Implementing Accessor

Accessor aims to make developing classes easy by providing a mechanism to get, set, and watch properties.

This guide provides a guideline for common Accessor usage patterns. Please follow the links below to get further information on how to implement classes derived from Accessor. Please see the working with properties guide topic for additional information on Accessor properties.

If working in TypeScript, you will want to install the ArcGIS API for JavaScript 4.x type definitions. You can access these typings with its command line syntax at the jsapi-resources Github repository.

Extend Accessor

Many classes in the API extend the Accessor class. These classes can expose watchable properties that may have unique characteristics, such as being read-only or computed. Under the hood, Accessor uses dojo/_base/declare to create classes.

Create a simple subclass

Simple subclass - TS

Simple subclass - JS

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import Accessor = require("esri/core/Accessor");

import { subclass } from "esri/core/accessorSupport/decorators";

@subclass("esri.guide.Color")
class Color extends Accessor {

}

Mixins with Accessor

The ArcGIS API for JavaScript uses mixins to build its classes. Read this excellent article that goes deep dive on mixins with TypeScript.

First we define our EventedMixin to add an event system to a class.

Defining an Accessor mixin - TS

Defining an Accessor mixin - JS

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import Accessor = require("esri/core/Accessor");
import { subclass } from "esri/core/accessorSupport/decorators";

// A type to represent a constructor function
type Constructor<T = object> = new (...args: any[]) => T;

// A type to represent a mixin function
// See for more details https://www.bryntum.com/blog/the-mixin-pattern-in-typescript-all-you-need-to-know/
type Mixin<T extends (...input: any[]) => any> = InstanceType<ReturnType<T>>;

// TBase extends Constructor<Accessor> indicates that `EventedMixin`
// expects the base class to extend `Accessor`, for example to be able to use the `watch` method.
export const EventedMixin = <TBase extends Constructor<Accessor>>(Base: TBase) => {

  @subclass("esri.guide.Evented")
  class Evented extends Base {

    /**
     * A first function defined by the mixin
     */
    emit(type: string, event?: any): boolean {
      // ...
    }

    /**
     * Another function defined by the mixin
     */
    on(type: string, listener: (event: any) => void): IHandle {
      // ...
    }
  }

  return Evented;
}

// define the type of the mixin. This is useful to type properties that extends this mixin
// eg: `myProperty: EventedMixin;`
export type EventedMixin = Mixin<typeof EventedMixin>;

A mixin is a function that creates the super class for final subclass. In this example we create a super class that extends Accessor and adds capabilities from EventedMixin. The Collection class then extends the final subclass.

Using an Accessor mixin - TS

Using an Accessor mixin - JS

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import Accessor = require("esri/core/Accessor");
import { subclass } from "esri/core/accessorSupport/decorators";

// import the newly created mixin
import { EventedMixin } from "esri/guide/EventedMixin";

@subclass("esri.guide.Collection")
export class Collection extends EventedMixin(Accessor) {
  // Collection extends a super class composed of Accessor and EventedMixin.
}

Properties

Define a simple property

The following syntax should be used when you want to have a simple, watchable, property that does not require any additional behavior. You can define both default values and types for primitive property values. If working with TypeScript, default property values can be set in the constructor.

Simple property - TS

Simple property - JS

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import Accessor = require("esri/core/Accessor");

import { subclass, property } from "esri/core/accessorSupport/decorators";

@subclass("esri.guide.Color")
class Color extends Accessor {

  @property()
  r: number = 255;

  @property()
  g: number = 255;

  @property()
  b: number = 255;

  @property()
  a: number = 1;

}

Define custom getter and setter

There may be times when you may need to verify, validate, or transform values set on a property. You may also need to do additional (synchronous) work when a property is being set. The following snippets show this.

Setter property - TS

Setter property - JS

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import Accessor = require("esri/core/Accessor");

import { subclass, property } from "esri/core/accessorSupport/decorators";

@subclass("esri.guide.Collection")
class Collection extends Accessor {

  private _items: any[] = [];

  // Example: Define a custom property getter.
  //   Accessor caches the values returned by the getters.
  //   At this point `length` will never change.
  //   See the "Notify a property change" section
  @property()
  get length(): number {
    return this._items.length;
  }

  set length(value: number) {
    // Example: perform  validation
    if (value <= 0) {
      throw new Error(`value of length not valid: ${value}`);
    }

    // internally you can access the cached value of `length` using `_get`.
    const oldValue = this._get<number>("length");

    if (oldValue !== value) {
      // a setter has to update the value from the cache
      this._set("length", value);

      // Example: perform additional work when the length changes
      this._items.length = value;
    }
  }

}

Define a read-only property

The following syntax shows how to set a read-only property.

Read-only - TS

Read-only - JS

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import Accessor = require("esri/core/Accessor");

import { subclass, property } from "esri/core/accessorSupport/decorators";

@subclass("esri.guide.Person")
class Person extends Accessor {

  // Example: read-only property may not be externally set
  @property({ readOnly: true })
  firstName: string;

  @property({ readOnly: true })
  lastName: string;

  updateName(firstName: string, lastName: string): void {
    // We may still update the read-only property internally, which will change
    // the property and notify changes to watchers
    this._set({
      firstName: firstName,
      lastName: lastName
    });
  }
}

Define a proxy property

Sometimes you need to proxy a property when both reading and writing, in addition to possibly performing a transformation on the value. For example, exposing an inner member property.

Proxy property - TS

Proxy property - JS

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import Accessor = require("esri/core/Accessor");

import { subclass, aliasOf } from "esri/core/accessorSupport/decorators";

@subclass("esri.guide.GroupLayer")
class GroupLayer extends Accessor {

  @property()
  sublayers: Collection = new Collection();

  // Define a property that reflects one in another object.
  @property({ aliasOf: "sublayers.length" })
  length: number;

  // Alternatively you can use the `@aliasOf` decorator
  //  @aliasOf
  //  length: number

  // You can also proxy a method from another object.
  @aliasOf("sublayers.add")
  add: (item: any) => void;

}

Computed properties

Define a computed property

You may need to use this when a property value depends on numerous other properties. These properties are always read-only.

Computed property - TS

Computed property - JS

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import Accessor = require("esri/core/Accessor");
import { subclass, property } from "esri/core/accessorSupport/decorators";

@subclass()
class Subclass extends Accessor {
  @property()
  firstName: string;

  @property()
  lastName: string;

  @property({
    readOnly: true,
    // define the property dependencies
    dependsOn: ["firstName", "lastName"]
  })
  get fullName(): string {
    return `${this.firstName} ${this.lastName}`;
  }
}

Define a writable computed property

Writable computed property - TS

Writable computed property - JS

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import Accessor = require("esri/core/Accessor");
import { subclass, property } from "esri/core/accessorSupport/decorators";

@subclass()
class Subclass extends Accessor {
  @property()
  firstName: string;

  @property()
  lastName: string;

  @property({
    // define the property dependencies
    dependsOn: ["firstName", "lastName"]
  })
  get fullName(): string {
    return `${this.firstName} ${this.lastName}`;
  }

  set fullName(value: string) {
    if (!value) {
      this._set("firstName", null);
      this._set("lastName", null);
      this._set("fullName", null);

      return;
    }

    const [firstName, lastName] = value.split(" ");
    this._set("firstName", firstName);
    this._set("lastName", lastName);
    this._set("fullName", value);
  }
}

Notify a property change

Sometimes properties cannot notify when changed. Accessor has an internal method to notify of any changes. This will mark the property as dirty. The next time the property is accessed its value is re-evaluated.

Writable computed property - TS

Writable computed property - JS

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import Accessor = require("esri/core/Accessor");

import { subclass, property } from "esri/core/accessorSupport/decorators";

@subclass("esri.guide.Collection")
class Collection extends Accessor {

  private _items: any[] = [];

  @property({
    readOnly: true
  })
  get length(): number {
    return this._items.length;
  }

  add(item: any): void {
    this._items.push(item);

    // We know the value of `length` is changed.
    // Notify so that at next access, the getter will be invoked
    this.notifyChange("length");
  }

}

Autocast

Define the property type

It is possible to define a type for a class' property.

Define the property type - TS

Define the property type - JS

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import Graphic = require("esri/Graphic");

import Accessor = require("esri/core/Accessor");
import Collection = require("esri/core/Collection");

import { subclass, property } from "esri/core/accessorSupport/decorators";

@subclass()
class GraphicsLayer extends Accessor {

  @property({
    // Define the type of the collection of Graphics
    // When the property is set with an array,
    // the collection constructor will automatically be called
    type: Collection.ofType(Graphic)
  })
  graphics: Collection<Graphic>;

}

Define a method to cast a property

Sometimes you need to ensure a property's value type when it is being set. A good example of this is having well-known, preset, names for specific values, such as map.basemap = "streets-vector".

The type metadata automatically creates an appropriate cast for Accessor and primitive types if it is not already set.

Define a casting method - TS

Define a casting method - JS

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import Accessor = require("esri/core/Accessor");
 import { subclass, property, cast } from "esri/core/tsSupport/declare";

 @subclass()
 class Color extends Accessor {

   @property()
   r: number = 0;

   @property()
   g: number = 0;

   @property()
   b: number = 0;

   @property()
   a: number = 1;

   @cast("r")
   @cast("g")
   @cast("b")
   protected castComponent(value: number): number {
     // cast method that clamp the value that
     // will be set on r, g or b between 0 and 255
     return Math.max(0, Math.min(255, value));
   }

   @cast("a")
   protected castAlpha(value: number): number {
     // cast method that clamp the value that
     // will be set on a between 0 and 1
     return Math.max(0, Math.min(1, value));
   }
 }

Additional information

Please refer to these additional links for further information:

PreviousMigrating to ArcGIS Platform location services

ArcGIS API for JavaScript

Implementing Accessor

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