Support plugins

CHANGELOG.md

@@ -26,12 +26,14 @@
       ! fullscreen
       border_width = 0
   ```
+* picom now has a rudimentary plugin system. At the moment, the only thing you can do with it is loading custom backends.
 
 ## Notable changes
 
 * `override_redirect` in rules now only matches top-level windows that doesn't have a client window. Some window managers (e.g. awesome) set override_redirect for all window manager frame windows, causing this rule to match against everything (#625).
 * Marginally improve performance when resizing/opening/closing windows. (#1190)
 * Type and format specifiers are no longer used in rules. These specifiers are what you put after the colon (':') in rules, e.g. the `:32c` in `"_GTK_FRAME_EXTENTS@:32c"`. Now this information is ignored and the property is matched regardless of format or type.
+* `backend` is now a required option. picom will not start if one is not specified explicitly.
 
 ## Deprecated features
 

include/meson.build

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: MPL-2.0
+# Copyright (c) Yuxuan Shui <[email protected]>
+subdirs('picom')

include/picom/api.h

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <[email protected]>
+
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#define PICOM_API_MAJOR (0UL)
+#define PICOM_API_MINOR (1UL)
+
+struct backend_base;
+
+/// The entry point of a backend plugin. Called after the backend is initialized.
+typedef void (*picom_backend_plugin_entrypoint)(struct backend_base *backend, void *user_data);
+struct picom_api {
+	/// Add a plugin for a specific backend. The plugin's entry point will be called
+	/// when the specified backend is initialized.
+	///
+	/// @param backend_name The name of the backend to add the plugin to.
+	/// @param major        The major version of the backend API interface this plugin
+	///                     is compatible with.
+	/// @param minor        The minor version of the backend API interface this plugin
+	///                     is compatible with.
+	/// @param entrypoint   The entry point of the plugin.
+	/// @param user_data    The user data to pass to the plugin's entry point.
+	bool (*add_backend_plugin)(const char *backend_name, uint64_t major, uint64_t minor,
+	                           picom_backend_plugin_entrypoint entrypoint,
+	                           void *user_data);
+};
+
+const struct picom_api *
+picom_api_get_interfaces(uint64_t major, uint64_t minor, const char *context);