Merge pull request #904 from Polymer/asyncQuery

Adds `@queryAsync` decorator

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 * 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)).
 * Added `enableUpdating()` to `UpdatingElement` to enable customizing when updating is enabled [#860](https://github.com/Polymer/lit-element/pull/860).
 * Added `queryAssignedNodes(slotName, flatten)` to enable querying assignedNodes for a given slot [#860](https://github.com/Polymer/lit-element/pull/860).
 * Added `getStyles()` to `LitElement` to allow hooks into style gathering for component sets [#866](https://github.com/Polymer/lit-element/pull/866).

src/lib/decorators.ts

@@ -207,6 +207,7 @@ export function internalProperty(options?: InternalPropertyDeclaration) {
  *         `;
  *       }
  *     }
+ *
  */
 export function query(selector: string) {
   return (protoOrDescriptor: Object|ClassElement,
@@ -225,6 +226,60 @@ export function query(selector: string) {
   };
 }
 
+// Note, in the future, we may extend this decorator to support the use case
+// where the queried element may need to do work to become ready to interact
+// with (e.g. load some implementation code). If so, we might elect to
+// add a second argument defining a function that can be run to make the
+// queried element loaded/updated/ready.
+/**
+ * A property decorator that converts a class property into a getter that
+ * returns a promise that resolves to the result of a querySelector on the
+ * element's renderRoot done after the element's `updateComplete` promise
+ * resolves. When the queried property may change with element state, this
+ * decorator can be used instead of requiring users to await the
+ * `updateComplete` before accessing the property.
+ *
+ * @param selector A DOMString containing one or more selectors to match.
+ *
+ * See: https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelector
+ *
+ * @example
+ *
+ *     class MyElement {
+ *       @queryAsync('#first')
+ *       first;
+ *
+ *       render() {
+ *         return html`
+ *           <div id="first"></div>
+ *           <div id="second"></div>
+ *         `;
+ *       }
+ *     }
+ *
+ *     // external usage
+ *     async doSomethingWithFirst() {
+ *      (await aMyElement.first).doSomething();
+ *     }
+ */
+export function queryAsync(selector: string) {
+  return (protoOrDescriptor: Object|ClassElement,
+          // tslint:disable-next-line:no-any decorator
+          name?: PropertyKey): any => {
+    const descriptor = {
+      async get(this: LitElement) {
+        await this.updateComplete;
+        return this.renderRoot.querySelector(selector);
+      },
+      enumerable: true,
+      configurable: true,
+    };
+    return (name !== undefined) ?
+        legacyQuery(descriptor, protoOrDescriptor as Object, name) :
+        standardQuery(descriptor, protoOrDescriptor as ClassElement);
+  };
+}
+
 /**
  * A property decorator that converts a class property into a getter
  * that executes a querySelectorAll on the element's renderRoot.

src/test/lib/decorators_test.ts

@@ -13,7 +13,7 @@
  */
 
 import {eventOptions, property} from '../../lib/decorators.js';
-import {customElement, html, LitElement, PropertyValues, query, queryAll, queryAssignedNodes} from '../../lit-element.js';
+import {customElement, html, LitElement, PropertyValues, query, queryAll, queryAssignedNodes, queryAsync} from '../../lit-element.js';
 import {generateElementName} from '../test-helpers.js';
 
 const flush =
@@ -460,6 +460,47 @@ suite('decorators', () => {
     });
   });
 
+  suite('@queryAsync', () => {
+
+    @customElement(generateElementName() as keyof HTMLElementTagNameMap)
+    class C extends LitElement {
+      @queryAsync('#blah') blah!: Promise<HTMLDivElement>;
+
+      @queryAsync('span') nope!: Promise<HTMLSpanElement|null>;
+
+      @property()
+      foo = false;
+
+      render() {
+        return html`
+          <div>Not this one</div>
+          ${this.foo ?
+            html`<div id="blah" foo>This one</div>` :
+            html`<div id="blah">This one</div>` }
+        `;
+      }
+    }
+
+    test('returns an element when it exists after update', async () => {
+      const c = new C();
+      container.appendChild(c);
+      let div = await c.blah;
+      assert.instanceOf(div, HTMLDivElement);
+      assert.isFalse(div.hasAttribute('foo'));
+      c.foo = true;
+      div = await c.blah;
+      assert.instanceOf(div, HTMLDivElement);
+      assert.isTrue(div.hasAttribute('foo'));
+    });
+
+    test('returns null when no match', async () => {
+      const c = new C();
+      container.appendChild(c);
+      const span = await c.nope;
+      assert.isNull(span);
+    });
+  });
+
   suite('@eventOptions', () => {
     test('allows capturing listeners', async function() {
       if (!supportsOptions) {