Use procedural macros to define events

The construction of an EventHandler object should not be directly done
by the client. Instead, it is easier to use procedural macro to
automatize the process, hence exposing a nice and simple interface. Such
use of procedural macros requires to crate an additional crate.

opensmtpd/src/event_handlers.rs

@@ -0,0 +1,51 @@
+use crate::entry::{Entry, Event};
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum MatchEvent {
+    Evt(Vec<Event>),
+    All,
+}
+
+#[derive(Clone)]
+pub struct EventHandler {
+    event: MatchEvent,
+    callback: (fn(&Entry) -> bool),
+}
+
+impl EventHandler {
+    fn get_events_from_string(event_str: &String) -> MatchEvent {
+        let mut events = Vec::new();
+        for name in event_str.split(" , ") {
+            match name {
+                "Any" | "All" => {
+                    return MatchEvent::All;
+                }
+                _ => match Event::from_str(name) {
+                    Ok(e) => {
+                        events.push(e);
+                    }
+                    Err(_) => {}
+                },
+            }
+        }
+        MatchEvent::Evt(events)
+    }
+
+    pub fn new(event_str: String, callback: (fn(&Entry) -> bool)) -> Self {
+        EventHandler {
+            event: EventHandler::get_events_from_string(&event_str),
+            callback,
+        }
+    }
+
+    pub fn is_callable(&self, event: Event) -> bool {
+        match &self.event {
+            MatchEvent::All => true,
+            MatchEvent::Evt(v) => v.contains(&event),
+        }
+    }
+
+    pub fn call(&self, entry: &Entry) -> bool {
+        (self.callback)(entry)
+    }
+}