Subscriptions

A re-frame app is 75% derived data.

app-db is the root, authoratative source of data but radiating from it is a graph of nodes all computing derived data. Ultimately, the leaf nodes of this graph are ViewFunctions which compute hiccup (yes, derived data) which is rendered into the application's UI. But sitting within the graph, between the root and these leaves, are intermediate computational nodes supplied by subscriptions which compute materialised views of the data in app-db.

The Domino Narative

In terms of the dominos narative, subscriptions are domino 4, and the leaf View Functions are domino 5. For tutorial purposes, we distinguish them - they serve different purposes - but they are, conceptually, all nodes in the same graph.

Graph Shapes

In the simplest version of a graph, subscriptions simply extract some part of the data in app-db, which then flows on into ViewFunctions unchanged.

In more complex examples, subscriptions are layered, with some obtaining data from one or more other subscriptions, before a ViewFunctions eventually recieve highly processed versions of what's in app-db.

The Layers

The layers in this graph are as follows:

• layer 1 is the root node, app-db.
• layer 2 subscriptions extract data directly from app-db.
• layer 3 subscriptions obtain data from other subsciptions (not app-db), and compute derived data.
• layer 4 the view functions which compute hiccup (more derived data)

As we'll see soon, there's efficency reasons to distinguish between layer 2 (extractor) and layer 3 (materialised view).

reg-sub

Subscription handlers are registered using reg-sub.

But note: just because you register a handler doesn't mean that node exists in the graph. You are only defining how the node would compute if it was needed.

Nodes in the signal graph are created and destroyed according to the demands of (leaf) ViewFunction nodes.

When a ViewFunction uses a subscription, the graph of nodes needed to service that subscription will be created and, later, when the ViewFunction is destroyed that part of the graph will also be destroyed (unless used for other purposes).

An Infographic Depiction

Please read the following infographic carefully because it contains important notes.

Example Use

To see reg-sub used in a real application, please read through the heavily commented subscription code in the todomvc example.

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