Introduce kiwmi_strata

With the migration to wlr_scene, some objects (desktop, output etc)
will no longer have any reason to care about individual shells and
similar; handling each one on its own would introduce code that is,
from their perspective, over-complicated.

Strata (sg. stratum; this name was chosen to avoid confusion with the
over-used 'layer') represent logical layers of surfaces on the desktop,
such as every layer shell layer, one for xdg shell, another for popups.

include/desktop/layer_shell.h

@@ -19,7 +19,7 @@
 struct kiwmi_layer {
     struct wl_list link;
     struct wlr_layer_surface_v1 *layer_surface;
-    uint32_t layer;
+    uint32_t layer; // enum zwlr_layer_shell_v1_layer
 
     struct kiwmi_output *output;
 

include/desktop/stratum.h

@@ -0,0 +1,33 @@
+/* Copyright (c), Niclas Meyer <niclas@countingsort.com>
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+#ifndef KIWMI_DESKTOP_STRATUM_H
+#define KIWMI_DESKTOP_STRATUM_H
+
+#include <stdint.h>
+
+/**
+ * A stratum is a layer in the scene-graph (the name was chosen to avoid
+ * confusion with layer-shell). The root node contains a scene_tree for each
+ * stratum, which itself contains a scene_tree per output.
+ */
+
+// There are some assumptions made about the values, don't mindlessly change.
+enum kiwmi_stratum {
+    KIWMI_STRATUM_LS_BACKGROUND, // LS == layer_shell
+    KIWMI_STRATUM_LS_BOTTOM,
+    KIWMI_STRATUM_NORMAL,
+    KIWMI_STRATUM_LS_TOP,
+    KIWMI_STRATUM_LS_OVERLAY,
+    KIWMI_STRATUM_POPUPS,
+    KIWMI_STRATA_COUNT,
+    KIWMI_STRATUM_NONE = KIWMI_STRATA_COUNT,
+};
+
+enum kiwmi_stratum stratum_from_layer_shell_layer(uint32_t layer);
+
+#endif /* KIWMI_DESKTOP_STRATUM_H */

kiwmi/desktop/stratum.c

@@ -0,0 +1,33 @@
+/* Copyright (c), Niclas Meyer <niclas@countingsort.com>
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+#include <stdint.h>
+
+#include <wlr/util/log.h>
+
+#include "desktop/stratum.h"
+#include "wlr-layer-shell-unstable-v1-protocol.h"
+
+enum kiwmi_stratum
+stratum_from_layer_shell_layer(uint32_t layer)
+{
+    switch (layer) {
+    case ZWLR_LAYER_SHELL_V1_LAYER_BACKGROUND:
+        return KIWMI_STRATUM_LS_BACKGROUND;
+    case ZWLR_LAYER_SHELL_V1_LAYER_BOTTOM:
+        return KIWMI_STRATUM_LS_BOTTOM;
+    case ZWLR_LAYER_SHELL_V1_LAYER_TOP:
+        return KIWMI_STRATUM_LS_TOP;
+    case ZWLR_LAYER_SHELL_V1_LAYER_OVERLAY:
+        return KIWMI_STRATUM_LS_OVERLAY;
+    default:
+        // We should update our codebase to support the layer
+        wlr_log(
+            WLR_ERROR, "No matching stratum for layer_shell layer %d", layer);
+        return KIWMI_STRATUM_NONE;
+    }
+}

kiwmi/meson.build

@@ -5,6 +5,7 @@ kiwmi_sources = files(
   'desktop/desktop.c',
   'desktop/layer_shell.c',
   'desktop/output.c',
+  'desktop/stratum.c',
   'desktop/view.c',
   'desktop/xdg_shell.c',
   'input/cursor.c',