New DGUS UI var / definition syntax (#18718)

Marlin/src/lcd/extui/lib/dgus/DGUSDisplay.cpp

   WriteVariable(adr, static_cast<const void*>(&value), sizeof(uint8_t));
 }
 
+void DGUSDisplay::WriteVariable(uint16_t adr, int8_t value) {
+  WriteVariable(adr, static_cast<const void*>(&value), sizeof(int8_t));
+}
+
 void DGUSDisplay::WriteVariable(uint16_t adr, long value) {
     union { long l; char lb[4]; } endian;
     char tmp[4];

Marlin/src/lcd/extui/lib/dgus/DGUSDisplay.h

   static void WriteVariable(uint16_t adr, int16_t value);
   static void WriteVariable(uint16_t adr, uint16_t value);
   static void WriteVariable(uint16_t adr, uint8_t value);
+  static void WriteVariable(uint16_t adr, int8_t value);
   static void WriteVariable(uint16_t adr, long value);
 
+  // Utility functions for bridging ui_api and dbus
+  template<typename T, float(*Getter)(const T), T selector, typename WireType=uint16_t>
+  static void SetVariable(DGUS_VP_Variable &var) {
+    WriteVariable(var.VP, (WireType)Getter(selector));
+  }
+
+  template<typename T, void(*Setter)(const float V, const T), T selector>
+  static void GetVariable(DGUS_VP_Variable &var, void *val_ptr) {
+    uint16_t newvalue = swap16(*(uint16_t*)val_ptr);
+    Setter(newvalue, selector);
+  }
+
   // Until now I did not need to actively read from the display. That's why there is no ReadVariable
   // (I extensively use the auto upload of the display)
 
   static void WritePGM(const char str[], uint8_t len);
   static void ProcessRx();
 
+  static inline uint16_t swap16(const uint16_t value) { return (value & 0xffU) << 8U | (value >> 8U); }
   static rx_datagram_state_t rx_datagram_state;
   static uint8_t rx_datagram_len;
   static bool Initialized, no_reentrance;
 };
 
+#define GET_VARIABLE(f, t, V...) (&DGUSDisplay::GetVariable<decltype(t), f, t, ##V>)
+#define SET_VARIABLE(f, t, V...) (&DGUSDisplay::SetVariable<decltype(t), f, t, ##V>)
+
 extern DGUSDisplay dgusdisplay;
 
 // compile-time x^y

Marlin/src/lcd/extui/lib/dgus/origin/DGUSDisplayDef.cpp

 #include "../../../../../module/planner.h"
 
 #include "../../../../ultralcd.h"
+#include "../../../ui_api.h"
 
 #if ENABLED(DGUS_UI_MOVE_DIS_OPTION)
   uint16_t distanceToMove = 10;
 #endif
+using namespace ExtUI;
 
 const uint16_t VPList_Boot[] PROGMEM = {
   VP_MARLIN_VERSION,
 
   // Temperature Data
   #if HOTENDS >= 1
-    VPHELPER(VP_T_E0_Is, &thermalManager.temp_hotend[0].celsius, nullptr, ScreenHandler.DGUSLCD_SendFloatAsLongValueToDisplay<0>),
-    VPHELPER(VP_T_E0_Set, &thermalManager.temp_hotend[0].target, ScreenHandler.HandleTemperatureChanged, &ScreenHandler.DGUSLCD_SendWordValueToDisplay),
+    VPHELPER(VP_T_E0_Is, nullptr, nullptr, SET_VARIABLE(getActualTemp_celsius, E0, long)),
+    VPHELPER(VP_T_E0_Set, nullptr, GET_VARIABLE(setTargetTemp_celsius, E0),
+                                   SET_VARIABLE(getTargetTemp_celsius, E0)),
     VPHELPER(VP_Flowrate_E0, nullptr, ScreenHandler.HandleFlowRateChanged, &ScreenHandler.DGUSLCD_SendWordValueToDisplay),
     VPHELPER(VP_EPos, &destination.e, nullptr, ScreenHandler.DGUSLCD_SendFloatAsLongValueToDisplay<2>),
     VPHELPER(VP_MOVE_E0, nullptr, &ScreenHandler.HandleManualExtrude, nullptr),