Add an overridable VCS Interface for the editor

The VCS interface can be thought of like a proxy system, where any call
to the API is redirected to the actual implementation of the VCS API
which may be existing in the form of a GDNative plugin which is marked
as a singleton and is not marked reloadable. If the implementation
doesn't exist in the file system, it only returns the default responses which contain
mostly empty containers of the data type that every API call returns.

EditorVCSInterface is used like a Godot object with a script attached to it. The script
is the implementation of the API and the object is the interface to the
script, which returns default responses if the script doesn't exist or
if the script doesn't define a function that handles that particular API call.

The entire system has been implemented using Object::call() and its
ability to switch to the script instance to handle the API call if the
script exists. Look for VersionControlEditorPlugin::_initialize() for
the essential API setup.

3 files changed, 341 insertions, 0 deletions

diff --git a/doc/classes/EditorVCSInterface.xml b/doc/classes/EditorVCSInterface.xml
new file mode 100644
index 0000000000..2f58e6d54e
--- /dev/null
+++ b/doc/classes/EditorVCSInterface.xml
@@ -0,0 +1,115 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="EditorVCSInterface" inherits="Object" category="Core" version="3.2">
+	<brief_description>
+		Version Control System (VCS) interface which reads and writes to the local VCS in use.
+	</brief_description>
+	<description>
+		Used by the editor to display VCS extracted information in the editor. The implementation of this API is included in VCS addons, which are essentially GDNative plugins that need to be put into the project folder. These VCS addons are scripts which are attached (on demand) to the object instance of [code]EditorVCSInterface[/code]. All the functions listed below, instead of performing the task themselves, they call the internally defined functions in the VCS addons to provide a plug-n-play experience.
+	</description>
+	<tutorials>
+	</tutorials>
+	<methods>
+		<method name="commit">
+			<return type="void">
+			</return>
+			<argument index="0" name="msg" type="String">
+			</argument>
+			<description>
+				Creates a version commit if the addon is initialized, else returns without doing anything. Uses the files which have been staged previously, with the commit message set to a value as provided as in the argument.
+			</description>
+		</method>
+		<method name="get_file_diff">
+			<return type="Array">
+			</return>
+			<argument index="0" name="file_path" type="String">
+			</argument>
+			<description>
+				Returns an [Array] of [Dictionary] objects containing the diff output from the VCS in use, if a VCS addon is initialized, else returns an empty [Array] object. The diff contents also consist of some contextual lines which provide context to the observed line change in the file.
+				Each [Dictionary] object has the line diff contents under the keys:
+				- [code]"content"[/code] to store a [String] containing the line contents
+				- [code]"status"[/code] to store a [String] which contains [code]"+"[/code] in case the content is a line addition but it stores a [code]"-"[/code] in case of deletion and an empty string in the case the line content is neither an addition nor a deletion.
+				- [code]"new_line_number"[/code] to store an integer containing the new line number of the line content.
+				- [code]"line_count"[/code] to store an integer containing the number of lines in the line content.
+				- [code]"old_line_number"[/code] to store an integer containing the old line number of the line content.
+				- [code]"offset"[/code] to store the offset of the line change since the first contextual line content.
+			</description>
+		</method>
+		<method name="get_is_vcs_intialized">
+			<return type="bool">
+			</return>
+			<description>
+				Returns [code]true[/code] if the VCS addon has been intialized, else returns [code]false[/code].
+			</description>
+		</method>
+		<method name="get_modified_files_data">
+			<return type="Dictionary">
+			</return>
+			<description>
+				Returns a [Dictionary] containing the path of the detected file change mapped to an integer signifying what kind of a change the corresponding file has experienced.
+				The following integer values are being used to signify that the detected file is:
+				- [code]0[/code]: New to the VCS working directory
+				- [code]1[/code]: Modified
+				- [code]2[/code]: Renamed
+				- [code]3[/code]: Deleted
+				- [code]4[/code]: Typechanged
+			</description>
+		</method>
+		<method name="get_project_name">
+			<return type="String">
+			</return>
+			<description>
+				Return the project name of the VCS working directory
+			</description>
+		</method>
+		<method name="get_vcs_name">
+			<return type="String">
+			</return>
+			<description>
+				Return the name of the VCS if the VCS has been intialized, else return an empty string.
+			</description>
+		</method>
+		<method name="initialize">
+			<return type="bool">
+			</return>
+			<argument index="0" name="project_root_path" type="String">
+			</argument>
+			<description>
+				Initialize the VCS addon if not already. Uses the argument value as the path to the working directory of the project. Creates the initial commit if required. Returns [code]true[/code] if no failure occurs, else returns [code]false[/code].
+			</description>
+		</method>
+		<method name="is_addon_ready">
+			<return type="bool">
+			</return>
+			<description>
+				Returns [code]true[/code] if the addon is ready to respond to function calls, else returns [code]false[/code].
+			</description>
+		</method>
+		<method name="shut_down">
+			<return type="bool">
+			</return>
+			<description>
+				Shuts down the VCS addon to allow cleanup code to run on call. Returns [code]true[/code] is no failure occurs, else returns [code]false[/code].
+			</description>
+		</method>
+		<method name="stage_file">
+			<return type="void">
+			</return>
+			<argument index="0" name="file_path" type="String">
+			</argument>
+			<description>
+				Stage the file which should be committed when [method EditorVCSInterface.commit] is called. Argument should contain the absolute path.
+			</description>
+		</method>
+		<method name="unstage_file">
+			<return type="void">
+			</return>
+			<argument index="0" name="file_path" type="String">
+			</argument>
+			<description>
+				Unstage the file which was staged previously to be committed, so that it is no longer committed when [method EditorVCSInterface.commit] is called. Argument should contain the absolute path.
+			</description>
+		</method>
+	</methods>
+	<constants>
+	</constants>
+</class>
diff --git a/editor/editor_vcs_interface.cpp b/editor/editor_vcs_interface.cpp
new file mode 100644
index 0000000000..db1c289ec9
--- /dev/null
+++ b/editor/editor_vcs_interface.cpp
@@ -0,0 +1,173 @@
+#include "editor_vcs_interface.h"
+
+EditorVCSInterface *EditorVCSInterface::singleton = NULL;
+
+void EditorVCSInterface::_bind_methods() {
+
+	// Proxy end points that act as fallbacks to unavailability of a function in the VCS addon
+	ClassDB::bind_method(D_METHOD("_initialize", "project_root_path"), &EditorVCSInterface::_initialize);
+	ClassDB::bind_method(D_METHOD("_get_is_vcs_intialized"), &EditorVCSInterface::_get_is_vcs_intialized);
+	ClassDB::bind_method(D_METHOD("_get_vcs_name"), &EditorVCSInterface::_get_vcs_name);
+	ClassDB::bind_method(D_METHOD("_shut_down"), &EditorVCSInterface::_shut_down);
+	ClassDB::bind_method(D_METHOD("_get_project_name"), &EditorVCSInterface::_get_project_name);
+	ClassDB::bind_method(D_METHOD("_get_modified_files_data"), &EditorVCSInterface::_get_modified_files_data);
+	ClassDB::bind_method(D_METHOD("_commit", "msg"), &EditorVCSInterface::_commit);
+	ClassDB::bind_method(D_METHOD("_get_file_diff", "file_path"), &EditorVCSInterface::_get_file_diff);
+	ClassDB::bind_method(D_METHOD("_stage_file", "file_path"), &EditorVCSInterface::_stage_file);
+	ClassDB::bind_method(D_METHOD("_unstage_file", "file_path"), &EditorVCSInterface::_unstage_file);
+
+	ClassDB::bind_method(D_METHOD("is_addon_ready"), &EditorVCSInterface::is_addon_ready);
+
+	// API methods that redirect calls to the proxy end points
+	ClassDB::bind_method(D_METHOD("initialize", "project_root_path"), &EditorVCSInterface::initialize);
+	ClassDB::bind_method(D_METHOD("get_is_vcs_intialized"), &EditorVCSInterface::get_is_vcs_intialized);
+	ClassDB::bind_method(D_METHOD("get_modified_files_data"), &EditorVCSInterface::get_modified_files_data);
+	ClassDB::bind_method(D_METHOD("stage_file", "file_path"), &EditorVCSInterface::stage_file);
+	ClassDB::bind_method(D_METHOD("unstage_file", "file_path"), &EditorVCSInterface::unstage_file);
+	ClassDB::bind_method(D_METHOD("commit", "msg"), &EditorVCSInterface::commit);
+	ClassDB::bind_method(D_METHOD("get_file_diff", "file_path"), &EditorVCSInterface::get_file_diff);
+	ClassDB::bind_method(D_METHOD("shut_down"), &EditorVCSInterface::shut_down);
+	ClassDB::bind_method(D_METHOD("get_project_name"), &EditorVCSInterface::get_project_name);
+	ClassDB::bind_method(D_METHOD("get_vcs_name"), &EditorVCSInterface::get_vcs_name);
+}
+
+bool EditorVCSInterface::_initialize(String p_project_root_path) {
+
+	WARN_PRINT("Selected VCS addon does not implement an initialization function. This warning will be suppressed.")
+	return true;
+}
+
+bool EditorVCSInterface::_get_is_vcs_intialized() {
+
+	return false;
+}
+
+Dictionary EditorVCSInterface::_get_modified_files_data() {
+
+	return Dictionary();
+}
+
+void EditorVCSInterface::_stage_file(String p_file_path) {
+
+	return;
+}
+
+void EditorVCSInterface::_unstage_file(String p_file_path) {
+
+	return;
+}
+
+void EditorVCSInterface::_commit(String p_msg) {
+
+	return;
+}
+
+Array EditorVCSInterface::_get_file_diff(String p_file_path) {
+
+	return Array();
+}
+
+bool EditorVCSInterface::_shut_down() {
+
+	return false;
+}
+
+String EditorVCSInterface::_get_project_name() {
+
+	return String();
+}
+
+String EditorVCSInterface::_get_vcs_name() {
+
+	return "";
+}
+
+bool EditorVCSInterface::initialize(String p_project_root_path) {
+
+	is_initialized = call("_initialize", p_project_root_path);
+	return is_initialized;
+}
+
+bool EditorVCSInterface::get_is_vcs_intialized() {
+
+	return call("_get_is_vcs_intialized");
+}
+
+Dictionary EditorVCSInterface::get_modified_files_data() {
+
+	return call("_get_modified_files_data");
+}
+
+void EditorVCSInterface::stage_file(String p_file_path) {
+
+	if (is_addon_ready()) {
+
+		call("_stage_file", p_file_path);
+	}
+	return;
+}
+
+void EditorVCSInterface::unstage_file(String p_file_path) {
+
+	if (is_addon_ready()) {
+
+		call("_unstage_file", p_file_path);
+	}
+	return;
+}
+
+bool EditorVCSInterface::is_addon_ready() {
+
+	return is_initialized;
+}
+
+void EditorVCSInterface::commit(String p_msg) {
+
+	if (is_addon_ready()) {
+
+		call("_commit", p_msg);
+	}
+	return;
+}
+
+Array EditorVCSInterface::get_file_diff(String p_file_path) {
+
+	if (is_addon_ready()) {
+
+		return call("_get_file_diff", p_file_path);
+	}
+	return Array();
+}
+
+bool EditorVCSInterface::shut_down() {
+
+	return call("_shut_down");
+}
+
+String EditorVCSInterface::get_project_name() {
+
+	return call("_get_project_name");
+}
+
+String EditorVCSInterface::get_vcs_name() {
+
+	return call("_get_vcs_name");
+}
+
+EditorVCSInterface::EditorVCSInterface() {
+
+	is_initialized = false;
+}
+
+EditorVCSInterface::~EditorVCSInterface() {
+}
+
+EditorVCSInterface *EditorVCSInterface::get_singleton() {
+
+	return singleton;
+}
+
+void EditorVCSInterface::set_singleton(EditorVCSInterface *p_singleton) {
+
+	singleton = p_singleton;
+}
diff --git a/editor/editor_vcs_interface.h b/editor/editor_vcs_interface.h
new file mode 100644
index 0000000000..baf196de6e
--- /dev/null
+++ b/editor/editor_vcs_interface.h
@@ -0,0 +1,53 @@
+#ifndef EDITOR_VCS_INTERFACE_H
+#define EDITOR_VCS_INTERFACE_H
+
+#include "core/object.h"
+#include "core/ustring.h"
+#include "scene/gui/panel_container.h"
+
+class EditorVCSInterface : public Object {
+
+	GDCLASS(EditorVCSInterface, Object)
+
+	bool is_initialized;
+
+protected:
+	static EditorVCSInterface *singleton;
+
+	static void _bind_methods();
+
+	// Implemented by addons as end points for the proxy functions
+	bool _initialize(String p_project_root_path);
+	bool _get_is_vcs_intialized();
+	Dictionary _get_modified_files_data();
+	void _stage_file(String p_file_path);
+	void _unstage_file(String p_file_path);
+	void _commit(String p_msg);
+	Array _get_file_diff(String p_file_path);
+	bool _shut_down();
+	String _get_project_name();
+	String _get_vcs_name();
+
+public:
+	static EditorVCSInterface *get_singleton();
+	static void set_singleton(EditorVCSInterface *p_singleton);
+
+	bool is_addon_ready();
+
+	// Proxy functions to the editor for use
+	bool initialize(String p_project_root_path);
+	bool get_is_vcs_intialized();
+	Dictionary get_modified_files_data();
+	void stage_file(String p_file_path);
+	void unstage_file(String p_file_path);
+	void commit(String p_msg);
+	Array get_file_diff(String p_file_path);
+	bool shut_down();
+	String get_project_name();
+	String get_vcs_name();
+
+	EditorVCSInterface();
+	virtual ~EditorVCSInterface();
+};
+
+#endif // !EDITOR_VCS_INTERFACE_H