Live Queries

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    Description — Couchbase mobile database live query concepts

    Overview

    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.

    Example 1. Starting a LIve Query
    CBLQuery *query = [CBLQueryBuilder select:@[[CBLQuerySelectResult all]]
                                         from:[CBLQueryDataSource database:database]];
    
    // Adds a query change listener.
    // Changes will be posted on the main queue.
    id<CBLListenerToken> token = [query addChangeListener:^(CBLQueryChange * _Nonnull change) // <.>{
        for (CBLQueryResultSet *result in [change results]) {
            NSLog(@"%@", result);
            /* Update UI */
        }
    }];
    
    // Start live query.
    [query execute: &error]; (1)
    1 Build the query statements using the QuerybUilder
    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
    3 Start the query
    This will immediately execute the live query and post the result to the change listener. When a change is detected the query automatically runs, and posts the new query result to any observers (change listeners).
    Example 2. Stop a LIve Query
    [query removeChangeListenerWithToken:token]; (1)
    1 Here we use the change lister token from Example 1 to remove the listeners. Doing so stops the live query.