Rajesh there is a separate event thread, that is always checking for events. Whenever it finds an event, it tells all of the objects that are listening for it that this happened. You absolutely do not have a thread for every listener though. The event thread is going to run regardless — Cruncher. Cruncher does it mean that it uses some notifyall kind of mechanism??? If so does this whole concept also have wait mechanism? Show 1 more comment. Active Oldest Votes. From Oracle's Introduction to Event Listeners : The event model, which you saw at its simplest in the preceding example, is quite powerful and flexible.
Improve this answer. Any example in that context? Thanks for your time. Yes, you can use listeners anywhere you want.
As you can see, there are dependencies between some of the pieces. The listener interface corresponds directly to the event. The event is necessarily the dispatch method's argument. The component corresponds directly with the event and listener. It needs to know about each so that it can create events, dispatch events, and register listeners.
Unlike the other two pieces, the event object is independent. As a result, many components are free to fire off the event type. Furthermore, multiple interfaces may define methods to dispatch the event. This story, "Events and listeners" was originally published by JavaWorld. Here are the latest Insider stories. More Insider Sign Out. Sign In Register. Sign Out Sign In Register. Event Adapters : If a program presents too many abstract methods for the listener to override, it can get difficult to compile it.
For example, if we want to close a frame, there are seven WindowListener abstract methods that we need to override. To reduce complexity and heavy coding, Java has event adapters.
Event adapters are already overridden abstract methods. It is important to remember that multiple event sources and listeners can interact with each other. So, multiple events belonging to the same class can be handled by a single listener.
This means that one listener can handle all those events that involve the same components that perform similar actions. Get in touch with us to learn about an extensive session on event handling in Java. Data Science. Data Science All Courses M. Sc in Data Science — University of Arizona. Software Engineering All Courses M.
Table of Contents. Leave a comment. Cancel reply Your email address will not be published. Comment Name Email Website. About unity in specific - there is no other way of checking player's input other than polling it every frame.
To create an event listener, you would still need an object like "event system" or "event manager" to do the polling, so it would only push the problem to a different class. Granted, once you have an event manager, you have only one class polling the input every frame, but this doesn't give any obvious performance advantages, since now this class has to iterate over listeners and call them, which, depending on your game design as in, how many listeners are there and how often the player uses input , might actually be more costly.
Apart from all this, remember the golden rule - premature optimisation is the root of all evil , which is especially true in video games, where often the process of rendering each frame costs so much, that small script optimisations like this are completely insignificant. But don't get turned away from this design pattern just because you don't have a performance benefit there right away.
Here are the reasons why you should use it regardless if you have underlaying support for event handling or not. Conclusion - you have been lucky to participate in the discussion and learned one alternative to polling.
Look for an opportunity to apply this concept in practice and you will appreciate how elegant the code could be. Most event loops are built above some polling multiplexing primitive provided by the operating system.
On Linux, that primitive is often the poll 2 system call but could be the old select one. In GUI applications, the display server e. Xorg , or Wayland is communicating thru a socket 7 or pipe 7 with your application. Such polling primitives are efficient; the kernel would in practice wake up your process when some input is done and some interrupt is handled. Concretely, your widget toolkit library communicates with your display server, waiting for messages, and dispatch these messages to your widgets.
Toolkit libraries like Qt or GTK are quite complex millions of lines of source code. Your keyboard and mouse are only handled by the display server process which translates such inputs to event messages sent to client applications. In a purely polling based system, subsystem that might want to know when some particular action happens will need to run some code any time that action might happen. If many of the things applications will watch for are rather similar, it may be more efficient to have a centralized one monitoring subsystem--perhaps table driven--which can watch many things and observe whether any of them have changed.
If there are 32 switches, for example, a platform might have a function to read all 32 switches at once into a word, making it possible for the monitor code to check whether any switches have changed between polls and--if not--not worry about what code might be interested in them.
If there are many subsystems that would want notification when something changes, having a dedicated monitoring subsystem notify other subsystems when events occur that they are interested in may be more efficient than having each subsystem poll its own events. Setting up a dedicated monitoring subsystem in cases where nobody is interested in any events, however, would represent a pure waste of resources.
If there are a only a few subsystems that are interested in events, the cost of having them watch for the events they're interested in may be less than the cost of setting up a general-purpose dedicated monitoring subsystem, but the break-even point will vary significant between different platforms. An event listener is like an ear waiting for a message. When the event occurs, the subroutine chosen as event listener works using the event arguments. There are always two important data: the moment where the event happens and the object where this event occurs.
Other argument are more data about what's happened. In its simplest form, a publishing object maintains a list of subscribers' instructions to be carried out when something needs to be published.
It will have a some kind of subscribe x method, where x depends on how the event handler is designed to handle the event. The publisher may contain all, some or none of the logic for handling the event. It's most likely to contain a mixture of both. The event handler adds the method to its list of subscriber callbacks. When an event occurs, the event handler iterates over its list and executes each callback.
For a real world example, when you subscribe to the newsletter on Stack Exchange, a reference to your profile will be added to a database table of subscribers.
When it's time to publish the newsletter, the reference will be used to populate a template of the newsletter and it will be sent to your email. In this case, x is simply a reference to you, and the publisher has a set of internal instructions used for all subscribers.
Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. How does an event listener work? Ask Question. Asked 3 years, 10 months ago. Active 7 months ago. Viewed 30k times. Based on the discussion in class, it seems that event listener works in a different way.
Improve this question. Gary Holiday Gary Holiday 1, 2 2 gold badges 7 7 silver badges 8 8 bronze badges. An event listener does not check at all. It gets called when the event that it is "listening" to fires. Yes, but how does it "listen", wouldn't it be constantly checking?
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