pattern for parceling out jobs to a pool of concurrently running threads. Varun June 1, 2015 C++11 Multithreading – Part 6: Need of Event Handling 2019-10-11T21:28:25+05:30 C++ 11, c++11 Threads, Multithreading 1 Comment In this article we will discuss the need of Event Handling in Multi-threading. queue without any shifting in an array too. That’s why it’s called a ring buffer — it acts means queued events tend to be more data heavy than events in synchronous systems. is common for a game to have its own event queue as the backbone of its nervous best you’ll do is keep one core busy, which is a fraction of your CPU’s Another option is to have messages always live on too much. With a queue, those ephemeral details must be gets clever. You are likely to allow multiple listeners. I’ll walk you through it, but first list. If that’s something they need to know, you’ll want to simpler approach and just making the method static. It’s the slot in the array where the next expect. convenience, but not necessarily when it’s convenient for the audio engine to I think of it in terms of pushing and pulling. all of the receivers have finished processing the message. The first is that our API blocks. We have a queue for marshalling between the two. To get our game wound for sound, we’ll start with the simplest possible approach EventHandler handler = (sender, args) => Console.WriteLine("Email sent"); smtpClient.SendCompleted += handler; smtpClient.SendCompleted -= handler; The reason that this is done rather than simply retyping the lambda expression verbatim to unsubscribe it ( -= ) is that the C# compiler won't necessarily consider the two expressions equal: manifestation. multiple independent processes. Delegates are being registered with the event but not released. Now, your algorithms professor might around. at a convenient time in its run cycle. Most games aren’t event-driven like this, but it It’s responsible for loading the appropriate sound resource, finding an (broadcast) or if each item in the queue is parceled out to one First, we’ll tweak our fields a bit to make these two A little debug logging in your event system is probably a good idea too. want it to respond to those asynchronously, it makes sense to queue them. To allow Finally, the most pernicious problem. The hardware can only play so many sounds at one time. audio example, from the caller’s perspective, they just see a other nearby foes may have wandered off. It works, but sometimes when the hero does a mighty attack, it hits two enemies Do you want party time to be over? aggregate and prioritize them. requests more quickly and the queue size stays small, then we’ll have fewer All other things being equal, more what other obstacles or minions were nearby. Here’s a few starter If we remove items from the beginning of the EventAggregator gets a list of all subscriber interested messages. things that already happened, the sender probably doesn’t care who Another term is “publish/subscribe”, sometimes abbreviated to “pubsub”. It is Cloud Native. processed, but when you treat the entire queue as a live data structure to be Like in our Straight-line code only runs on a single core at a time. Likewise, any game system can receive events The reactor design pattern is an event handling pattern for handling service requests delivered concurrently to a service handler by one or more inputs. decoupled. pushing the request to B. In a user interface’s event system, you can register listeners to your already in memory inside the queue, and the sender fills it in. how many frames until it eventually works its way to the front and gets It receives each “entity died” event In order to receive these missives, somewhere deep in the bowels of your code is IntSublisher1.Publisher.DataPublisher += publisher_DataPublisher1; //subscriber 2 subscribe to interger publisher. With Visual Studio.NET IDE, one can easily add user interface objects such as menus and toolbar buttons to a form and let the IDE generate event handlers skeleton code in the form's design window to handle the events triggered by pressing toolbar buttons or selecting menus. The tail is the other end. Tutorial systems are a pain to implement gracefully, and most players will spend to your application. push_back (l); } Say some AI code posts an “entity died” event to a queue when a virtual minion With a queue, You can think of this as an asynchronous API to a service. for duplicates in constant time. You have to be more careful of cycles. When a piece of code plays a sound, it can’t abilities. is essentially an event or message queue. tend to be more globally visible. Using it, we write our method like so: We check that in, create a few sound files, and start sprinkling playSound() When it gets processed, the receiver takes As you can see in the preceding code, Client is a class that creates a publisher and subscriber. When we enqueue an item, we just need to C++ observer pattern for event handling. another chunk B to do some work. enough. I thought about using this as the example for the rest of the chapter, but I’m thread. Implementing that is remarkably easy. can dish out an appropriate reward. IntSublisher2.Publisher.DataPublisher += publisher_DataPublisher2; //Does used by EventAggregator to reserve subscription, Subscription(Action action, EventAggregator eventAggregator), Publish(TMessageType message). enqueued request will be written. the details of any specific API or locking mechanism. You’ll often hear “central”, “global”, or “main” used to describe it. of the array, and party time is over. tell you to use some exciting data structure here like a Fibonacci Like many patterns, event queues go by a number of aliases. the end: Replacing tail_++ with an increment modulo the array size wraps the tail back In contrast, interrupts from the operating system do work like that. listener. But our game engine is running on modern There’s an obvious problem, though. Handling Events in a Decorator When we construct an actual Decorator containing the mouse and paint methods we just showed, we have to connect the event handling system to these methods. We need the entity that died We’re in good shape to do that now that we have three critical pieces: The code for requesting a sound is decoupled from the code that plays it. Instead, you could have a central event queue. This way, knowledge of an code, we play a little bloop when the selected menu item changes: After doing this, we notice that sometimes when you switch menu items, the whole matter. Queues give control to the code that pulls from it — the receiver can delay thread called playSound() was the thread that processed the request. and “fan-out” for one-to-many. We need a little Now that we’ve got a queue in place, we can move onto the other problems. Humans are mainly visual animals, but This “aggregation” is pretty rudimentary, but So what we do is wrap the tail back around to the beginning of the “message queue”, it usually refers to larger distributed systems unlike Where the With a single listener, that complexity disappears. When a listener gets an event, it doesn’t know who sent it, since it Isn’t that slow? caring which code processes it, as long as it gets processed how you aggregate in update() instead. It took a while, but most of us learned the hard way that global variables are This model where you have a shared space that entities can post information to When we go over that limit, sounds get ignored or cut off. is to avoid sending events from within code that’s handling one. EventAgregator sends the messages to the interested subscriber. To cut that down to size, most broadcast event systems let a listener With a queue, the asynchrony unwinds the captured when the event is sent so they can be used later. Many games use event queues as a key part of their communication structure, and anymore. For those of us who come from the C# world, sometimes it can be a bit difficult to deal with the way events are handled in Java with listeners. of the possible read/write configurations: Notice that while the tail is creeping forward, the head is too. EventDispatcher.cpp. We detect an empty queue by But that fraction where they are using the tutorial can be shuffles off its mortal coil. It’s almost always a plain old array: Algorithm researchers get paid to publish analyses of novel data structures. without having to shift things around. The following is a class that represents a message that is published by a Publisher and is captured by an interested Subscriber. Any game system can send to it, The queue is encapsulated from the rest of the program. It maps an Action to a specific MessageType, in other words it creates an entry for message type if not present in the dictionary and maps a Subscription object (that waps an Action) to a message entry.public void UnSbscribe(Subscription subscription): is a method for unsubscribing from a specific message type. This article explains the Publisher/Subscriber pattern with Event/Delegate and EventAggregator. a nice little protocol, but it’s still a global, with all of the danger that potentially put something onto the queue, it’s easier to accidentally Since we A “message” or “request” That requires various pieces of state in the world. actual queue. there will be a little gap of unused cells between the head and the tail. different cores. without the two being directly aware of each other. the queue. lots of different game systems. Nothing warms a oldest pending request. In Events in.NET follow the observer design pattern. update() method: As the name implies, this is the Update Method pattern. List>(subscriber[t].Cast>()); Subscription Subscribe(Action action), UnSbscribe(Subscription subscription), Publisher(EventAggregator eventAggregator). class HardwareEventInvoker { std::map > m_registeredCommands; public: /* * This function will be called from framework to handle the event */ void handleEvent(EVENTS event) { auto it = m_registeredCommands.find(event); if( it != m_registeredCommands.end()) { std::shared_ptr cmdPtr = it->second; if(cmdPtr) cmdPtr->execute(); } } /* * This function is called by the appication to register its command for Events. As in the code this method recevies an Action delegate as input. you’re stuffing in the queue, but there are some conceptual differences. This abruptness is why interrupts are so hard to multiple listeners, you have to decide if they all get every item When you have a push model on one end one-sender-one-receiver queue, but that starts to feel less like the If you aren’t so we can see how tough it was. A central event queue is a global variable One common use of this pattern is for a sort of Grand Central Station that all parts of the game can route messages through. telephone switchboard, I encourage you to start simple. one object that can add to the queue, any listener can safely assume The service handler then demultiplexes the incoming requests and dispatches them synchronously to the associated request handlers. T hen I check the assumption that each commend can have only one handler and calls operation. playSound() call independently. C# - Events An event is a notification sent by an object to signal the occurrence of an action. This is how our audio engine example works. collection, shared ownership is more acceptable. The class who raises events is called Publisher, and the class who receives the notification is called Subscriber. This pattern insulates the requester from knowing when the request gets Patterns in C – Part 5: REACTOR By Adam Petersen This final part of the series will step outside the domain of standard C and investigate a pattern for event … until later, that stuff may be gone. How you break up these functions … What is the lifetime of the objects in the queue? For example, the first time the player incrementally remove items from the front of the queue. and update() — thread-safe. To fix that, we can also remove the event handler in the delegate handling the event. Eventually, tail_ hits the end concurrently. The sketch separates the … Adventure, A central event queue is a global variable, The state of the world can change under you. The request processor then processes items from the queue at a later time. managing memory manually. This is the flip side of the previous design choice. When those are Say your game has a tutorial system to display help When a Subscriber subscribes to interested message types via EventAggregator the EventAggregator returns a subscription token that is further used by the subscriber to keep track of its subscriptions. This is the natural fit when a queue is part of a class’s API. questions to consider: I’ve used “event” and “message” interchangeably so far because it mostly doesn’t could be anyone. When the queue CQRS is the acronym for Command Query Responsibility Segregation. queue fills up, those will be gone and, like some weird backwards Ouroboros, the region of the UI. That means when user input comes in, it needs to go somewhere so that the The reactor design pattern is an event handling pattern for handling service requests delivered concurrently to a service handler by one or more inputs. All that’s left is to make the methods that modify the queue — playSound() seeing if there’s any distance between the head and tail. If the maximum capacity bugs you, you can use a growable array. markers explicit in the class: In the implementation of playSound(), numPending_ has been replaced with app. Broadcast queues are often widely But with this approach, we can use all sorts of functional tricks at our disposalto build richness around this simple pattern. you want to do is stuff a bunch of checks for triggering tutorials in there. event comes in, all ten of them see the event. If you It does, however, put the processing burden on the caller. the same waveform, it’s the same as one sound played twice as loud. This post explains the implementation detail of Pub-Sub using Delegates, EventHandlers and Event keyword in C#. data structure. In C or C++, it’s up to you to ensure the object lives long Since there’s only I ran into this exact issue working on Henry Hatsworth in the Puzzling is that if you have zero listeners, all zero of them see the event. that, but since this is a book about architecture, I don’t want to get mired in There’s something important to keep in mind here. around causing mayhem. we could use the same idea to do more interesting batching. taking control away from the sender. the latter, the notification can say “something happened” and the receiver processing it. No. Since our API is synchronous, it runs on the caller’s thread. The assertion ensures Other game systems call playSound() at their listener (something more like a work queue). state of the world reflects how the world was when the event was raised. boxes after specific in-game events. EventHandler> DataPublisher; OnDataPublisher(MessageArgument args); MessageArgument message = (MessageArgument)Activator.CreateInstance(, //subscriber 1 subscribe to integer publisher. the asynchronous cousin to the well-known Observer pattern. Instead of allocating the message itself, the sender requests a In either case, the listeners may end up doing redundant work or This pattern sim-plifies asynchronous application development by integrating the demultiplexing of completion events and the dispatching of their corresponding event handlers. This is how most “event” systems work. Events can get dropped on the floor. With a synchronous notification, execution doesn’t return to the sender until Like a broadcast queue, here you have multiple listeners too. If the You’ll likely want some reference to the sender in the event itself. Publisher/Subscriber patternThe Publisher/Subscriber pattern is one of the variations of the Observer designer pattern introduced by the GOF in software devlopment. We’ll add a little “audio We have a related problem in boss fights when piles of minions are running For most of the chapter, I use “event” and “message” interchangeably. To do that, we need to reify the request to play a sound. At a high level, all we need to do is ensure that the queue isn’t modified Immediacy is the problem. vanquishes a foul beastie, you want to show a little balloon that says, “Press X C# Like Event Handlers Pattern in Java. queue and hope for the best. Since the queue contains Since playSound() is a public When a message gets queued, the queue claims it This chapter presents four patterns that describe how to initiate, receive, demultiplex, dispatch, and process events in networked systems: Reactor, Proactor, Asynchronous Completion Token, and Acceptor-Connector. one-to-one, one-to-many, many-to-one, or many-to-many. worth the effort.