Support DGUS Display with DWIN OS (#13253)

Marlin/src/lcd/extensible_ui/lib/dgus/DGUSVPVariable.h

@@ -0,0 +1,47 @@
+/**
+ * Marlin 3D Printer Firmware
+ * Copyright (C) 2019 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ *
+ * Based on Sprinter and grbl.
+ * Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+#pragma once
+
+/**
+ * DGUSVPVariable.h
+ *
+ *  Created on: Feb 9, 2019
+ *      Author: tobi
+ */
+
+struct DGUS_VP_Variable {
+  uint16_t VP;
+  void*    memadr;  // If nullptr, the value cannot be uploaded to the display.
+  uint8_t  size;
+
+  // Callback that will be called if the display modified the value.
+  // nullptr makes it readonly for the display.
+  void (*set_by_display_handler)(DGUS_VP_Variable &var, void *val_ptr);
+  void (*send_to_display_handler)(DGUS_VP_Variable &var);
+
+  template<typename T>
+  DGUS_VP_Variable& operator =(T &o) {
+    *(T*)memadr = o;  // warning this is not typesafe.
+    // TODO: Call out the display or mark as dirty for the next update.
+    return *this;
+  }
+};