It's working more or less...

src/controller.cpp

@@ -1,13 +1,100 @@
 #include "controller.h"
+#include "esmp3.h"
 
 void Controller::handle() {
-
+	if (last_rfid_check + 500 < millis() || last_rfid_check > millis()) {
+		handle_rfid();
+		last_rfid_check = millis();
+	}
 }
 
 void Controller::handle_buttons() {
-
+	if (is_button_pressed(PIN_BTN_VOL_UP)) {
+		audio.setVolume(min(audio.getVolume()+1, 21));
+	} else if (is_button_pressed(PIN_BTN_VOL_DOWN)) {
+		audio.setVolume(max(audio.getVolume()-1, 1));
+	} else if (is_button_pressed(PIN_BTN_TRACK_NEXT)) {
+		next_track();
+	} else if (is_button_pressed(PIN_BTN_TRACK_PREV)) {
+		prev_track();
+	}
 }
 
 void Controller::handle_rfid() {
-	
+	if (is_rfid_present) {
+		byte buffer[2];
+		byte buffer_size = 2;
+		MFRC522Constants::StatusCode status = rfid->PICC_WakeupA(buffer, &buffer_size);
+		if (status == MFRC522Constants::STATUS_OK) {
+			// Card is still present.
+			rfid->PICC_HaltA();
+		} else {
+			Serial.printf("RFID status is %s\n", MFRC522Debug::GetStatusCodeName(status));
+			is_rfid_present = false;
+			Serial.println("No more RFID card.\n");
+			stop();
+		}
+	} else {
+		if (rfid->PICC_IsNewCardPresent()) {
+			if (rfid->PICC_ReadCardSerial()) {
+				uint32_t uid = rfid->uid.uidByte[0]<<24 | rfid->uid.uidByte[1]<<16 | rfid->uid.uidByte[2]<<8 | rfid->uid.uidByte[3];
+				Serial.printf("Found new rfid card with uid %x\n", uid);
+				is_rfid_present = true;
+				if (uid > 0) {
+					String temp = String(uid, HEX);
+					String s_uid = "";
+					for (int i=0; i<(8-temp.length()); i++) {
+						s_uid.concat("0");
+					}
+					s_uid.concat(temp);
+					play(s_uid);
+				}
+				rfid->PICC_HaltA();
+			}
+		}
+	}
+}
+
+void Controller::play(String rfid_id) {
+	if (!rfid_id.equals(current_playlist.get_rfid_id())) {
+		if (pm->has_playlist(rfid_id)) {
+			current_playlist = pm->get_playlist(rfid_id);
+			play();
+		} else {
+			Serial.printf("There is no playlist for rfid_id %s\n", rfid_id.c_str());
+		}
+	} else {
+		if (!audio.isRunning()) {
+			play();
+		}
+	}
+}
+
+void Controller::play() {
+	String file = current_playlist.get_current_file_name();
+	Serial.printf("Playing file: %s\n", file.c_str());
+	audio.connecttoFS(SD, file.c_str(), current_playlist.get_current_time());
+}
+
+void Controller::next_track() {
+	if (current_playlist.next_track()) {
+		play();
+	}
+}
+
+void Controller::prev_track() {
+	if (audio.getAudioCurrentTime() <= 5) {
+		current_playlist.restart();
+		play();
+	} else {
+		if (current_playlist.prev_track()) {
+			play();
+		}
+	}
+}
+
+void Controller::stop() {
+	if (audio.isRunning()) {
+		current_playlist.set_current_time(audio.stopSong());
+	}
 }