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# Behaviour Builder

In the previous pages about updates handling and was mentioned that currently in the most cases you should use Flows. So, there is an improvement for that system which hide direct work with flows and allow you to create more declarative logic of your bot.

## Main parts of Behaviour Builder

There are several things you should know for better understanding of behaviour builder:

- BehaviourContext - it is the thing which contains all necessary tools for working with bots
- Triggers - `on*` extensions for `BehaviourContext` which allow you to create reaction on some update
- Expectations (or waiters) - `wait*` extensions which you may use in buildBehaviour function, but it is recommended to use it in bodies of triggers

## Initialization

As was said above, there is buildBehaviour function which allow you set up your bot logic. Let’s see an example:

    val bot = telegramBot("TOKEN")

    bot.buildBehaviour {
      onCommand("start") { // creating of trigger
        val message = it
        val content = message.content

        reply(message, "Ok, send me one photo") // send text message with replying on incoming message

        val photoContent = waitPhoto().first() // waitPhoto will return List, so, get first element

        val photo = downloadFile(photoContent) // ByteArray of photo

        // some logic with saving of photos
      }
    }

## Filters

In most cases there are opportunity to filter some of messages before starting of main logic. Let’s look at this using the example above:

    val bot = telegramBot("TOKEN")

    bot.buildBehaviour {
      onCommand(
        "start",
        initialFilter = {
          it.content.textSources.size == 1 // make sure that user has sent /start without any additions
        }
      ) {
        // ...
      }
    }

OR

    val bot = telegramBot("TOKEN")

    bot.buildBehaviour {
      onCommand(
        "start",
        requireOnlyCommandInMessage = true // it is default, but you can overwrite it with `requireOnlyCommandInMessage = false`
      ) {
        // ...
      }
    }