This sample demonstrates how to query for statistics in a FeatureLayerView by geometry and display the results of the query in a chart.
This application displays 2010 population density by census tracts. The population is queried by age and gender among census tracts intersecting a buffer, and is displayed in a population pyramid chart.
How it works
The population pyramid chart is updated as the user moves the buffer by dragging its the center point or resizes the buffer by moving its edge point. As the user moves one of the two points, a geodesic buffer is recalculated and used to query the statistics of all features in the layer view that intersect the buffer. The center and edge points of the buffer graphic are updated by listening to the update event on SketchViewModel.
// Listen to SketchViewModel's update event so that population pyramid chart
// is updated as the graphics are updated
sketchViewModel.on("update", (event) => {
// If the edge graphic is moving, keep the center graphic
// at its initial location. Only move edge graphic
if (
event.toolEventInfo && event.toolEventInfo.mover.attributes.edge
) {
const toolType = event.toolEventInfo.type;
if (toolType === "move-start") {
centerGeometryAtStart = centerGraphic.geometry;
}
// keep the center graphic at its initial location when edge point is moving
else {
centerGraphic.geometry = centerGeometryAtStart;
}
}
// the center or edge graphic is being moved, recalculate the buffer
const vertices = [
[centerGraphic.geometry.x, centerGraphic.geometry.y],
[edgeGraphic.geometry.x, edgeGraphic.geometry.y]
debounceOnMove(vertices).catch((error) => {
if (!promiseUtils.isAbortError(error)) {
throw error;
}
});
if (event.state == "complete") {
sketchViewModel.update([edgeGraphic, centerGraphic], {
tool: "move"
});
}
});
/*********************************************************************
* Edge or center graphics are being moved. Recalculate the buffer with
* updated geometry information and run the query stats again.
*********************************************************************/
const debounceOnMove = promiseUtils.debounce(async (vertices) => {
// client-side stats query of features that intersect the buffer
const buffer = await calculateBuffer(vertices);
// Query female and male age groups of the census tracts that intersect
// the buffer polygon on the client
const newData = await queryLayerViewAgeStats(buffer);
updateChart(newData);
// user is clicking on the view... call update method with the center and edge graphics
if (event.state === "complete") {
sketchViewModel.update([edgeGraphic, centerGraphic], {
tool: "move"
});
}
});The updated buffer polygon is then used to query statistics to get total population belonging to different age categories by gender among census tracts. Once the statistics query results are returned, the information is displayed in the population pyramid chart.
/*********************************************************************
* Spatial query the census tracts feature layer view for statistics
* using the updated buffer polygon.
*********************************************************************/
function queryLayerViewAgeStats(buffer) {
// Data storage for the chart
let femaleAgeData = [],
maleAgeData = [];
// Client-side spatial query:
// Get a sum of age groups for census tracts that intersect the polygon buffer
const query = featureLayerView.layer.createQuery();
query.outStatistics = statDefinitions;
query.geometry = buffer;
// Query the features on the client using FeatureLayerView.queryFeatures
return featureLayerView
.queryFeatures(query)
.then((results) => {
// Statistics query returns a feature with 'stats' as attributes
const attributes = results.features[0].attributes;
// Loop through attributes and save the values for use in the population pyramid.
for (const key in attributes) {
if (key.includes("FEM")) {
femaleAgeData.push(attributes[key]);
} else {
// Make 'all male age group population' total negative so that
// data will be displayed to the left of female age group
maleAgeData.push(-Math.abs(attributes[key]));
}
}
// Return information, separated by gender
return [femaleAgeData, maleAgeData];
})
.catch((error) => {
console.error(error);
});
}