How to use the @comunica/bus-rdf-resolve-quad-pattern.getDataSourceValue function in @comunica/bus-rdf-resolve-quad-pattern

const proxyIt: AsyncIterator> = sourcesIt.map((source) => {
      remainingSources++;
      sourcesCount++;

      // If we can predict that the given source will have no bindings for the given pattern,
      // return an empty iterator.
      const pattern: Algebra.Pattern = this.algebraFactory.createPattern(
        FederatedQuadSource.nullToVariable(subject, 's'),
        FederatedQuadSource.nullToVariable(predicate, 'p'),
        FederatedQuadSource.nullToVariable(object, 'o'),
        FederatedQuadSource.nullToVariable(graph, 'g'),
      );

      // Prepare the context for this specific source
      const context: ActionContext = this.contextDefault.set(KEY_CONTEXT_SOURCE,
        { type: getDataSourceType(source), value: getDataSourceValue(source) });

      return new PromiseProxyIterator(async () => {
        let output: IActorRdfResolveQuadPatternOutput;
        if (this.isSourceEmpty(source, pattern)) {
          output = { data: new EmptyIterator(), metadata: () => Promise.resolve({ totalItems: 0 }) };
        } else {
          output = await this.mediatorResolveQuadPattern.mediate({ pattern, context });
        }
        if (output.metadata) {
          output.metadata().then((subMetadata: {[id: string]: any}) => {
            if ((!subMetadata.totalItems && subMetadata.totalItems !== 0) || !isFinite(subMetadata.totalItems)) {
              metadata.totalItems = Infinity;
              remainingSources = 0; // We're already at infinite, so ignore any later metadata
              checkEmitMetadata(Infinity, source, pattern, subMetadata);
            } else {
              metadata.totalItems += subMetadata.totalItems;