex_gui_frameless_drag: %d failure(s)", failures);
        return 1;
    }
    n_log(LOG_NOTICE, "ex_gui_frameless_drag: all checks passed");
    return 0;
}