# Events

## Introduction

Although many branches of **Logic** can be defined in **Incari's** **Logic Editor**, a branch will never be executed without being initiated by an **Event**. These **Events** define when, and under what conditions, functionality should be triggered. In both **Incari** and computer science in general, event systems define a relationship between the fulfillment of criteria and the set of instructions to be performed once those criteria are met. They are made up of two parts: *triggers* and *listeners*.

*Triggers* can be thought of as being like announcements. A real-world example of a *trigger* is a school teacher announcing that an exam's time limit has been reached. The other staff members and students are *listeners* in this case. The **Event** of the teacher's announcement prompts the students to put down their pencils, and staff members to begin collecting exam papers.

**Events** in **Incari** can be directly triggered by the user's input, as is the case with **Keyboard** and **Mouse Events**, or triggered automatically when something happens relating to an **Object**, **Variable**, **Scene**, or **Screen**. One of the most powerful aspects of **Incari's** **Events** is that you can even define your own custom *triggers* and *listeners*, to establish the criteria under which one part of your **Logic** causes the execution of others.

## Subscribing and Unsubscribing

![An Example of a Node with a Subscribe input and an Event ID output.](/files/pXSuod0je9XzW6RchMd7)

Most **Event** **Nodes** have an additional `Subscribe` input **Pulse** and some have an `Event ID` output **Socket**.

`Subscribe` is used to start listening to an **Event** when it is triggered by some arbitrary **Node** chosen by the user. This does not apply to **Event Nodes** that subscribe on initialization.

Once an **Event** is subscribed to, it can be unsubscribed from. This requires the **Event's** *ID*. If you recall, all items in **Incari** have a unique ID. In this case, an **Event ID** is a unique ID to a specific **Event**. This can be attached to the [**Unsubscribe Node**](/incari-studio/toolbox/events/unsubscribe.md), thus ending a connection to the **Event**. For **Nodes** with a `Subscribe` input, this can create a loop of subscribing and unsubscribing. **Events** that are subscribed to on initialization can only be unsubscribed from once.

![An Example of Subscribing and Unsubscribing.](/files/konHIK8Vfjq0VH4dL8Ag)

## External Links

[*What is event programming?*](https://mortoray.com/2017/06/26/what-is-event-programming/) on Musing Mortoray.


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