Live Queries

Couchbase mobile database live query concepts

Activating a Live Query

A live query is a query that, once activated, remains active and monitors the database for changes; refreshing the result set whenever a change occurs. As such, it is a great way to build reactive user interfaces — especially table/list views — that keep themselves up to date.

So, a simple use case may be: A replicator running and pulling new data from a server, whilst a live-query-driven UI automatically updates to show the data without the user having to manually refresh. This helps your app feel quick and responsive.

To activate a LiveQuery just add a change listener to the query statement. It will be immediately active. When a change is detected the query automatically runs, and posts the new query result to any observers (change listeners).

Example 1. Starting a Live Query

query = QueryBuilder
    .select(SelectResult.all())
    .from(DataSource.collection(collection)) (1)

// Adds a query change listener.
// Changes will be posted on the main queue.
let token = query.addChangeListener { (change) in (2)
    for result in change.results! {
        print(result.keys)
        /* Update UI */
    }
} (3)

1	Build the query statements
2	Activate the live query by attaching a listener. Save the token in order to detach the listener and stop the query later — se Example 2

Example 2. Stop a LIve Query

query.removeChangeListener(withToken: token) (1)

1	Here we use the change lister token from Example 1 to remove the listener. Doing so stops the live query.