Observe query

[poojamat]

Issue #, if available:

Description of changes:

• Adding observe query api which

  • Return a stream that emits a snapshot with the initial data in the local DB, and subsequent snapshots containing data (items) synced from the cloud and isSynced status

  • Allow for processing updates in batches
    Sample Kotlin code:
    private fun observeQuery() {
    val tag = "ObserveQuery"
    val onQuerySnapshot: Consumer<DataStoreQuerySnapshot> =
    Consumer<DataStoreQuerySnapshot> { value: DataStoreQuerySnapshot ->
    Log.d(tag, "success on snapshot")
    Log.d(tag, "num records: " + value.items.size)
    Log.d(tag, "sync status: " + value.isSynced)
    }

    val observationStarted =
    Consumer { _: Cancelable ->
    Log.d(tag, "success on cancelable")
    }
    val onObservationError =
    Consumer { value: DataStoreException ->
    Log.d(tag, "error on snapshot$value")
    }
    val onObservationComplete = Action {
    Log.d(tag, "complete")
    }

    val predicate: QueryPredicate =
    NAME.contains("1").or(NAME.contains("2")).or(NAME.contains("3"))
    val QuerySortBy = QuerySortBy("car", "name", QuerySortOrder.ASCENDING)

    val options = ObserveQueryOptions(predicate, listOf(QuerySortBy))
    Amplify.DataStore.observeQuery(
    Blog::class.java,
    options,
    observationStarted,
    onQuerySnapshot,
    onObservationError,
    onObservationComplete
    )
    }
    By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rjuliano]

Left a couple of comments, but it LGTM!