reactiveUtils

Method Overview

Method Details

on(getTarget, eventName, callback, options){WatchHandle}

Watches the value returned by the getTarget function for changes and automatically adds or removes an event listener for a given event, as needed.

Parameters

getTarget ReactiveOnExpression Function which returns the object to which the event listener is to be added.

eventName String The name of the event to add a listener for.

callback ReactiveOnCallback The event handler callback function.

options ReactiveListenerOptions optional Options used to configure how the tracking happens and how the callback is to be called.

Returns

Type	Description
WatchHandle	A watch handle.

Examples

// Adds a click event on the view when it changes
reactiveUtils.on(
  () => view,
  "click",
  (event) => {
    console.log("Click event emitted: ", event);
  });

// Adds a drag event on the view and adds a callback
// to check when the listener is added and removed.
// Providing `once: true` in the ReactiveListenerOptions
// removes the event after first callback.
reactiveUtils.on(
  () => view,
  "drag",
  (event) => {
    console.log(`Drag event emitted: ${event}`);
  },
  {
    once: true,
    onListenerAdd: () => console.log("Drag listener added!"),
    onListenerRemove: () => console.log("Drag listener removed!")
  });

once(getValue, signal){Promise}

Tracks any properties being evaluated by the getValue function. When getValue changes, it returns a promise containing the value. This method only tracks a single change.

Parameters

getValue ReactiveWatchExpression Expression to be tracked.

signal AbortSignal optional Abort signal which can be used to cancel the promise from resolving.

Returns

Type	Description
Promise	A promise which resolves when the tracked expression changes.

Examples

// Observe for the first time a property equals a specific string value
// Equivalent to watchUtils.once()
reactiveUtils.once(
  () => featureLayer.loadStatus === "loaded")
  .then(() => {
    console.log("featureLayer loadStatus is loaded.");
  });

// Use a comparison operator to observe for a first time
// difference in numerical values
const someFunction = async () => {
  await reactiveUtils.once(() => view.zoom > 20));
  console.log("Zoom level is greater than 20!");
}

// Use a comparison operator and optional chaining to observe for a
// first time difference in numerical values.
reactiveUtils.once(
  () => map?.allLayers?.length > 2)
  .then((value) => {
    console.log(`The map now has ${value} layers.`);
  });

watch(getValue, callback, options){WatchHandle}

Tracks any properties accessed in the getValue function and calls the callback when any of them change.

Parameters

getValue ReactiveWatchExpression Function used to get the current value. All accessed properties will be tracked.

callback ReactiveWatchCallback The function to call when there are changes.

options ReactiveWatchOptions optional Options used to configure how the tracking happens and how the callback is to be called.

Returns

Type	Description
WatchHandle	A watch handle.

Examples

// Watching for changes in a boolean value
// Equivalent to watchUtils.watch()
reactiveUtils.watch(
 () => view.popup.visible,
 () => {
   console.log(`Popup visible: ${visible}`);
 });

// Watching for changes within a Collection
reactiveUtils.watch(
 () => view.map.allLayers.length,
 () => {
   console.log(`Layer collection length changed: ${view.map.allLayers.length}`);
 });

// Watch for changes in a numerical value.
// Providing `initial: true` in ReactiveWatchOptions
// checks immediately after initialization
// Equivalent to watchUtils.init()
reactiveUtils.watch(
 () => view.zoom,
 () => {
   console.log(`zoom changed to ${view.zoom}`);
 },
 {
   initial: true
 });

// Watch properties from multiple sources
const handle = reactiveUtils.watch(
 () => [view.stationary, view.zoom],
 ([stationary, zoom]) => {
   // Only print the new zoom value when the view is stationary
   if(stationary){
     console.log(`Change in zoom level: ${zoom}`);
   }
 }
);

when(getValue, callback, options){WatchHandle}

Watches the value returned by the getValue function and calls the callback when it becomes truthy.

Parameters

getValue ReactiveWatchExpression Function used to get the current value. All accessed properties will be tracked.

callback ReactiveWatchCallback The function to call when the value becomes truthy.

options ReactiveWatchOptions optional Options used to configure how the tracking happens and how the callback is to be called.

Returns

Type	Description
WatchHandle	A watch handle.

Examples

// Observe for when a boolean property becomes not truthy
// Equivalent to watchUtils.whenFalse()
reactiveUtils.when(
  () => !layerView.updating,
  () => {
    console.log("LayerView finished updating.");
  });

// Observe for when a boolean property becomes true
// Equivalent to watchUtils.whenTrue()
reactiveUtils.when(
  () => view?.stationary === true,
  async () => {
    console.log("User is no longer interacting with the map");
    await drawBuffer();
  });

// Observe a boolean property for truthiness.
// Providing `once: true` in ReactiveWatchOptions
// only fires the callback once
// Equivalent to watchUtils.whenFalseOnce()
reactiveUtils.when(
  () => !widget.visible,
  () => {
    console.log(`The widget visibility is ${widget.visible}`);
  },
  {
    once: true
  });

whenOnce(getValue, signal){Promise}

Tracks any properties being evaluated by the getValue function. When getValue becomes truthy, it returns a promise containing the value. This method only tracks a single change.

Parameters

getValue ReactiveWatchExpression Expression to be tracked.

signal AbortSignal optional Abort signal which can be used to cancel the promise from resolving.

Returns

Type	Description
Promise	A promise which resolves once the tracked expression becomes truthy.

Examples

// Check for the first time a property becomes truthy
// Equivalent to watchUtils.whenOnce()
reactiveUtils.whenOnce(
  () => view.popup?.visible)
  .then(() => {
    console.log("Popup used for the first time");
  });

// Check for the first time a property becomes not truthy
// Equivalent to watchUtils.whenFalseOnce()
const someFunction = async () => {
  await reactiveUtils.whenOnce(() => !layerview.updating));
  console.log("LayerView is no longer updating");
}

// Check for the fist time a property becomes truthy
// And, use AbortController to potentially cancel the async callback
const abortController = new AbortController();

// Observe the View's animation state
reactiveUtils.whenOnce(
  () => view?.animation, abortController.signal)
  .then((animation) => {
    console.log(`View animation state is ${animation.state}`)
  });

// Cancel the async callback
const someFunction = () => {
  abortController.abort();
}

Type Definitions

ReactiveEqualityFunction(newValue, oldValue){Boolean}

Function used to check whether two values are the same, in which case the watch callback isn't called.

Parameters

newValue * The new value.

oldValue * The old value.

Returns

Type	Description
Boolean	Whether the new value is equal to the old value.

ReactiveListenerChangeCallback(target)

Callback to be called when a event listener is added or removed.

Parameter

target * optional The event target to which the listener was added or from which it was removed.

ReactiveListenerOptions Object

Options used to configure the behavior of reactiveUtils.

Properties

sync Boolean optional Whether to fire the callback synchronously or on the next tick.

once Boolean optional Whether to fire the callback only once.

onListenerAdd ReactiveListenerChangeCallback optional Called when the event listener is added.

onListenerRemove ReactiveListenerChangeCallback optional Called when the event listener is removed.

ReactiveOnCallback(event)

Function called to be called when an event is emitted or dispatched.

Parameter

event * The event emitted by the target.

ReactiveOnExpression(){*}

Function which is auto-tracked and should return an event target to which an event listener is to be added.

Returns

Type	Description
*	The event target.

ReactiveWatchCallback(newValue, oldValue)

Function to be called when a value changes.

Parameters

newValue * The new value.

oldValue * The old value.

ReactiveWatchExpression(){*}

Function which is auto-tracked and should return a value to pass to the ReactiveWatchCallback

Returns

Type	Description
*	The new value.

ReactiveWatchOptions Object

Options used to configure how auto-tracking is performed and how the callback should be called.

Properties

initial Boolean optional Whether to fire the callback immediately after initialization, if the necessary conditions are met.

sync Boolean optional Whether to fire the callback synchronously or on the next tick.

once Boolean optional Whether to fire the callback only once.

equals ReactiveEqualityFunction optional Function used to check whether two values are the same, in which case the callback isn't called.