Hey I have a problem getting my head around how custom GWT event Handlers work. I have read quite a bit about the topic and it still is some what foggy. I have read threads here on Stackoverflow like this one GWT Custom Event Handler. Could someone explain it in an applied mannar such as the following.
I have 2 classes a block and a man class. When the man collides with the block the man fires an event ( onCollision() ) and then the block class listens for that event.
Thanks
Events in general:
Events are always sent to inform about something (e.g. a change of state). Let's take your example with a man and a wall. Here we can imagine that there is a game where a user can walk as a man in a labyrinth. Every time a user hits the wall it should be informed about the collision so that it can react to it (e.g. a wall can render itself as a destroyed wall). This can be achieved by sending a collision event every time the collision with a wall is detected. This event is sent by a man and every object in the system interested in the event receives it and can react to it accordingly. Objects which want to receive events must register themselves as interested with event.
This is how events work in general in every system or framework (not only in GWT). In order to send and receive events in such systems you have to define:
What is sent (what do events look like)
Who receives events (event receivers)
Who sends events (event senders)
Then you can:
Register event receivers which want to receive events
Send events
Events in GWT:
Here I will show an example of using custom events in GWT. I will use an example of a system which is responsible for checking a mailbox and inform a user if there are new mails. Let's assume that in the system there are at least 2 components:
message checker responsible for checking the mailbox and
message displayer responsible for displaying new mails
Message checker sends events when a new mail is received and message displayer receives these events.
Step 1: Define events
Information about a new mail will be sent as an instance of MessageReceivedEvent class. The class contains a new mail (for the simplicity let's assume it is just a String).
Full source code of this class is presented below (the comment for it is below the source code).
public class MessageReceivedEvent extends GwtEvent<MessageReceivedEventHandler> {
public static Type<MessageReceivedEventHandler> TYPE = new Type<MessageReceivedEventHandler>();
private final String message;
public MessageReceivedEvent(String message) {
this.message = message;
}
#Override
public Type<MessageReceivedEventHandler> getAssociatedType() {
return TYPE;
}
#Override
protected void dispatch(MessageReceivedEventHandler handler) {
handler.onMessageReceived(this);
}
public String getMessage() {
return message;
}
}
MessageReceivedEventHandler is an interface that represents event receivers. Don't bother with it at the moment, this will be discussed later.
Every class representing a GWT event has to extend GwtEvent class. This class contains two abstract methods which must be implemented: getAssociatedType and dispatch. However in every event class they are usually implemented in a very similar way.
The class stores information about a received message (see constructor). Every event receiver can get it using getMessage method.
Step 2: Define event receivers
Each event type in GWT is associated to an interface representing receivers of this event type. In GWT receivers are called handlers. In the example an event receiver interface for MessageReceivedEvent will be named MessageReceivedEventHandler. The source code is below:
public interface MessageReceivedEventHandler extends EventHandler {
void onMessageReceived(MessageReceivedEvent event);
}
Each handler has to extend EventHandler interface. It should also define a method which will be invoked when an event occurs (it should take at least one parameter - an event). Here the method is named onMessageReceived. Each receiver can react on an event by implementing this method.
The only event receiver in the example is MessageDisplayer component:
public class MessageDisplayer implements MessageReceivedEventHandler {
#Override
public void onMessageReceived(MessageReceivedEvent event) {
String newMessage = event.getMessage();
// display a new message
// ...
}
}
Step 3: Define event senders
In the example the only event sender is a component responsible for checking mails - EventChecker:
public class MessageChecker implements HasHandlers {
private HandlerManager handlerManager;
public MessageChecker() {
handlerManager = new HandlerManager(this);
}
#Override
public void fireEvent(GwtEvent<?> event) {
handlerManager.fireEvent(event);
}
public HandlerRegistration addMessageReceivedEventHandler(
MessageReceivedEventHandler handler) {
return handlerManager.addHandler(MessageReceivedEvent.TYPE, handler);
}
}
Every event sender has to implement HasHandlers interface.
The most important element here is a HandlerManager field. In GWT HandlerManager as the name suggest manages event handlers (event receivers). As it was said at the beginning every event receiver that wants to receive events must register itself as interested. This is what handler managers are for. They make it possible to register event handlers an they can send a particular event to every registered event handler.
When a HanlderManager is created it takes one argument in its constructor. Every event has a source of origin and this parameter will be used as a source for all events send by this handler manager. In the example it is this as the source of events is MessageChecker.
The method fireEvent is defined in HasHandlers interface and is responsible for sending events. As you can see it just uses a handler manager to send (fire) and event.
addMessageReceivedEventHandler is used by event receivers to register themselves as interested in receiving events. Again handler manager is used for this.
Step 4: Bind event receivers with event senders
When everything is defined event receivers must register themselves in event senders. This is usually done during creation of objects:
MessageChecker checker = new MessageChecker();
MessageDisplayer displayer = new MessageDisplayer();
checker.addMessageReceivedEventHandler(displayer);
Now all events sent by checker will be received by displayer.
Step 5: Send events
To send an event, MessageChecker must create an event instance and send it using fireEvent method. This cane be done in newMailReceived method:
public class MessageChecker implements HasHandlers {
// ... not important stuff omitted
public void newMailReceived() {
String mail = ""; // get a new mail from mailbox
MessageReceivedEvent event = new MessageReceivedEvent(mail);
fireEvent(event);
}
}
I hope it is clear and will help :)
Since this question and the answer from Piotr GWT has added support for a slightly different way to create custom events. This event implementation is specific build to be used with the GWT's EventBus in the package com.google.web.bindery.event.shared. An example on how to build a custom event for GWT 2.4:
import com.google.web.bindery.event.shared.Event;
import com.google.web.bindery.event.shared.EventBus;
import com.google.web.bindery.event.shared.HandlerRegistration;
/**
* Here is a custom event. For comparison this is also a MessageReceivedEvent.
* This event extends the Event from the web.bindery package.
*/
public class MessageReceivedEvent extends Event<MessageReceivedEvent.Handler> {
/**
* Implemented by methods that handle MessageReceivedEvent events.
*/
public interface Handler {
/**
* Called when an {#link MessageReceivedEvent} event is fired.
* The name of this method is whatever you want it.
*
* #param event an {#link MessageReceivedEvent} instance
*/
void onMessageReceived(MessageReceivedEvent event);
}
private static final Type<MessageReceivedEvent.Handler> TYPE =
new Type<MessageReceivedEvent.Handler>();
/**
* Register a handler for MessageReceivedEvent events on the eventbus.
*
* #param eventBus the {#link EventBus}
* #param handler an {#link MessageReceivedEvent.Handler} instance
* #return an {#link HandlerRegistration} instance
*/
public static HandlerRegistration register(EventBus eventBus,
MessageReceivedEvent.Handler handler) {
return eventBus.addHandler(TYPE, handler);
}
private final String message;
public MessageReceivedEvent(String message) {
this.message = message;
}
#Override
public Type<MessageReceivedEvent.Handler> getAssociatedType() {
return TYPE;
}
public String getMessage() {
return message;
}
#Override
protected void dispatch(Handler handler) {
handler.onMessageReceived(this);
}
}
The event is used as follows:
To register your handler for this event with the eventbus call the static register method on the MessageReceivedEvent class:
MessageReceivedEvent.register(eventbus, new MessageReceivedEvent.Handler() {
public void onMessageReceived(MessageReceivedEvent event) {
//...do something usefull with the message: event.getMessage();
}
});
Now to fire the event on the eventbus call fireEvent with a newly constructed event:
eventBus.fireEvent(new MessageReceivedEvent("my message"));
Another implementation can be found in GWT's own EntityProxyChange event class. That implementation uses a alternative option of the EventBus. It uses the ability to add handlers that are bound to a specific source, via addHandlerToSource and can be triggered via eventBus.fireEventFromSource.
The event implementation given here is also more suitable when working with GWT's Activities.
I created my own widget by extending GWT's Composite class. I wanted to create my own custom event in this class. I wanted the events to be accessible to GWT's WindowBuilder Editor.
I learned a lot of from the answers on this page, but I had to make some changes.
I wanted to start from the Hilbrand Bouwkamp answer, because it was newer. But I ran into a couple of problems. 1) That answer made reference to the event bus. The even bus is a global variable owned by the main program. It's not clear how a widget library could get access to that. 2) I wasn't starting from scratch. I was was extending GWT library code. In order to make that work, I had to start from the GwtEvent class, rather than the Event class.
Piotr's answer is essentially correct, but it was very long. My class (indirectly) extends GWT's Widget class. Widget takes care of many details, such as creating a HandlerManager object. (I looked through the source code, and that's exactly how standard Widgets work, not by using an EventBus.)
I only had to add two things to my widget class to add a custom event handler. Those are shown here:
public class TrackBar extends Composite {
public HandlerRegistration addValueChangedHandler(TrackBarEvent.Handler handler)
{
return addHandler(handler, TrackBarEvent.TYPE);
}
private void fireValueChangedEvent(int value)
{
final TrackBarEvent e = new TrackBarEvent(value);
fireEvent(e);
}
My new event is almost exactly the same as Piotr's event class, shown above. One thing is worth noting. I started with getValue(), based on that example. Later I added getTrackBar() to give a lot more information. If I was starting from scratch I'd focus on the latter, not the former. The complete event class is shown below.
import com.google.gwt.event.shared.EventHandler;
import com.google.gwt.event.shared.GwtEvent;
public class TrackBarEvent extends GwtEvent< TrackBarEvent.Handler >
{
public interface Handler extends EventHandler {
void onTrackBarValueChanged(TrackBarEvent event);
}
static final Type<TrackBarEvent.Handler> TYPE =
new Type<TrackBarEvent.Handler>();
private final int value;
public TrackBarEvent(int value) {
this.value = value;
}
#Override
public Type<TrackBarEvent.Handler> getAssociatedType() {
return TYPE;
}
public int getValue() {
return value;
}
public TrackBar getTrackBar()
{
return (TrackBar)getSource();
}
#Override
protected void dispatch(Handler handler) {
handler.onTrackBarValueChanged(this);
}
}
If you happen to be using the GWTP framework on top of GWT, refer to this Stack.
GWTP is "A complete model-view-presenter framework to simplify your next GWT project."
Related
I am very new to Axon so bear with me. I have a command that starts the sessionCreated event in my aggregate. After that happens some other external code is executed multiple times. For each iteration I would like an event to be published without having to send a new command to the aggregate. I tried using EventGateway.publish and #EventHandler as shown below.
Code:
#Aggregate
public class SessionAggregate {
// aggregate logic
#EventHandler
public void on(OtherEvent event){
// code that never runs
}
}
public class ExternalLogic {
private final EventGateway eventGateway;
public void execute() {
// other code
eventGateway.publish(new OtherEvent());
}
}
This won't work. The Aggregate only reacts to CommandHandler calls and (in case you are using eventSourcing as storage) can replay its internal state via EventSourcingHandlers.
In addition, an aggregate is only active while handling a command, it is not permanently present in memory, so even if it would be notified, you wouldn't have any guarantee that it was called.
In my application I have multiple custom Spring Application Events. I want to log every instance of these events occurring. Is there a way to do this automatically or do I have to manually log it every time?
You might create an event listener for the type of events you're interested in and log the event. Spring will register this listener and you'll basically done:
#Component
public class CustomEventListener implements ApplicationListener<CustomEvent> {
private static final Logger logger = ...;
#Override
public void onApplicationEvent(CustomEvent event) {
LOGGER.info("Got event: {}" , event);
}
}
However, you can't really use the same listener subscribed to multiple types of event unless they have an hierarchy (inheritance from some base event) You might be interested to read this thread it provides technical solutions in various cases.
I am using a POJO as a BootStrapNotifier instead of Application class. The POJO has reference to the context. Will background detection start in this way?
I am also using this class as MonitorNotifier when the app is in foreground.
Is it mandatory to use BootStrapNotifier and RangeNotifier in the same class like shown in the Reference Application?
Is this a correct approach? If the app is killed, will the beacon detection start only when power is connected or disconnected or re booted?
Yes, it is possible to use a POJO to receive the callbacks from a RegionBootsrap, but you must still use the onCreate method of an Android Application class to construct this POJO and set it up.
The Application class is needed because its onCreate method is the first user-executable code that executes when an Android application starts up. The Android Beacon Library’s RegionBootstrap works because the library sets up a broadcast receiver that looks for BOOT_COMPLETED, ACTION_POWER_CONNECTED and ACTION_POWER_DISCONNECTED events. This broadcast receiver doesn’t do much, but if the app is not running when one of these events happens, it causes the Application class’ onCreate method to get executed. It is the creation of a RegionBootstrap at this time that causes beacon scanning to start in the background and then notify user code when beacons of interest are located.
The code below shows how you set up a POJO called MyPojo to receive the callbacks from the RegionBootstrap, and register that POJO in the Application’s onCreate method. The first parameter of the RegionBootstrap is the class that will receive the callbacks when beacons are detected.
You can use a POJO like this to set up ranging or do anything else you want -- there is no reason that such code has to reside in an Android Application class.
public void onCreate() {
super.onCreate();
MyPojo myPojo = new MyPojo(this);
Region region = new Region("backgroundRegion",
null, null, null);
regionBootstrap = new RegionBootstrap(myPojo, region);
}
...
public class MyPojo implements BootstrapNotifier {
private Context mContext;
public MyPojo(Context context) {
mContext = context;
}
public Context getApplicationContext() {
return mContext;
}
public void didEnterRegion(Region region) {
...
}
public void didExitRegion(Region region) {
...
}
public void didDetermineStateForRegion(int state, Region region) {
...
}
}
I am trying to use events as a form of communication between the tiers in a 3 tier application since events are recommended way to avoid coupling between tiers. However I am not able to figure how to use events when synchronous response is needed in the result like in case of a MVC web application. Consider below example
//1. Repository for Employee (Database Tier)
interface IEmployeeRepository
{
//event to signal add complete in repository
public event EventHandler event_EmployeeAdded_To_Repository;
void AddEmployee(Employee emp);
}
public class EmployeeRepository: IEmployeeRepository
{
//implementation of event
public event EventHandler event_EmployeeAdded_To_Repository;
public void AddEmployee(Employee emp)
{
//Save to database
//Fire event to signal add complete in repository
event_EmployeeAdded_To_Repository(null,e);
}
}
//2. Business Tier for Employee
interface IEmployee
{
//event to signal add complete in business tier
public event EventHandler event_EmployeeAdded_To_Entity;
public void AddEmployee();
}
public class Employee : IEmployee
{
public event EventHandler event_EmployeeAdded_To_Entity;
IEmployeeRepository _empRepository;
//constructor injection
public Employee(IEmployeeRepository empRepository)
{
this._empRepository = empRepository;
//Add event handler for the repository event
this._empRepository.event_EmployeeAdded_To_Repository += OnEmployeeAddedToRepository;
}
public void AddEmployee()
{
//Call repository method for adding to database
_empRepository.AddEmployee(this);
}
//Event handler for the repository event
public void OnEmployeeAddedToRepository(Object sender, EventArgs e)
{
//Fire event to signal add complete in business tier
event_EmployeeAdded_To_Entity(null, e);
}
}
//3. Presentation Tier
public class EmployeeController : Controller
{
IEmployee _emp;
//constructor injection
public EmployeeController(IEmployee emp)
{
this._emp = emp;
//Add event handler for business tier event
this._emp.event_EmployeeAdded_To_Entity += OnEmployeeAddedToEntity;
}
//This is an Ajax method call
public ActionResult Add()
{
//Call add method of business tier
_emp.AddEmployee();
//What do I do here ?
}
//Event handler for the business tier event
public void OnEmployeeAddedToEntity(Object sender, EventArgs e)
{
//What do I do here ?
}
}
In above example, I have defined event "event_EmployeeAdded_To_Repository" in the Repository (Database tier) to notify addition completion in repository. In the business tier I have defined event handler "OnEmployeeAddedToRepository" that handles this event. The "OnEmployeeAddedToRepository" event handler in turn fires the event "event_EmployeeAdded_To_Entity". Finally in the presentation tier I have defined event handler "OnEmployeeAddedToEntity" for handling the business tier event.
But in the controller I have an action "Add" that needs to return a response synchronously (after "AddEmployee" has completed) to notify user (or maybe call another ajax action). Firing an event will simply change the flow of control from the action to the event handler.
So then how will you use events in this scenario ?
So then how will you use events in this scenario?
The short answer is: you won't :)
You are in a situation where you have a hammer and you are looking for a nail. Rather determine why you need to do this.
Typically you will have very little use within a domain of knowing when something happened in the domain itself. You could take a look at domain events. But once again, a particular domain will probably be more interested in something happening in another domain.
If you do find yourself needing to know on the web-server that an employee was added then I would go with domain events anyway. A domain event would typically be used to publish the event on a service bus that other systems subscribe to. For instance, once you add an employee in the HR system the access control system would like to know so that it can start issuing a card. In this case the access control system would have it's endpoint subscribe to the Company.HR.Messages.EmployeeAddedEvent message.
I have a Mvc3-Project where I want to register to custom event hooks.
So that I can register to an event like "User logon". I do not want to do it in the controller, because there is a business logic behind it in an other project.
So in my Mvc3-Project I want to write some classes that will have the code that has to be executed when a User is loged on. But how do I register these classes (or an instance of them) to the event. Is it a good idea to use reflection an search for all classes inherited from a special base class, or is there an other smarter way?
So again, I do not want to monitor the action that is called, I want that the business logic triggers some classes in my Mvc3-Project.
EDIT
As Chris points out in the comments below, MVC3 is stateless, meaning that with this solution you would have to resubscribe for these events on every request. This is probably not a very good solution for MVC.
Have you considered an global event service?
Rough example:
class Example : IEventReceiver
{
public void Init()
{
EventService.Subscribe("Logon", this);
}
private void OnEvent(string eventName)
{
// Do logon stuff here.
}
}
You would need to create the EventService class, which might be a singleton or service. It might have interface similar to the following:
public interface IEventService
{
void Subscribe(string eventName, IEventReceiver receiver);
void Unsubscribe(string eventName, IEventReceiver receiver);
void DispatchEvent(string eventName);
}
public interface IEventReceiver
{
void OnEvent(string eventName);
}