Return an empty array when an observation request returns no data

[philip-khaisman]

I found an issue where if a data source has no data and i try to replay it with other non-empty data sources (with a data synchronizer) no data streams. This is because in the DataSynchronizerAlgoReplay class we gate the streaming of data with this check:

if ((nbFetch + nbSkip) === totalDataSources) { start processing data }

nbFetch increments for every data source that has a status of FETCH_STARTED. The empty data source does not have this status. The status is set in the DelegateReplayHandler class in the startLoop() method:

let data = await this.context.nextBatch();
this.context.onChangeStatus(Status.FETCH_STARTED);

Previously nextBatch() (defined in the SweApiReplayContext class) was implemented like this:

    async nextBatch(properties, masterTimestamp, status = {cancel:false}) {
        let version = this.properties.version;
        return new Promise(async (resolve, reject) => {
            try {
                let data;
                let results = [];

                const moveTimeCursor = async () => {...}

                const fetchNext = async () => {
                    data = await this.collection.nextPage();
                    if (status.cancel) {
                        reject('Status has been cancelled');
                    }
                    if (data.length > 0) {
                        results = data;
                        for(let d of results) {
                            d.version = version;
                        }
                        if (status.cancel) {
                            reject('Status has been cancelled');
                        } else {
                            // start startTime cursor
                            this.relativeStartTimestamp = results[results.length-1].timestamp;
                            resolve(data);
                        }
                    }
                }

                await moveTimeCursor();
                await fetchNext();
            } catch (ex) {
                reject(ex);
            }
        });
    }

Note that the condition where data.length === 0) leaves this promise hanging resulting in the FETCH_STARTED status never being set. By resolving the promise to an empty array this promise allows the logic in the startLoop() method to continue, setting the FETCH_STARTED status allowing the replay algorithm to process data. I also added an early return in the startLoop() method since subsequent logic is unnecessary if there is no data returned.

Testing:

• Load data with data sources that have data and at least one that does not
• I used the webtak plugin to test
  • Toggle to replay mode
  • Start playback
  • Verify data is visualized