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_