Keep Unit of Measurement in Flash. (#4719)

Co-authored-by: Your Name <you@example.com>

esphome/core/entity_base.cpp

@@ -84,4 +84,13 @@ std::string EntityBase_DeviceClass::get_device_class() {
 
 void EntityBase_DeviceClass::set_device_class(const char *device_class) { this->device_class_ = device_class; }
 
+std::string EntityBase_UnitOfMeasurement::get_unit_of_measurement() {
+  if (this->unit_of_measurement_ == nullptr)
+    return "";
+  return this->unit_of_measurement_;
+}
+void EntityBase_UnitOfMeasurement::set_unit_of_measurement(const char *unit_of_measurement) {
+  this->unit_of_measurement_ = unit_of_measurement;
+}
+
 }  // namespace esphome

esphome/core/entity_base.h

@@ -74,4 +74,15 @@ class EntityBase_DeviceClass {
   const char *device_class_{nullptr};  ///< Device class override
 };
 
+class EntityBase_UnitOfMeasurement {
+ public:
+  /// Get the unit of measurement, using the manual override if set.
+  std::string get_unit_of_measurement();
+  /// Manually set the unit of measurement.
+  void set_unit_of_measurement(const char *unit_of_measurement);
+
+ protected:
+  const char *unit_of_measurement_{nullptr};  ///< Unit of measurement override
+};
+
 }  // namespace esphome