set.changesOn()

set.changesOn() is a deprecated. Use set.eventsOn() instead.

Creates an event source that tracks changes to specified document fields in a supported Set.

Signature

changesOn(...fields: A => Any) => EventSource<A>

Description

The changesOn() method creates an event source token that tracks changes to specified document fields in a Set. The token has the EventSource type:

"g9WD1YPG..."

You can only call changesOn() on a supported Set. The exact behavior of the method depends on this source. The calling Set isn’t changed.

Sets for event sources support a limited number of transformations and filters.

Parameters

Parameter Type Required Description

fields

Any

Yes

Comma-separated list of document field accessors (using dot notation). The event source tracks changes to the values of these document field.

Return value

Type Description

EventSource

A string-encoded token that represents the event source. Use the token to consume events as an Event Feed or Event Stream.

Examples

Collection event sources

You can use changesOn() to only track changes to specific fields for documents in a collection.

Product.all().changesOn(.description)

The query returns an event source.

"g9WD1YPG..."

Index event sources

Event sources for indexes only send events for changes to the index’s terms or values fields.

For example, the following Product collection’s byName() index has:

  • A term field of name

  • Value fields of name and price

collection Product {
  *: Any

  index byName {
    terms [.name]
    values [desc(.stock)]
  }
  ...
}

When called on an index, changesOn() only accepts the index’s terms or values fields as arguments.

For example, in the following query, changesOn() only accepts .name and .stock as arguments.

Product.byName("limes").changesOn(.name, .stock)

Document event sources

You can use changesOn() to track changes to a Set containing a single document. The following query only tracks changes to the name or price field of a single document.

let product = Product.byId("111")!
Set.single(product).changesOn(.name, .price)

See also

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