MCMessagingUtils is an Apex class used to trigger journey builder actions and push messages from Salesforce service and sales cloud.

Invocable_TriggerMCEvent.cls

public with sharing class Invocable_TriggerMCEvent {

  @InvocableMethod(label='Send Message with new Comment to Reporter' description='Send Message with new Comment to Reporter')
  public static void TriggerMCEvent(List<String> contactKey) {
    MCMessagingUtils.triggerMCInteractionEvent(contactKey[0]);
  }

}

MCMessagingUtils.cls

public with sharing class MCMessagingUtils {
  //inner classes
  public class MCMessagingUtilsException extends Exception{}
  public class accessTokenResult{
    public String accessToken;
    public integer expiresIn;
  }

  public class TriggerInteractionEventObj {
    String ContactKey {get;set;}
    String EventDefinitionKey {get;set;}
    public TriggerInteractionEventObj(String incomingContactKey){
      this.ContactKey = incomingContactKey;
      this.EventDefinitionKey = credentials.eventDefinitionKey__c;
    }
  }

  // Appropriate HTTP methods for various calls
  private static final String kAuthMethod = 'POST';
  private static final String kFireInteractionMethod = 'POST';
  private static final String kFirePushMessageMethod = 'POST';

  // Custom Setting Name to use:
  private static final String kCredsName = 'df16';

  // Push Message to reuse:
  private static final String kMessageId = 'MjoxMTQ6MA';

  // URI's
  private static final String kAPIAuthBaseURI = 'https://auth.exacttargetapis.com';
  private static final String kAPIBaseURI = 'https://www.exacttargetapis.com';
  private static final String kAuthURI = kAPIAuthBaseURI + '/v1/requestToken';
  private static final String kFireInteractionEventURI = kAPIBaseURI + '/interaction/v1/events';
  private static final String kFirePushMessageURI = '/messageContact/'+ kMessageId +'/send';
  private static final String kActivityTriggerMCInteractionEvent = 'kActivityTriggerMCInteractionEvent';
  private static final String kActivityTriggerMCPushEvent = 'kActivityTriggerMCPushEvent';

  private static McCreds__c credentials = McCreds__c.getInstance(kCredsName);

  private static String preservedActivity;
  private static String preservedContactKey;
  private static String preservedCommentBody;

  public static void triggerMCInteractionEvent(String contactKey){
    MCMessagingUtils.accessToken();
    TriggerInteractionEventObj payloadObj= new TriggerInteractionEventObj(contactKey);
    String payloadString = JSON.serialize(payloadObj);
    system.debug('*** - method: ' + kFireInteractionMethod);
    system.debug('*** - payload: ' + payloadString);
    system.debug('*** - uri: ' + kFireInteractionEventURI);
    system.debug('*** - token: ' + MCMessagingUtils.accessToken() );
    sendHTTPRequest(kFireInteractionMethod,
      payloadString,
      kFireInteractionEventURI);
  }

  public static void sendMCPushMessage(String commentBody, String contactKey){
    MCMessagingUtils.accessToken();
    sendHTTPRequest(kFirePushMessageMethod,
      generateMessageSendBody(commentBody, contactKey),
      kFirePushMessageURI);
  }

  //Note, I have to use a string building method, rather than serializing an object
  // because override is a reserved word in apex.
  private Static String generateMessageSendBody(String CommentBody, String SubscriberKey){
    return '{' +
      '"MessageText": "'+CommentBody+'",' +
      '"Override": true,'+
      '"sound": "default",'+
      '"SendTime": "2012-10-31 09:00",'+
      '"content-available": 1'+
      '"subscriberKeys": ["' + SubscriberKey + '"]' +
      '}';
  }

  private static string accessToken(){
    if(credentials.accessToken__c == null || DateTime.now() > credentials.expiresIn__c){
      return makeAuthenticationCall();
    } else {
      return credentials.accessToken__c;
    }
  }

  private static string makeAuthenticationCall(){
    if(credentials != null && String.isNotEmpty(credentials.clientId__c) && String.isNotEmpty(credentials.clientSecret__c)){
      String authJson = '{"clientId":"' + credentials.clientId__c + '","clientSecret":"' + credentials.clientSecret__c + '"}';
      HttpResponse response = sendAuthRequest(authJson);
      accessTokenResult atr = (accessTokenResult) System.JSON.deserialize(response.getBody(), accessTokenResult.class);
      credentials.accessToken__c = atr.accessToken;
      credentials.expiresIn__c = Datetime.now().addSeconds(atr.expiresIn);
      update credentials;
    }
    return credentials.accessToken__c;
  }

  private static HttpResponse sendAuthRequest(String body){
    Http h = new Http();
    HttpRequest req = generateRequest(kAuthMethod, body, kAuthURI);
    HttpResponse res = h.send(req);
    return res;
  }

  private static HttpRequest generateRequest(String method, String body, String url){
    HttpRequest req = new HttpRequest();
    req.setHeader('Content-Type', 'application/json');
    req.setEndpoint(url);
    req.setBody(body);
    req.setMethod(method);
    system.debug('%%% - ' + body);
    return req;
  }

  @future(Callout=true)
  Public Static void sendHTTPRequest(String Method, String Body, String URL) {
    Http h = new Http();
    HttpRequest req = generateRequest(method, body, url);
    req.setHeader('Authorization', 'Bearer ' + MCMessagingUtils.accessToken());
    HttpResponse res = h.send(req);
    system.debug('#### - ' + res.getBody());
  }
}