@geodaoyu/accessor

Accessor

Class: Accessor

Accessor is an abstract class that facilitates the access to instance properties as well as a mechanism to watch for property changes. Every sub-class of Accessor defines properties that are directly accessible or by using the get() and set() methods. It is possible to watch for a property changes by using the watch() method.

Installation

npm install @geodaoyu/accessor

Property Overview

Name	Type	Summary	Class
declaredClass	String	The name of the class.	Accessor

Property Details

declaredClass String readonly

The name of the class.

Method Overview

Name	Return Type	Summary	Class
get()	*	Gets the value of a property.	Accessor
set()	*	Sets the value of a property.	Accessor
watch()	WatchHandle	Watches for property changes on the instance.	Accessor

Method Details

get(path){*}

Gets the value of a property.

The name of the property can refer to a property in the instance.

view.get("scale"); 

It can also be a path to a property deeper in the instance. get() returns undefined if a property in the path doesn't exist.

var title = map.get("basemap.title");

// equivalent of
var title = map.basemap && map.basemap.title || undefined;

Parameter:

path	String
The path of the property to get.

Returns:

Type	Description
*	The property's value.

set(path, value){*}

Sets the value of a property.

Call set() with a property name and a value to change the value of the property.

// setting the basemap of the map
map.set("basemap", "topo-vector");
// is equivalent to
map.basemap = "topo-vector";

// currying set
var updateViewScale = view.set.bind(view, "scale");
updateViewScale(5000);

set() can be called with the path to a property and a value. The property is not set if a property in the path doesn't exist.

// updating the title of the basemap
map.set("basemap.title", "World Topographic Map");

// is equivalent to
if (map.basemap != null) {
  map.basemap.title = "World Topographic Map";
}

An object with key-value pairs may be passed into set() to update multiple properties at once.

// setting a viewpoint on the view
view.set({
  center: [-4.4861, 48.3904],
  scale: 5000
});

// currying set
var updateView = view.set.bind(view);

updateView({
  center: [-4.4861, 48.3904],
  scale: 5000
});

Parameters:

path	String | Object
The path to the property to set, or an object of key-value pairs.
value	*
*The new value to set on the property.

Returns:

Type	Description
*	The instance.

watch(path, callback){WatchHandle}

Watches for property changes on the instance.

Watching for property changes is essential for tracking changes on objects. To start watching for changes on a property, call watch() with the property name and a callback function that will execute each time the property changes.

var handle = mapview.watch("scale", function(newValue, oldValue, propertyName, target) {
  console.log(propertyName + " changed from " + oldValue + " to " + newValue);
})

To stop watching for changes, call the remove() method on the object that watch() returns.

handle.remove();

It is important to store the resulting objects from watch() to properly clean up the references.

var viewHandles = [];
function setView(view) {
  // remove the handles for the current view.
  viewHandles.forEach(function(handle) {
    handle.remove();
  });
  viewHandles.length = 0;

  this.view = view;

  // watch for properties on the newly set view.
  if (view) {
    viewHandles.push(
      view.watch("scale", scaleWatcher);
    );
  }
}

setView(mapView);
setView(null);

Like get() and set(), it is possible to watch for a property deep in the object hierarchy by passing a path. If a property in the path doesn't exist the watch callback is called with undefined.

var view = new SceneView({
  map: new Map({
    basemap: "streets-vector"
  })
});

view.watch("map.basemap.title", function(newValue, oldValue) {
  console.log("basemap's title changed from " + oldValue + " to " + newValue);
});

view.map.basemap = "topo-vector";
// output: "basemap's title changed from Streets to Topographic"

view.map = null;
// output: "basemap's title changed from Topographic to undefined"

Pass a comma delimited list of property paths, or an array of property paths, to watch multiple properties with the same callback. Use the third parameter of the callback call to determine what property changed.

view.watch("center, scale, rotation", function(newValue, oldValue, propertyName) {
  console.log(propertyName + " changed");
});

// equivalent of
view.watch(["center", "scale", "rotation"], function(newValue, oldValue, propertyName) {
  console.log(propertyName + " changed");
});

// equivalent of
var callback = function(newValue, oldValue, propertyName) {
  console.log(propertyName + " changed");
}
view.watch("center", callback);
view.watch("scale", callback);
view.watch("rotation", callback);

Accessor doesn't call the watch callbacks for a property immediately after its value changes. Instead, when a property's value changes and if that property is watched, Accessor schedules a notification which is then processed at a later time. Properties that change frequently like view.scale can be watched without having to throttle the callback.

// Divides the view.scale three times
view.watch("scale", function(newValue, oldValue) {
  console.log("view's scale changed from " + oldValue + " to " + newValue);
});
console.log("current view scale: " + view.scale);
view.scale = view.scale / 2;
view.scale = view.scale / 2;
view.scale = view.scale / 2;
console.log("current view scale: " + view.scale);

// output the following:
// current view scale: 36978595.474472
// current view scale: 4622324.434309
// view's scale changed from 36978595.474472 to 4622324.434309

Parameters:

path	String | String[]
The property or properties to watch. Multiple properties can be specified as a comma-separated list.
callback	watchCallback
The callback to execute when the property value has changed.

Returns:

Type	Description
WatchHandle	A watch handle

Type Definitions

watchCallback(newValue, oldValue, propertyName, target)

Callback to be called when a watched property changes.

Parameters:

newValue	*
The new value of the watched property.
oldValue	*
The old value of the watched property.
propertyName	String
The property name.
target	Accessor
The object containing the property being watched.

WatchHandle Object

Represents a watch created when an object invokes watch().

Property:

remove	Function
Removes the watch handle.

Example:

var handle = map.watch('basemap', function(newVal){
  // Each time the value of map.basemap changes, it is logged in the console
  console.log("new basemap: ", newVal);
});

// When remove() is called on the watch handle, the map no longer watches for changes to basemap
handle.remove();