Show Global Expressions

The Tracks menu at the top of a Show window lets you define expressions that can manage values you want to make available to all your other expressions within the Show. The previous section explained how to share functions with all the expressions in your show. Now we will look at how to manage values for them during the show lifecycle.

All Show expressions have access to a Clojure atom named globals that is shared with all other Show 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. Show expressions also all have access to the Triggers window globals atom, under the name trigger-globals.

Also, all expressions that run in a show have access to a value show that contains everything Beat Link Trigger knows about the show, which can be useful.

Global Setup Expression

This is run when the Show file is opened, either because you explicitly opened it using the Triggers window File menu, or because Beat Link Trigger automatically reopened it at launch because you had it open the last time you used the program. You can use it to open network connections or set up other values for your Track and Cue expressions to use.

Came Online Expression

This is run whenever the show is opened when Beat Link Trigger is already connected to a DJ Link network, or if you have told it manually to go online with the show already open. When opening the show 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 show and triggers globals if you need to use anything in them.

Default Enabled Filter Expression

The basic concept of an Enabled Filter is described in the introduction to the Expressions section; shows use them in a similar way. Each Track that you are watching in the Show can be enabled separately. If the track’s Enabled menu is set to Default, that track will look to the show itself to decide whether it should be enabled. There is an Enabled Default menu at the top of the Show window that is used by all tracks whose Enabled mode is Default. If the Show’s Enabled Default is set to Custom, it will run your Default Enabled Filter Expression to decide what to do. If your expression returns a true value, all these tracks will be enabled; otherwise they will be disabled.

Disabled Tracks do not respond to being played, and all of their configured cues are disabled.

Enhanced On-Air Example

As an example of a useful Default Enabled Filter Expression, one of the most convenient Default Enabled Filter options is On-Air, which will prevent your tracks from being inadvertently enabled when a DJ is just previewing tracks in his headphones, but will enable them once they are actually playing through the house speakers. But this setting runs into trouble if the DJ also likes to fade out tracks to coax the audience to sing along: your On-Air filter will interrupt your cues while the track is faded out completely.

To fix this, we can write a Default Enabled Filter expression that enables the tracks both when they are On-Air, and when they temporarily go off the air, as long as they are still playing, and were previously enabled. Here’s a first draft of a Default Enabled Filter expression code that implements this goal (but read on before you use it, for an even better version):

(let [new-state (swap! globals update-in [:previously-enabled device-number]
                       (fn [was-enabled?]
                         (and playing? (or on-air? was-enabled?))))]
    (or (get-in new-state [:previously-enabled device-number])

This works pretty well under most circumstances, but you will discover that if the DJ happens to have the same track loaded into more than one player, this expression will get confused, and the track will flash between enabled and disabled when only one player is on-air. To fix that, we need to extend the expression so that we track the "on-air locked" state for each such player separately. Here is how we can do that:

(let [signature   (get-in show [:loaded device-number])
      any-on-air? ((set (vals (:on-air show))) signature)
      new-state   (swap! globals update :enable-locked
                         (fn [old-locks]
                           (let [locked (get old-locks device-number)]
                             (if (and playing?
                                      (or on-air? (= locked signature)))
                               (assoc old-locks device-number signature)
                               (dissoc old-locks device-number)))))]
  (or (get-in new-state [:enable-locked device-number])
These kinds of multi-player issues presented some of the biggest challenges when adding the Show feature to Beat Link Trigger.

Going Offline Expression

This is run whenever the show is open, Beat Link Trigger was online, and you manually take it offline by choosing Network  Online?, when you close the Show file, or exit the program. 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 show and triggers globals if you need to use anything in them.

Global Shutdown Expression

This is run when the Show file is closed, either because you closed the window, or because Beat Link Trigger is shutting down. You can use it to close any network connections or clean up any other resources your Global Setup Expression allocated.