Add support for expiring values

src/sensesp/system/expiring_value.h

@@ -0,0 +1,56 @@
+#ifndef SENSESP_SRC_SENSESP_SYSTEM_EXPIRING_VALUE_H_
+#define SENSESP_SRC_SENSESP_SYSTEM_EXPIRING_VALUE_H_
+
+namespace sensesp {
+
+/**
+ * @brief Value container that keeps track of its expiration time.
+ *
+ * The value is considered expired if the time since the last update is greater
+ * than the expiration duration. When expired, the value is replaced with an
+ * expiration placeholder value.
+ *
+ * @tparam T
+ */
+template <typename T>
+class ExpiringValue {
+ public:
+  ExpiringValue()
+      : value_{},
+        expiration_duration_{1000},
+        last_update_{0},
+        expired_value_{T{}} {}
+
+  ExpiringValue(T value, unsigned long expiration_duration, T expired_value)
+      : value_{value},
+        expiration_duration_{expiration_duration},
+        expired_value_{expired_value},
+        last_update_{millis()} {}
+
+  void update(T value) {
+    value_ = value;
+    last_update_ = millis();
+  }
+
+  T get() const {
+    if (!is_expired()) {
+      return value_;
+    } else {
+      return expired_value_;
+    }
+  }
+
+  bool is_expired() const {
+    return millis() - last_update_ > expiration_duration_;
+  }
+
+ private:
+  T value_;
+  T expired_value_;
+  unsigned long expiration_duration_;
+  unsigned long last_update_;
+};
+
+}  // namespace sensesp
+
+#endif  // SENSESP_SRC_SENSESP_SYSTEM_EXPIRING_VALUE_H_