Triggers Global Expressions

The Triggers menu at the top of the window lets you define two expressions that can manage values you want to make available to all your other Trigger expressions.

To support that, all Trigger expressions have access to a Clojure atom named globals that is shared with all other Trigger expressions, so that’s a great place to put things for them to find. The atom starts out holding an empty map, which allows you to add key/value pairs to organize the information you want to share across expressions.

Global Setup Expression

This is run when Beat Link Trigger starts up, or when you open a new Trigger file, so it runs before any of your individual trigger expressions. As a simple example of what you might want to do, here is an expression opens the Player Status window:


Setting this as your Global Setup Expression will automatically open the Player Status window whenever Beat Link Trigger launches. (This also makes sure subsystems like the TimeFinder are running, which can be helpful if you are writing other expressions that depend on knowing the playback position of tracks.)

If you ever start up BLT in offline mode with this Global Setup Expression in place, however, you will be presented with an error dialog, complaining “Must be Online to show Player Status window.” To avoid being inconvenienced by that, we can make the expression smarter, so it only tries to open Player Status when already Online:

(when (.isRunning (VirtualCdj/getInstance))  (1)
  (beat-link-trigger.triggers/show-player-status))  (2)
1 A when clause in Clojure only executes its body if a test expression returns a true value. In this case we ask the Beat Link VirtualCdj object if it is running, which is only true when Beat Link Trigger is online.
2 This means the expression body, which is the same as our first attempt at opening Player Status, is only run when it is safe to do so.

But there is an even better way to do this, now that version 0.5.4 has added the Came Online Expression which runs once Beat Link Trigger has successfully connected to a DJ Link network. Using that expression to show the Player Status window means it will always be safe to try, and — even better — the window will be reopened every time you go back online, not just when initially starting the program.

Here is an even more sophisticated example that creates a DatagramSocket for sending a remote trigger command to the ChamSys MagicQ lighting control software using its remote protocol:

(let [chamsys-address (InetSocketAddress.  (1)
                       (InetAddress/getByName "") 6553)
      trigger-on (byte-array (map int "71,1H"))  (2)
      trigger-off (byte-array (map int "71,0H"))]
  (swap! globals assoc  (3)
         :chamsys-socket (DatagramSocket.)  (4)
         :chamsys-on (DatagramPacket. trigger-on (count trigger-on) (5)
         :chamsys-off (DatagramPacket. trigger-off (count trigger-off)

This begins with a let binding which sets up some values that will be used later in the expression.

1 chamsys-address gets set to a SocketAddress representing port 6553 (the default port used by MagicQ) on the broadcast address for the Deep Symmetry show network (you will need to use the value appropriate for your own network).
2 trigger-on and trigger-off are arrays of bytes containing the characters that make up the commands for turning a MagicQ remote programming trigger on and off.
3 Those values are then used in the swap! call, which is the way you modify a Clojure atom. In this case we are modifying the map in globals by using assoc to add some new key-value pairs to it:
4 :chamsys-socket gets associated with a newly-allocated DatagramSocket that triggers will be able to use for sending UDP messages to MagicQ,
5 and the actual command packets are created as DatagramPacket objects preconfigured with the correct command bytes and destination address and port, in :chamsys-on and :chamsys-off.

The Beat Expression below shows how these globals are actually used.

Came Online Expression

This is run whenever Beat Link Trigger has successfully connected to a DJ Link network, either at initial startup or when you have told it manually to go online. At initial startup, or whenever you open a new Triggers file, it will run after the Global Setup expression as long as you are online. It runs by itself when you choose Network  Online? to successfully transition from an offline to an online state.

You have access to the expression globals if you need to use anything in them.

This is the perfect place to take actions that can only succeed when Beat Link Trigger is online, such as opening the Player Status window. The simplest approach described above is safe to use in this expression, unlike in the Global Setup expression:


With this in place, every time you successfully go online, either at startup or using Network  Online?, the Player Status window will open for you.

Another example came up in the community chat room, from a user who wanted to set up a turn-key lighting system that automatically started Beat Link Trigger and Carabiner. If you are in a situation like that, you can set up a Came Online Expression to automatically open the Carabiner window and connect to it:

(beat-link-trigger.carabiner/show-window nil)  (1)
(beat-link-trigger.carabiner/connect)  (2)
(beat-link-trigger.carabiner/sync-mode :passive)  (3)
(beat-link-trigger.carabiner/sync-link true)  (4)
(beat-link-trigger.carabiner/align-bars true)  (5)
1 Opens the Carabiner window.
2 Tries to connect to the Carabiner daemon.
3 Allows the DJ Link network to become Tempo Master for Ableton Link. Other choices are :off, :triggers, and :full.
4 Causes Ableton Link to actually follow the DJ Link Tempo Master.
5 Aligns Ableton Link and the DJ Link network at the level of entire four-beat measures, rather than simple beat alignment.
If you are using the above Carabiner configuration functions in an expression, you need to perform the first three steps in the order they are shown, because each will only work when the preceding step has been succesfully accomplished. Functions 4 and 5 can be called in any order.

Going Offline Expression

This is run whenever Beat Link Trigger is online and you manually take it offline by choosing Network  Online?, exit the program, open a new Trigger file, or choose to delete all triggers. In any of the situations where you are doing more than just going offline, it runs right before the Global Shutdown expression. It gives you a chance to gracefully close any connections and release any system resources you allocated in your Came Online Expression.

You have access to the expression globals if you need to use anything in them.

Global Shutdown Expression

This is run when Beat Link Trigger is exiting, or before it opens a new Trigger file (or when you choose to delete all triggers). It gives you a chance to close any connections and release any system resources that you allocated in your Global Setup Expression. Here is how we would do that for the ChamSys MagicQ example we started above:

(.close (:chamsys-socket @globals))

This simply looks up the DatagramSocket that was created in the setup expression, and closes it. There is no need to remove the key/value pairs themselves from the globals atom because Beat Link Trigger will reset it to hold an empty map once the shutdown expression finishes.