[sim] Expose GUI control functions via HAL_RegisterExtension (#4034)

simulation/halsim_gui/src/main/native/include/HALSimGuiExt.h

@@ -0,0 +1,26 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <functional>
+
+namespace halsimgui {
+
+// These functions can be used to hook into the GUI, and can be accessed
+// via HAL_RegisterExtensionListener
+
+#define HALSIMGUI_EXT_ADDGUIINIT "halsimgui::AddGuiInit"
+using AddGuiInitFn = void (*)(std::function<void()> initialize);
+
+#define HALSIMGUI_EXT_ADDGUILATEEXECUTE "halsimgui::AddGuiLateExecute"
+using AddGuiLateExecuteFn = void (*)(std::function<void()> execute);
+
+#define HALSIMGUI_EXT_ADDGUIEARLYEXECUTE "halsimgui::AddGuiEarlyExecute"
+using AddGuiEarlyExecuteFn = void (*)(std::function<void()> execute);
+
+#define HALSIMGUI_EXT_GUIEXIT "halsimgui::GuiExit"
+using GuiExitFn = void (*)();
+
+}  // namespace halsimgui