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+---
+# File generated by dokgen. Do not edit. 
+# Edit 'src/main/kotlin/docs/40_Interaction/C90_Events.kt' instead.
+layout: default
+title: Events
+parent: Interaction
+last_modified_at: 2024.03.05 16:52:37 +0100
+nav_order: 90
+has_children: false
+---
+
+# Events
+
+Events are notifications sent by a sender to one or more subscribers indicating 
+that an action has taken place.
+
+Mouse and keyboard events are among the most commonly used events in OPENRNDR. 
+Before we get to those in the next chapter, let's examine how to create 
+our own custom events.
+
+We will often use events in class instances, so let's create
+a simple class called `Thing`. This class will include an update method 
+that will emit an event every 60 times it's called. Anyone listening to this
+event will receive it. 
+
+```kotlin
+class Thing {
+    val timeEvent = Event<Boolean>()
+
+    private var frame = 0
+
+    fun update() {
+        if (++frame % 60 == 0) {
+            timeEvent.trigger(Random.bool())
+            timeEvent.deliver()
+        }
+    }
+}
+``` 
+
+### Sending an event        
+
+Notice how events carry a payload, in this case `Boolean`. This is convenient
+because it allows us to transmit information together with the event.
+Mouse and Keyboard events contain details about the mouse position or 
+the key pressed. In this program we are free to choose any type, so lets
+just broadcasting a message containing a random boolean value.
+
+Another thing to observe is that `timeEvent` is a public variable. If it
+was private we couldn't listen to it from outside 
+by calling `thing.timeEvent.listen { ... }`.    
+
+At some point in our program execution we need to call `.trigger()` 
+to queue an event. We can call it as many times as needed.
+
+Finally, we call `.deliver()` to deliver the queued events to those
+listening to them.
+
+### Listening to an event
+
+The following small program shows how to listen to an event emitted by a class.
+
+First, let's create one instance of the class (`Thing` in this case).
+
+Next, listen to an event this instance can emit (`timeEvent` here).
+
+
+```kotlin
+fun main() = application {
+    program {
+        val thing = Thing()
+        extend {
+            thing.update()
+        }
+        thing.timeEvent.listen {
+            println("timeEvent triggered! It contains a: $it")
+        }
+    }
+}
+``` 
+
+There are multiple approaches to including events in our class. For example, a
+class could have separate `loadStart` and `loadComplete` events, or instead,
+have just one `loadEvent` and have the payload describe if it was
+a `start` or a `complete` event. Having two separate events arguably produces 
+more readable code. 
+
+[edit on GitHub](https://github.com/openrndr/openrndr-guide/blob/main/src/main/kotlin/docs/40_Interaction/C90_Events.kt){: .btn .btn-github }