Makes createProperty easier to use to customize properties

CHANGELOG.md

@@ -19,7 +19,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## Unreleased
 
 ### Changed
-* The value returned by `render` is always rendered, even if it isn't a `TemplateResult`. ([#712](https://github.com/Polymer/lit-element/issues/712)
+* Added a static `getPropertyDescriptor` method to allow easier customization of property accessors. This method should return a a `PropertyDescriptor` to install on the property. If no descriptor is returned, a property accessor is not be created. ([#911](https://github.com/Polymer/lit-element/issues/911))
+* The value returned by `render` is always rendered, even if it isn't a `TemplateResult`. ([#712](https://github.com/Polymer/lit-element/issues/712))
 
 ### Added
 * Added `@queryAsync(selector)` decorator which returns a Promise that resolves to the result of querying for the given selector after the element's `updateComplete` Promise resolves ([#903](https://github.com/Polymer/lit-element/issues/903)).

src/demo/ts-element.ts

@@ -4,7 +4,7 @@ class TSElement extends LitElement {
   @property() message = 'Hi';
 
   @property(
-      {attribute: 'more-info', converter: (value: string) => `[${value}]`})
+      {attribute: 'more-info', converter: (value: string|null) => `[${value}]`})
   extra = '';
 
   render() {

src/lib/updating-element.ts

@@ -51,7 +51,7 @@ export interface ComplexAttributeConverter<Type = unknown, TypeHint = unknown> {
 }
 
 type AttributeConverter<Type = unknown, TypeHint = unknown> =
-    ComplexAttributeConverter<Type>|((value: string, type?: TypeHint) => Type);
+    ComplexAttributeConverter<Type>|((value: string|null, type?: TypeHint) => Type);
 
 /**
  * Defines options for a property accessor.
@@ -112,6 +112,10 @@ export interface PropertyDeclaration<Type = unknown, TypeHint = unknown> {
    * the property changes.
    */
   readonly noAccessor?: boolean;
+
+  // Allows extension while preserving the ability to use the
+  // @property decorator.
+  [index: string]: unknown;
 }
 
 /**
@@ -281,10 +285,25 @@ export abstract class UpdatingElement extends HTMLElement {
   }
 
   /**
-   * Creates a property accessor on the element prototype if one does not exist.
+   * Creates a property accessor on the element prototype if one does not exist
+   * and stores a PropertyDeclaration for the property with the given options.
    * The property setter calls the property's `hasChanged` property option
    * or uses a strict identity check to determine whether or not to request
    * an update.
+   *
+   * This method may be overridden to customize properties; however,
+   * when doing so, it's important to call `super.createProperty` to ensure
+   * the property is setup correctly. This method calls
+   * `getPropertyDescriptor` internally to get a descriptor to install.
+   * To customize what properties do when they are get or set, override
+   * `getPropertyDescriptor`. To customize the options for a property,
+   * implement `createProperty` like this:
+   *
+   * static createProperty(name, options) {
+   *   options = Object.assign(options, {myOption: true});
+   *   super.createProperty(name, options);
+   * }
+   *
    * @nocollapse
    */
   static createProperty(
@@ -304,7 +323,38 @@ export abstract class UpdatingElement extends HTMLElement {
       return;
     }
     const key = typeof name === 'symbol' ? Symbol() : `__${name}`;
-    Object.defineProperty(this.prototype, name, {
+    const descriptor = this.getPropertyDescriptor(name, key, options);
+    if (descriptor !== undefined) {
+      Object.defineProperty(this.prototype, name, descriptor);
+    }
+  }
+
+  /**
+   * Returns a property descriptor to be defined on the given named property.
+   * If no descriptor is returned, the property will not become an accessor.
+   * For example,
+   *
+   *   class MyElement extends LitElement {
+   *     static getPropertyDescriptor(name, key, options) {
+   *       const defaultDescriptor = super.getPropertyDescriptor(name, key, options);
+   *       const setter = defaultDescriptor.set;
+   *       return {
+   *         get: defaultDescriptor.get,
+   *         set(value) {
+   *           setter.call(this, value);
+   *           // custom action.
+   *         },
+   *         configurable: true,
+   *         enumerable: true
+   *       }
+   *     }
+   *   }
+   *
+   * @nocollapse
+   */
+  protected static getPropertyDescriptor(name: PropertyKey,
+    key: string|symbol, _options: PropertyDeclaration) {
+    return {
       // tslint:disable-next-line:no-any no symbol in index
       get(): any {
         return (this as {[key: string]: unknown})[key as string];
@@ -317,7 +367,23 @@ export abstract class UpdatingElement extends HTMLElement {
       },
       configurable: true,
       enumerable: true
-    });
+    };
+  }
+
+  /**
+   * Returns the property options associated with the given property.
+   * These options are defined with a PropertyDeclaration via the `properties`
+   * object or the `@property` decorator and are registered in
+   * `createProperty(...)`.
+   *
+   * Note, this method should be considered "final" and not overridden. To
+   * customize the options for a given property, override `createProperty`.
+   *
+   * @final
+   */
+  protected static getPropertyOptions(name: PropertyKey) {
+    return this._classProperties && this._classProperties.get(name) ||
+      defaultPropertyDeclaration;
   }
 
   /**
@@ -563,8 +629,7 @@ export abstract class UpdatingElement extends HTMLElement {
     const ctor = (this.constructor as typeof UpdatingElement);
     const propName = ctor._attributeToPropertyMap.get(name);
     if (propName !== undefined) {
-      const options =
-          ctor._classProperties!.get(propName) || defaultPropertyDeclaration;
+      const options = ctor.getPropertyOptions(propName);
       // mark state reflecting
       this._updateState = this._updateState | STATE_IS_REFLECTING_TO_PROPERTY;
       this[propName as keyof this] =
@@ -585,8 +650,7 @@ export abstract class UpdatingElement extends HTMLElement {
     // If we have a property key, perform property update steps.
     if (name !== undefined) {
       const ctor = this.constructor as typeof UpdatingElement;
-      const options =
-          ctor._classProperties!.get(name) || defaultPropertyDeclaration;
+      const options = ctor.getPropertyOptions(name);
       if (ctor._valueHasChanged(
               this[name as keyof this], oldValue, options.hasChanged)) {
         if (!this._changedProperties.has(name)) {

src/test/lib/updating-element_test.ts

@@ -12,8 +12,8 @@
  * http://polymer.github.io/PATENTS.txt
  */
 
-import {property} from '../../lib/decorators.js';
-import {ComplexAttributeConverter, PropertyDeclarations, PropertyValues, UpdatingElement} from '../../lib/updating-element.js';
+import {property, customElement} from '../../lib/decorators.js';
+import {ComplexAttributeConverter, PropertyDeclarations, PropertyValues, UpdatingElement, PropertyDeclaration, defaultConverter} from '../../lib/updating-element.js';
 import {generateElementName} from '../test-helpers.js';
 
 // tslint:disable:no-any ok in tests
@@ -1802,6 +1802,134 @@ suite('UpdatingElement', () => {
         assert.equal(sub.getAttribute('foo'), '5');
       });
 
+    test('can provide a default property declaration', async () => {
+
+        const SpecialNumber = {};
+
+        const myPropertyDeclaration = {
+          type: SpecialNumber,
+          reflect: true,
+          converter: {
+            toAttribute: function(value: unknown, type?: unknown): unknown {
+              switch (type) {
+                case String:
+                  return value === undefined ? null : value;
+                default:
+                return defaultConverter.toAttribute!(value, type);
+              }
+            },
+            fromAttribute: function(value: string|null, type?: unknown) {
+              switch (type) {
+                case SpecialNumber:
+                  return Number(value) + 10;
+                default:
+                return defaultConverter.fromAttribute!(value, type);
+              }
+            }
+          }
+        };
+
+        @customElement(generateElementName())
+        class E extends UpdatingElement {
+
+          static createProperty(
+            name: PropertyKey,
+            options: PropertyDeclaration) {
+            // Always mix into defaults to preserve custom converter.
+            options = Object.assign(Object.create(myPropertyDeclaration), options);
+            super.createProperty(name, options);
+          }
+
+          @property()
+          foo = 5;
+
+          @property({type: String})
+          bar?: string = 'bar';
+        }
+
+        const el = new E();
+        container.appendChild(el);
+        el.setAttribute('foo', '10');
+        el.setAttribute('bar', 'attrBar');
+        await el.updateComplete;
+        assert.equal(el.foo, 20);
+        assert.equal(el.bar, 'attrBar');
+        el.foo = 5;
+        el.bar = undefined;
+        await el.updateComplete;
+        assert.equal(el.getAttribute('foo'), '5');
+        assert.isFalse(el.hasAttribute('bar'));
+      });
+
+  test('can customize property options and accessor creation', async () => {
+
+    interface MyPropertyDeclaration<TypeHint = unknown> extends PropertyDeclaration {
+      validator?: (value: any) => TypeHint;
+      observer?: (oldValue: TypeHint) => void;
+    }
+
+    @customElement(generateElementName())
+    class E extends UpdatingElement {
+
+      static getPropertyDescriptor(name: PropertyKey, key: string|symbol, options: MyPropertyDeclaration) {
+        const defaultDescriptor = super.getPropertyDescriptor(name, key, options);
+        return {
+          get: defaultDescriptor.get,
+          set(this: E, value: unknown) {
+            const oldValue =
+              (this as unknown as {[key: string]: unknown})[name as string];
+            if (options.validator) {
+              value = options.validator(value);
+            }
+            (this as unknown as {[key: string]: unknown})[key as string] = value;
+            (this as unknown as UpdatingElement).requestUpdate(name, oldValue);
+          },
+
+          configurable: defaultDescriptor.configurable,
+          enumerable: defaultDescriptor.enumerable
+        };
+      }
+
+      updated(changedProperties: PropertyValues) {
+        super.updated(changedProperties);
+        changedProperties.forEach((value: unknown, key: PropertyKey) => {
+          const options = (this.constructor as typeof UpdatingElement)
+            .getPropertyOptions(key) as MyPropertyDeclaration;
+          const observer = options.observer;
+          if (typeof observer === 'function') {
+            observer.call(this, value);
+          }
+        });
+      }
+
+      @property({type: Number, validator: (value: number) => Math.min(10, Math.max(value, 0))})
+      foo = 5;
+
+      @property({})
+      bar = 'bar';
+
+      // tslint:disable-next-line:no-any
+      _observedZot?: any;
+
+      @property({observer: function(this: E, oldValue: string) { this._observedZot = {value: this.zot, oldValue}; } })
+      zot = '';
+    }
+
+    const el = new E();
+    container.appendChild(el);
+    await el.updateComplete;
+    el.foo = 20;
+    assert.equal(el.foo, 10);
+    assert.deepEqual(el._observedZot, {value: '', oldValue: undefined});
+    el.foo = -5;
+    assert.equal(el.foo, 0);
+    el.bar = 'bar2';
+    assert.equal(el.bar, 'bar2');
+    el.zot = 'zot';
+    await el.updateComplete;
+    assert.deepEqual(el._observedZot, {value: 'zot', oldValue: ''});
+  });
+
   test('attribute-based property storage', async () => {
     class E extends UpdatingElement {
       _updateCount = 0;