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A Signal is a value that changes over time and is implemented by signum.signal/signal

A Reaction is a mechanism for wrapping computation around one or more Signals, that runs every time one of the referenced Signals changes. The output of a Reaction is also a Signal.

Signum introduces the concept of a Subscription (similar to the one provided by re-frame) that formalizes the use of Signals and Reactions.

These basic building blocks and be used to create sophisticated graphs that update in real time as values change.


The core abstraction provided is a Subscription. A Subscription is used to manage the lifecycle of external Signals as well as wrap the input Signals with computation.

Signals can either be obtained from other Subscriptions or via de-referencing externally allocated signum.signal/signals from the Reaction graph.

A Subscription is declared via signum.subs/reg-sub. reg-sub is available in several arities. The core of a Subscription is a query-id and a computation function. Additional arities support the specification of init and dispose functions as well.

The following example declares a basic Reaction graph.

(require '[signum.signal :as s])
(require '[signum.subs :refer [reg-sub subscribe]])

 (fn [query-v]
   (let [counter (s/signal 0)
         counter-loop (future
                        (loop []
                          (s/alter! counter inc)
                          (Thread/sleep 1000)
     {:counter counter
      :counter-loop counter-loop}))
 (fn [{:keys [counter-loop]} _query-v]
   (future-cancel counter-loop))
 (fn [{:keys [counter]} _query-v]

 (fn [[_ some-text]]
   (let [value @(subscribe [:api.example/my-counter])]
     {:value value
      :str (str some-text "-" value)})))

The Subscription for :api.example/my-counter declares init, dispose and computation functions, where is the Subscription for :api.example/auto-increment-string only declares a computation function.

The init and dispose functions are used to manage resources outside of the Subscription graph. In the case of :api.example/my-counter the init function is allocating a counter and a future that will increment it every second. The dispose function is used to shut these down. The value returned from the init funtion is passed into the computation and dispose functions along with the query vector.

If there are no init and dispose functions in a reg-sub, the computation function will only be passed in the query-vector.

The subscribe function is used within :api.example/auto-increment-string to reference the Signal for :api.example/my-counter.

Any signum.signal/signal dereferenced within a computation function will cause the computation function to automatically run if the signal changes.

Implementation details

The Signal returned from a subscribe receives it's value asynchronously and it is important to watch it so that the updated values can be received.

The first time an add-watch is called, the upstream Subscription graph gets instantiated. When the last remove-watch is called, the upstream Subscription graph is disposed.

If an add-watch is never called, the Subscription graph will never be instantiated and the value will always be nil. This will also cause the output-signal to be retained forever.

This is only important to keep track of if you're implementing a library on top of Signum. No special handling is required if using com.7theta/via, com.7theta/servo etc.

Copyright and License

Copyright © 2020 7theta

Distributed under the MIT License.

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